Enzootic in tropical areas. Transmitted by insect vectors. Episodic epizootics in summer in incursive areas probably initiated by wind-borne transmission of insect vector. High morbidity but low case fatality
Disease of cattle with fever, respiratory distress, muscular shivering, stiffness, lameness and enlargement of the peripheral lymph nodes
Generally spontaneous recovery in three days and low case–fatality rate
Leukocytosis, hyperfibrinogenemia, hypocalcemia, elevated creatine kinase. Blocking ELISA for serology
Ephemeral fever is associated with an arthropod-borne rhabdovirus which is the type species of the genus Ephemerovirus. There are a number of strains that vary antigenically.1,2 Other antigenically related but non-pathogenic species of Ephemerovirus occur in the same environment in Australia. The BEF virus is closely associated with the leukocyte–platelet fraction of the blood, and can be maintained deep frozen or on tissue culture and chick embryos.
A disease of cattle, ephemeral fever is enzootic in the tropical areas of Africa, in most of Asia, the Middle East, the East Indies, and in much of Australia with extensions into the subtropics and some temperate regions. In these areas the disease presents as episodic epidemics. Area outbreaks can last several months, with the spread of infection following prevailing winds, and during this period most herds within a region will be infected. The proportion of herds affected in outbreaks in the Jordan Valley in Israel in 1990 and 1999 were 78.5% and 97.7% respectively.3
The morbidity rate in outbreaks is usually between 25% and 45%, but if the population is highly susceptible or the infecting strain virulent, the morbidity rate may reach 100%. In enzootic areas, only 5–10% will be affected. A case–fatality and loss from involuntary culling rate of 1% is usual with low virulence strains but can approach 10%.4,5
The source of infection is the animal affected with the clinical disease and biological vectors.
A great deal of work in recent years has not clearly defined the vector list which probably includes the mosquitoes Aedes spp., Culex annulirostris, Anopheles bancroftii and A. annulipes, and the biting midge Culicoides brevitarsis.6,7 Culex annulirostris has been identified as a biological vector in Australia.8,9 This mosquito can transmit infection within a week of feeding on an infected animal and the epidemiology of the disease in Australia supports transmission by mosquitoes rather than Culicoides spp.10
The reservoir host, other than cattle, has not been identified. This is of particular importance when the epidemiological pattern of occurrence of the disease changes as it has done in Australia. The disease now occurs annually in areas where it used to occur only once each decade, probably because of establishment of the virus in indigenous vectors.11
Spread by wind-borne carriage of vectors is suspected.12,13 Epidemiological studies suggest that outbreaks in Japan originate from Korea.13 Spread is largely independent of cattle movement, and transmission does not occur through contact with infected animals or their saliva or ocular discharge. The disease is not spread through semen, nor is intrauterine administration of the virus a suitable route of transmission.
The disease can be transmitted by the injection of whole blood or the leukocyte fraction of it. Experimental reproduction in cattle requires intravenous administration and viremia lasts 3 days with a maximum of 2 weeks. There is no carrier state.
The disease occurs in the summer months, outbreaks are clustered and relatively short lived,13 and spread depends largely on the insect vector population and the force and direction of prevailing winds.3 The disease tends to disappear for long periods to return in epizootic form when the resistance of the population is diminished.
Recurrence depends primarily on suitable environmental conditions for increase and dissemination of the insect vector.6 During periods of quiescence the disease is still present but the morbidity is reputed to be very low. However, in many enzootic areas the degree of surveillance is less than intense and clinical cases may occur without being observed. Temporary protection against infection is provided by subclinical infections by other unrelated arboviruses, e.g. Akabane, Aino, and others.11
Among domestic animals, only cattle are known to be naturally affected but antibodies can be found in African ruminant wildlife. All age groups of cattle are susceptible but calves less than 3 to 6 months old are not affected by the natural disease. With experimental infections calves as young as 3 months old are as susceptible as adults to experimental infection but do not show clinical disease.
In dairy cattle, higher producing cows are at greater risk and clinical disease may be minimal in cows under 2 years of age. A recent Israeli study in 10 beef herds found an average morbidity and mortality rate of 46.2% and 4.8% respectively with higher rates in bulls than cows and a higher morbidity in cows 2 to 5 years of age than in heifers less than 2 years of age.14 In natural outbreaks there is no breed susceptibility.12
In Africa, based on serological results, the virus is thought to be cycling in populations of wild ruminants between epidemics in domestic cattle. Buffalo (Bubalus bubalis) are susceptible to experimental infection, but it is unlikely that they play any part as a reservoir host.15 After experimental infection of cattle there is solid immunity against homologous strains for up to 2 years. Immunity against heterologous strains is much less durable which probably accounts for the apparent variations in immunity following field exposure.
Although the case–fatality rate is very low, considerable loss occurs in dairy herds due to the depression of milk flow – up to 80% in cows in late lactation. In an Israeli study of eight infected dairy herds, the decline in milk yield from preinfection levels varied between cows and ranged from 30% to 70% with the highest yielding cows having the greatest drop. Following recovery from disease, milk production was still less than that of preinfection levels.4
There is also a lowered resistance to mastitis. Reproductive inefficiency is associated with a significant delay in the occurrence of estrus, abortion in cows and temporary sterility in bulls. Occasional animals die of intercurrent infection, usually pneumonia, or prolonged recumbency. BEF can have a serious effect on the agricultural economy in countries where cattle are used as draught animals. For cattle-exporting countries such as Australia, BEF causes interference with movement of cattle when receiving countries insist on evidence of freedom from the disease.
Experimental production of the disease requires the IV route of transmission. Virus multiplication probably occurs primarily within the vascular system.8 After an incubation period of 2–10 days, there is a biphasic fever with peaks 12–24 h apart.16 The fever lasts 2 d and increased respiratory rate, dyspnea, muscle trembling, limb stiffness and pain are characteristic at this time.17
There is generalized inflammation with vasculitis and thrombosis, serofibrinous inflammation in serous and synovial cavities, and increased endothelial permeability at the same sites.18 The virus can be detected in circulating neutrophils and plasma, the serosal and synovial fluids, the mesothelial cells of synovial membrane and epicardium, and in neutrophils in the fluids.8 Clinical signs are believed caused by the expression of mediators of inflammation coupled with a secondary hypocalcemia.14
Calves are least affected, those less than 3 to 6 months of age showing no clinical signs. Overweight cows, high producing cows, and bulls are affected the most.
In most cases the disease is acute. After an incubation period of 2–4 d, sometimes as long as 10 d, there is a sudden onset of fever (40.5–41°C; 105–106°F), which may be biphasic or have morning remissions. Anorexia and a sharp fall in milk yield occur. There is severe constipation in some animals and diarrhea in others. Respiratory and cardiac rates are increased, and stringy nasal and watery ocular discharges are evident. The animals shake their heads constantly and muscle shivering and weakness are observed. There may be swellings about the shoulders, neck and back.
Muscular signs become more evident on the second day with severe stiffness, clonic muscle movements and weakness in one or more limbs. A posture similar to that of acute laminitis, with all four feet bunched under the body, is often adopted. On about the third day, the animal begins eating and ruminating, and the febrile reaction disappears, but lameness and weakness may persist for 2–3 more days.
Some animals remain standing during the acute stages, but the majority go down and assume a position reminiscent of parturient paresis, associated with hypocalcemia, with the hindlegs sticking out and the head turned into the flank. Occasionally, animals adopt a posture of lateral recumbency. Some develop clinically detectable pulmonary and SC emphysema, possibly related to a nutritional deficiency of selenium.19 In most cases recovery is rapid and complete after an illness of 3–5 d unless there is exposure to severe weather, or unless aspiration of a misdirected drench or ruminal contents occurs. Some cases have a second episode of clinical disease 2 to 3 weeks after recovery.
Occasional cases show persistent recumbency and have to be destroyed and abortion occurs in a small proportion of cases. Affected bulls are temporarily sterile. Milder cases, with clinical signs restricted to pyrexia and lack of appetite, may occur at the end of an epizootic.
Blood taken from cattle in the febrile stage clots poorly.20 A marked leukocytosis with a relative increase in neutrophils occurs during the acute stage of the disease. There is a shift to the left and a lymphopenia. Plasma fibrinogen levels are elevated for about 7 d17 and there is a marked increase in creatine kinase. In natural cases, but not experimentally produced ones, a significant hypocalcemia occurs.17,21,22 Available serological tests include a complement fixation test, serum neutralization, fluorescent antibody test, agar gel immunodiffusion (AGID) test,19 and a blocking ELISA, which is reported to be simple and the preferred test.23
Postmortem lesions are not dramatic. The most consistent lesions are a serofibrinous polyserositis, involving synovial, pericardial, pleural and peritoneal cavities, with a characteristic accumulation of neutrophils in these fluids and surrounding tissues. Hemorrhage may also be observed in the periarticular tissues, and there may be foci of necrosis in the musculature of the limbs and back. All lymph nodes are usually enlarged and edematous. Pulmonary emphysema and fibrinous bronchiolitis are standard findings and subcutaneous emphysema along the dorsum may be observed. Characteristic microscopic findings consist of a mild vasculitis of small vessels, with perivascular neutrophils and edema fluid plus intravascular fibrin thrombi.
Necropsy examinations of animals that develop persistent recumbency have shown severe degenerative changes in the spinal cord similar to those produced by physical compression but the pathogenesis of these lesions remains uncertain. Although nucleic acid sequences of the agent are known, PCR tests are not yet widely utilized.
Antigen in reticuloendothelial cells can be detected by immunoperoxidase and immunofluorescent techniques.24
• Virology – chilled lung, spleen, synovial membrane, pericardium (ISO)
Palliative intramuscular treatment with the non-steroidal anti-inflammatory drugs phenylbutazone (8 mg/kg at 8-hour intervals) or flunixin meglumine (2.2 mg/kg/d) or with ketoprofen (3 mg/kg/d) result in remission of signs without in any way influencing the development of the disease.21,22,25,26 There is little effect on the respiratory manifestations of the disease but a major effect on stiffness, lameness and anorexia. All treatments are continued for 3 days. Phenylbutazone may be most effective but the injection frequency is less practical. Parenteral treatment with calcium borogluconate should be given to cows that show signs of hypocalcemia and field observations are that parenteral treatment with calcium solutions often helps to get a recumbent cow to her feet. Proper nursing of the recumbent animal is required.
Restriction of movement from infected areas is practiced but vaccination is the only effective method of control. Vaccines prepared from attenuated tissue culture virus or in mouse brain and adjuvanated in Freund’s incomplete or Quil A adjuvants are commercially available in Australia, Japan, Taiwan and South Africa. Two vaccinations are required and are effective in preventing disease in natural outbreaks for periods up to 12 months.27 The use of vaccination in Japan is credited with preventing further major outbreaks.13 Attenuated vaccines are expensive to produce, have a short shelf-life, and breakdowns are recorded after their use. Formalin killed vaccines are used, with and without adjuvants, and give protection for approximately 6 months, requiring frequent boosting for effect.2,5 Vaccines prepared from the envelope glycoprotein (G protein) protect against experimental challenge.21 Immunity is positively correlated with the level of specific antibody measured with a blocking ELISA or as virus neutralizing antibody.
1 Wang-Funln, et al. J Vet Diag Invest. 2001;13:462.
2 Nandi S, Negi BS. Comp Immunol Microbiol Infect Dis. 1999;22:81.
3 Yeruham I, et al. Vet Rec. 2002;151:117.
4 Yeruham I, et al. Vet Rec. 2003;153:180.
5 Wang F-I, et al. J Vet Diag Invest. 2001;13:462.
6 Davies EG, et al. Bull WHO. 1985;63:941.
7 Standfast HA, Miller MJ. Della-Porta AJ, editor. Veterinary viral diseases. New York: Academic Press. 1985:394.
8 St George TD. Proc 1st Int Symp ACIAR Bejing. 1993;44:13.
9 Kirkland PD. Proc 1st Intl Symp ACIAR Bejing. 1993;44:33.
10 Murray MD. Aust Vet J. 1997;75:20.
11 St George TD. Vet Microbiol. 1985;10:493.
12 Abu Elzein EME, et al. Vet Rec. 1997;140:630.
13 Shirakawa H, et al. Aust Vet J. 1994;71:50.
14 Yeruham I, et al. Vet Rec. 2003;152:86.
15 Wenbin B, et al. Aust Vet J. 1989;66:373.
16 Young PL, Spradbrow PB. Vet Rec. 1990;126:86.
17 Uren MF, Murphy GM. Vet Microbiol. 1985;10:505.
18 Young PL, Spradbrow PB. J Comp Pathol. 1990;102:105.
19 Gard GP, Melville LF. Aust Adv Vet Sci. 1985:152.
20 St George TD. Aust Vet J. 2000;78:857.
21 Uren MF, et al. Vaccine. 1994;12:845.
22 Fenwick DC, Daniel RCW. Aust Vet J. 1996;74:37.
23 Zakrzewski H, et al. J Immunol Methods. 1992;151:289.
24 Shehab GG, et al. Vet Med J Giza. 2004;52:135.
25 Uren MF, et al. Vet Microbiol. 1989;19:99.
Infectious, non-contagious, arthropod-borne disease of horses, donkeys and mules endemic to sub-Saharan Africa. Epizootics occur in the Iberian peninsula, Mediterranean coasts, Middle East and Indian subcontinent
Pulmonary form: fever, respiratory distress, frothy nasal discharge, death. Cardiac form: fever, edema of the head and ventral chest, hydropericardium. Mixed form has characteristics of both pulmonary and cardiac forms. Horse fever: mild fever, often inapparent infection
Leukopenia, disseminated intravascular coagulation. Serology often negative in horses that die acutely
African horse sickness is an important disease of horses and mules in southern and central Africa and, during epizootics, in northern Africa and the Arabian and Iberian peninsulas. The disease in southern Africa occurs as frequent, intermittent small outbreaks and as periodic epidemics that kill large numbers of horses. An epidemic during 1854–55 killed over 17 000 horses, 40% of the horse population, in the Western Cape region. During pre-mechanized exploration and development of southern and central Africa and during the Boer war the disease had a major economic and military impact. For example, during a single campaign in the Boer War, of 1732 British horses involved 323 died of African Horse Sickness within a 17 day period in late April, 1901.1
African horse sickness (AHS) is associated with a viscerotropic orbivirus (family Reoviridae) of which nine antigenic strains (serotypes) are recognized. The serotypic differences are due to variations in the capsid proteins, predominantly VP2 and to a lesser extent VP5. VP2 contains the predominant neutralizing epitopes although antibodies to VP5 are one of the earliest serologic markers of infection and have neutralizing activity.2 Lineages are also evident within serotypes and the resultant clades are grouped geographically, at least for the serotypes studied.3 Identification of clades facilitates epidemiologic studies.3 There are also variants of each serotype with attenuated virulence.4 No new serotypes have been identified since 1960 and all virtually all epidemics outside of southern Africa, with the exception of that in the Iberian peninsula in 1987–90 (serotype 4), were caused by serotype 9.5 The virus is similar to other animal orbiviruses including bluetongue virus, enzootic hemorrhagic disease virus, and equine encephalosis virus. The host range includes equids (horses, donkeys, mules, zebra), elephants, camels, sheep, goats, and predatory or scavenging carnivores.4 Infection produces disease in horses and mules, and less commonly African donkeys, but rarely in the other herbivorous hosts.
The virus is inactivated by heating at 50°C for 3 hours or 60°C for 15 minutes, is stable at 4°C, and survives for 37 days at 37°C. It remains viable at pH of 6–12, but is inactivated by acid and in 48 hours by 0.1% formalin or phenol, sodium hypochlorite, and iodophores.6,7
Disease associated with African horse sickness virus is on List A of the Office International des Epizooties.6
African horse sickness is an infectious but not contagious disease of Equidae. It is spread by the bite of blood-feeding insects.
The disease is enzootic in sub-Saharan Africa, causing clinical disease in horses, donkeys, mules and dogs, and infecting zebras, elephants and perhaps other wildlife.8 The disease occurs from Senegal through sub-Saharan Africa to Somalia.5 The disease makes occasional incursions into Iran, Pakistan, India, Turkey and the eastern Mediterranean and Cyprus. The virus occurs in the Middle East, including Saudi Arabia and the Yemen. It does not appear to be enzootic to Saudi Arabia9 although the long-term status of this region is uncertain. In 1987 the disease recurred in Spain through introduction of infected zebras into a game park. By 1990 the disease had spread throughout Spain and Portugal but was eliminated by 1991.
The disease has been recognized in South Africa since shortly after introduction of domesticated horses in the 1600s. The disease occurred throughout what is now South Africa in the 19th and early 20th centuries, but as an enzootic disease became restricted to the north east of the country in the middle and later part of the 20th century. The geographic contraction of disease was associated with elimination of large herds of wild zebra from all except the game parks of the north east of the country. Elimination of zebra, the reservoir of infection, reduced the occurrence of the disease dramatically.10 Outbreaks of disease outside of the endemic areas in the north east of South Africa are associated with introductions of virus from endemic areas at times of high abundance of Culicoides spp., the vector. The disease does not overwinter in the essentially zebra-free non-endemic areas. Serotype 9 causes enzootic disease in central Africa in the absence of zebra – the wildlife host has not been identified.
African horse sickness occurred in 1999 in the surveillance zone of the Cape Province of South Africa surrounding the disease-free area of Cape Town. The virus (serotype 7) was of a clade identical to that found in Kwazulu Natal Province13 and its introduction was by the movement of infected horses from that region into the Cape Province.
African horse sickness virus (AHSV) is transmitted by the bite of hematophagous insects including midges (Culicoides spp.), ticks (Hyalomma dromadarii and the brown dog tick, Rhipicephalus sanguineus), and mosquitoes (various species in laboratory studies).11 Midges are by far the most important vector in the spread of the spontaneous disease. The source of virus for midges is blood of infected horses, donkeys, mules and zebra. Horses and mules have clinical signs of disease while viremic, but donkeys are often and, most importantly, zebra are always, inapparently infected. Zebras may remain viremic for 6 weeks, donkeys for 12 d, and horses for 18–21 d.10,12,13 Dogs are infected by eating infected animals, although transmission to and from dogs by ticks can occur.
Transmission of the virus to areas where it does not usually exist occurs both by movement of infected animals, such as zebras and horses, and by transportation of midges by wind. Mechanical transmission of the virus on contaminated surgical instruments and needles should be considered a possibility.
In areas in which the disease is enzootic, the virus persists by cycling between the mammalian host, the zebra, and vectors year round.10,12 Zebra in enzootic areas can seroconvert during any month of the year, indicating that persistence of the virus is associated with sequential infection of zebra within a herd or region. Persistence of the virus in a region is attributable to the long period of viremia in zebra and the presence of a herd of sufficient size to support cycling of infection among animals. The minimum size of a zebra population to maintain an enzootic infection is unknown.10 However, in areas in which the disease is not enzootic, the virus does not persist over the cooler winter months when viremic animals recover and the vectors die. Concern exists that reintroduction of zebra to areas of the country currently free of enzootic AHS might permit reestablishment of the virus and disease in horses.5,10
Midges are infected with AHSV, that is they are not mechanical vectors but rather the virus infects and replicates in the midge, although transovarial transmission of infection between generations of midges does not occur. C. imicola is the vector responsible for the transmission of AHSV within its enzootic area, and during epizootics. C. bolitinos is also a vector of AHSV in southern Africa14 while a number of other Culicoides spp. are unlikely to be vectors as they are unable to maintain infection with virus 10 days after ingesting a meal of infected blood.15 However, C. varipenis, C. pulicaris, and C. obsoletus are competent and likely important vectors because of their ability to maintain infection over winter, as demonstrated in Portugal.16
The abundance of midges can be predicted from measures of soil moisture content and land surface temperature.4 Midges breed in damp soils that are rich in organic material, such as irrigated pastures, that provide soil moisture adequate for completion of the life cycle (at least 7 to 10 days).5 Higher temperatures increase the rates of infection of midges, virogenesis within midges, and transmission rate but decrease midge longevity.5 Replication of AHSV in midges does not occur at temperatures <15°C although midges continue to be active at 12°C. Midges can be transported by winds for up to 700 km.17
The incidence of the disease is often seasonal because of the seasonal variations in the number of Culicoides spp. present and possibly other weather related factors such as host (zebra) behavior. Vector activity is favored by temperatures between 12.5 and 29°C and it is likely that several cool or cold episodes, rather than one ‘killing frost’, are necessary to kill all or most vectors.12 Local factors, including topography, influence the distribution of midges within their overall range and therefore the disease has a geographical distribution: the areas most severely affected are low lying and swampy.
Epizootics of AHS occur in southern Africa in association with variations in the El Nino/southern oscillation.18 Epizootics of the disease occur in years in which the oscillation produces drought followed by heavy rains. The reason for this association, which was first anecdotally reported in the 1800s, is unknown but could be related to congregation of zebra around water holes during the drought. Congregation of large numbers of zebra might increase the infection rate among midges which then disseminate the infection when rains produce widespread conditions favorable to their reproduction.18
Natural infection occurs in Equidae, the most severe disease occurring in horses, with mules, donkeys and zebras showing lesser degrees of susceptibility in that order. The virus causes severe disease in dogs.19 Elephants seroconvert when exposed to infection, but are probably not an important reservoir.20 The case fatality rate varies depending on the severity of disease (see under ‘Clinical signs’) but can be as high as 90% in susceptible horses, but is lower in mules and donkeys.
After natural infection or vaccination immunity to that strain, but not to heterologous strains, is solid. The development of immunity is slow and may require 3 weeks to be appreciable: titers may continue to rise for 6 months after infection. Foals from immune dams derive passive immunity from the colostrum and are immune until 5–6 months of age.
The disease was of tremendous economic concern in southern Africa when horses were important for transportation and as draft animals. The disease is currently an economic concern because of the costs associated with preventive measures in enzootic areas, monitoring for introduction of disease in neighboring unaffected areas, and restrictions on importation of horses from countries in which the disease is enzootic. The high case fatality rate and morbidity of the disease in outbreaks is another source of loss. The cost of disease epizootics can be large, as demonstrated by the outbreak in the Iberian Peninsula where control of the disease in Portugal in 1990–1991 was achieved at a cost of US$2 000 000.21
African horse sickness caused encephalitis and chorioretinitis in eight workers in an AHS vaccine factory. Infection was likely be inhalation of freeze-dried virus.22
AHSV affects vascular endothelium and monocytes/macrophages.23,24 The tissue tropism of the infecting serotype determines which organs are most severely affected, with virus serotypes affecting endothelium in different organs, resulting in a variety of ‘forms’ or clinical presentations of the disease. After infection, the virus multiplies in local lymph nodes and a primary viremia ensues with dissemination of infection to endothelial cells and intravascular macrophages of lung, spleen and lymphoid tissues. Viral multiplication then results in a secondary cell-associated (red cell and white cell) viremia in horses of up to 9 days duration.5 Fever and viremia occur at the same time and resolution of the viremia is associated with defervescence. Localization of antigen depends on the form of the disease – horses with horse sickness have most of the antigen in the spleen whereas horses with the more severe cardiopulmonary form have abundant antigen in cardiovascular and lymphatic systems.5
Infection of endothelial cells results in degenerative changes, increases in vascular permeability, impaired intercellular junctions, loss of endothelium, subendothelial deposition of cell debris and fibrin, and evidence of vascular repair.23 Edema, hemorrhage, and microthrombi are associated with the vascular lesions. Abnormalities in the lungs include development of alveolar and interstitial edema, sequestration of neutrophils and platelet aggregates and formation of fibrinous microthrombi.24 Combined these changes likely result in a coagulopathy, systemic inflammatory response syndrome, edema, impaired cardiovascular and pulmonary function and hypovolemia.
The incubation period in natural infections is about 5–7 d. Three or four clinical forms of the disease occur, an acute or pulmonary form, a cardiac or subacute form, a mixed form, and a mild form known as ‘horse sickness fever’. An intermittent fever of 40–41°C (105–106°F) is characteristic of all forms.
This is the most common form in epizootics and has a case fatality rate of 95%. Fever is followed by labored breathing, severe paroxysms of coughing and a profuse nasal discharge of yellowish serous fluid and froth. Profuse sweating, profound weakness and a staggery gait progress to recumbency. Death usually occurs after a total course of 4–5 d although it can be so acute as to be without observed premonitory signs in some horses. Severe respiratory distress persists for many weeks in surviving animals. This is the form of the disease that occurs naturally in dogs.
Subacute (cardiac) horse sickness is most common in horses in enzootic areas and has a case fatality rate of 50%. The incubation period may be up to 3 weeks, and the disease has a more protracted course than does the acute, pulmonary form. There is edema in the head, particularly in the temporal fossa, the eyelids and the lips, and the chest which may not develop until the horse has been febrile for a week. The oral mucosa is bluish in color and petechiae may develop under the tongue. Examination of the heart and lungs reveals evidence of hydropericardium, endocarditis and pulmonary edema. Restlessness and mild abdominal pain and paralysis of the esophagus, with inability to swallow and regurgitation of food and water through the nose, is not uncommon. Recovery is prolonged. A fatal course may last as long as 2 weeks.
A mixed form of the disease, with both pulmonary and cardiac signs, is evident as an initial subacute cardiac form that suddenly develops acute pulmonary signs. Also, a primary pulmonary syndrome may subside but cardiac involvement causes death. This mixed form is not common in field outbreaks.
A mild form of horse sickness fever that may be easily overlooked, and is common in enzootic areas. The disease occurs in horses with some immunity or infection by serotypes of low virulence. This is the only form of the disease that occurs in zebras. The temperature rises to 40.5°C (105°F) over a period of 1–3 d but returns to normal about 3 d later. The appetite is poor, there is slight conjunctivitis and moderate respiratory distress.
Leukopenia, with lymphopenia, neutropenia and a left shift, mild thrombocytopenia and hemoconcentration are characteristic of the acute forms of AHS.25 Serum biochemical abnormalities include increases in creatine kinase, lactate dehydrogenase, and alkaline phosphatase activities and creatinine and bilirubin concentrations.25 There is evidence of activation of coagulation cascade and fibrinolysis although disseminated intravascular coagulation is unusual.25
Serological diagnosis of the acute disease may be difficult because many horses die before they mount a detectable antibody response.26 In horses that survive for at least 10 d, agar gel immunodiffusion (AGID), indirect fluorescent antibody (IFA), complement fixation (CF), virus neutralization (VN) and ELISA tests are all effective in detecting antibody to the virus.27,28 An indirect ELISA (I-ELISA) is more sensitive in detecting early immunological responses to vaccination or infection and the declining immunity in foals.29 However, in outbreaks of disease early and accurate diagnosis of disease and identification of the serotype involved is important to guide selection of vaccine and thereby control spread of the disease. Diagnosis early in the course of the disease can be achieved by demonstration of viral antigen or nucleic acid in blood or tissue samples by any of a number of ELISA tests. A recent RT-PCR test enables rapid identification and differentiation of viral serotype from both live and formalin inactivated virus.30 This test has utility in the early diagnosis of outbreaks of AHS. Viral isolation can be achieved in suckling mice or cell culture. Suitable samples are blood collected into heparin during the febrile stage of the disease or lung, spleen or lymphoid tissue collected at necropsy.
Tests approved for testing horses for international trade include a complement fixation test and an indirect sandwich ELISA.31
The fulminant disease in groups of horses is characteristic, although acute intoxication by monensin, salinomycin, or similar compounds can produce similar signs. Individual horses affected with purpura hemorrhagica and groups of horses affected with equine viral arteritis can have signs similar to horses with AHS. Piroplasmosis (B. caballi or T. equi) and trypanosomiasis cause fever and depression. Anthrax can cause acute deaths in solitary horses or groups of horses.
Gross findings in acute cases include severe hydrothorax and pulmonary edema and moderate ascites. The liver is acutely congested and there is edema of the bowel wall. The pharynx, trachea and bronchi are filled with yellow serous fluid and froth. In cases of cardiac horse sickness there is marked hydropericardium, endocardial hemorrhage and myocardial degeneration. Edema of the head and neck is common, especially of the supraorbital fossa and nuchal ligament. Microscopic lesions are minimal in the acute form; pulmonary edema but no obvious vascular injury. Myocardial damage, including foci of necrosis, hemorrhage, and mild leukocytic infiltrates, may be seen during histologic examination of many cardiac (subacute) cases.
There is no specific treatment for AHS. Supportive care and treatment of complication of the disease should be provided.
The principles of control in enzootic areas are vaccination and reduction of exposure of horses to biting insects, whereas in non-enzootic areas the aim is to prevent introduction of the disease, and eradication if it is introduced. The objectives of a control program for African horse sickness are:5
• Prevention of introduction of infection by clinically ill or inapparently infected animals
• Slaughter of viremic animals where animal welfare and economic considerations permit this course of action
• Management changes to reduce exposure to midges
• Induction of active immunity in animals at risk of disease.
Infection can be introduced into an area free of AHSV by infected animals or midges. Control of midges is discussed below. Infected animals can be horses incubating the disease, clinically ill animals, or animals, including donkeys and zebras, that have no clinical signs of illness but are infected and viremic, as was the case of the Portuguese epizootic. Appropriate control measures to prevent movement of animals at risk of being infected should be instituted and include32: completion of a vaccination protocol effective against all important serotypes at least 42–60 days before introduction of the horse, positive identification of all horses by microchipping and passport documenting vaccination status, and a veterinary certificate confirming health and issued no more than 48 hours before introduction. Equids imported from areas in which the disease is enzootic, or from neighboring regions, should be housed in isolation in insect proof enclosures for 60 days. Recommendations that call for vaccination of all equids within 10 miles (16 km) of imported horses are not appropriate for most countries to which the disease is exotic.
This extreme measure is appropriate in controlling infection recently introduced into areas previously free of the disease. It is an effective adjunct in control of spread of infection, as demonstrated in Portugal.21 There are obvious economic, animal welfare and public relations aspects to this practice, especially in areas where horses have high intrinsic worth or are companion animals.
Horses should be housed in insect proof buildings or, at a minimum, buildings that limit exposure of horses to midges by closure of doors and covering of windows with gauze.33 Impregnation of gauze with an insecticide further reduces biting rates.5 Stables should be situated in areas such as on hill tops or well-drained sites, that have minimal midge populations. Midge numbers on individual farms should be reduced by habitat alteration, so that areas of damp, organically enriched soils are eliminated. Widespread use of insecticides is unlikely to be environmentally acceptable.5
The feeding pattern of midges is such that housing of horses during the crepuscular periods and at night will significantly reduce biting rates and likelihood of infection.5 Horses kept at pasture should have insect repellents applied regularly and especially to provide protection during periods of high insect biting activity. DEET (N,N-diethy-meta-toluamide) is the only commercially available repellent with documented activity against Culicoides spp.34
Vaccination is used in two circumstances – in areas in which the disease is endemic and in regions with an epizootic of the disease. Vaccination can be used in enzootic or neighboring regions to provide active immunity of all resident equids because of the continual risk of the disease in these areas. Vaccination in this instance is initiated as soon as foals no longer have passive immunity to the virus, and continues annually throughout the horse’s life. Alternatively, vaccination may be used in the face of an epizootic to induce active immunity in horses in contact or in regions surrounding the outbreak. In this instance, vaccination is stopped when the infection is eradicated from the area.
Early attenuated virus vaccines, while effective in preventing AHS, were associated with significant side-effects, such as encephalitis. More recent vaccines of virus attenuated by passage through tissue culture are effective in preventing disease but do not prevent viremia. They were used to control the most recent outbreak in Spain and Portugal. Currently available vaccines are polyvalent or monovalent preparations containing attenuated strains of the virus. Protection against heterologous serotypes is usually weak and most vaccines are polyvalent. The polyvalent vaccines contain serotypes 1, 3, and 4 or serotypes 2, 6, 7, and 8, respectively.35 AHSV-9 is not included as serotype 6 is cross protective.5 A monovalent vaccine containing attenuated serotype 9 is used in western Africa where this is the only serotype present.5 Inactivated vaccines are effective in preventing viremia in most animals and disease without side-effects.27 Inactivated vaccines are no longer available and vaccination with sub-unit vaccines and DNA vaccination are experimental at the time of writing.
The recommended vaccination program for horses in South Africa is:36
• The primary vaccination consists of Horse Sickness Vaccine I (AHS I) and Horse Sickness Vaccine II (AHS II) administered at least 3 weeks apart to foals between 1 February and 31 July
• The primary vaccination should preferably not be administered before foals are 6 months of age to avoid the effect of maternally derived passive immunity on vaccine efficacy
• Revaccination with AHS I and AHS II at least 3 weeks apart to yearlings between 1 August and 31 January
• Subsequent revaccination with AHS I and AHS II either at intervals not exceeding 12 months, or every year between 1 July and 31 December.
Immunity after vaccination is protective for at least 1 year but annual revaccination of all horses, mules and donkeys is recommended.
There is concern over the use of attenuated virus vaccines in epizootic situations, that is in regions where AHSV is not enzootic. These reasons include the lack of vaccines approved for use in the European Community, the availability of only two types of polyvalent vaccines and one type of monovalent vaccine, delays in availability of vaccine for emergency vaccination, introduction of virus, even attenuated virus, into regions in which it is not present, attenuated virus viremia in some vaccinated horses, and reversion of vaccine strains to virulence.5 These concerns have heightened the need for availability of inactivated virus or subunit vaccines.
1 Smith F. Veterinary history of the War in South Africa 1899–1902. London: H&W Brown, 1914;166.
2 Martinez-Torrecuadrada JL, et al. Virology. 1999;257:449.
3 Koekemoer JJO, et al. Virus Res. 2003;93:159.
4 Calisher CH, Mertens PPC. African horse sickness. Wein, Austria: Springer-Verlag, 1998;3.
5 Mellor PS, Hamblin C. Vet Res. 2004;35:445.
6 http://www.oie.int/eng/maladies/fiches/a_A110.htm. Accessed 21 February 2005.
7 Shirai J, et al. J Vet Med Sci. 2000;62:85.
8 Binepal VS, et al. Vet Microbiol. 1992;31:19.
9 Al-Afaleq AI, et al. Rev Sci Tech Off Int Epiz. 1998;17:777.
10 Barnard BJH. African horse sickness. Wein, Austria: Springer-Verlag, 1998;14.
11 Mellor PS, Boorman J. Annals Trop Med Parasitol. 1995;89:1.
12 Skowronek AJ, et al. Vet Pathol. 1995;32:112.
13 Hamblin C, et al. African horse sickness. Wein, Austria: Springer-Verlag, 1998;37.
14 Meiswinkel R, Paweska JT. Prev Vet Med. 2003;60:243.
15 Venter GJ, et al. Med Vet Entom. 2000;14:245.
16 Capela R, et al. Med Vet Entom. 2003;17:165.
17 Sellers RF. J Hyg. 1980;85:65.
18 Baylis M, et al. Nature. 1999;397:574.
19 van Rensburg IBJ. J S Afr Vet Assoc. 1981;52:323.
20 Barnard BJH, et al. Onderst J Vet Res. 1995;62:271.
21 Portas M, et al. Epidemiol Infect. 1999;123:337.
22 van der Meyden CH, et al. Sth Afr Med J. 1992;81:451.
23 Gomez-Villamandos JC, et al. J Comp Path. 1999;121:101.
24 Carrasco L, et al. J Comp Path. 1999;121:25.
25 Mellor PS. Comp Immun Infect Dis. 1994;17:287.
26 Martinez-Torrecuadrada JL, et al. J Clin Microbiol. 1997;35:531.
27 House C, et al. Ann N Y Acad Sci. 1992;653:228.
28 Kweon CH, et al. J Virol Meth. 2003;113:13.
29 Maree S, Paweska JT. J Virol Meth. 2005;125:55.
30 Sailleau C, et al. J Gen Virol. 2000;81:831.
31 http://www.oie.int/eng/normes/mmanual/A_00034.htm. Accessed 22 February 2005.
32 http://www.freyja.co.za/ahscontrolpolicy.htm. Accessed 22 February 2005.
33 Meiswinkel R, et al. Bull Entom Res. 2000;90:509.
34 Braverman Y, et al. Med Vet Entamol. 1997;11:355.
35 House JA. African horse sickness. Wein, Austria: Springer-Verlag, 1998;297.
36 www.jockeyciubsa.co.za. accessed 22 February 2005
Encephalomyocarditis is a viral infection of rodents, transmissible to domestic animals and man but it is only a significant pathogen in pigs and elephants. It is postulated as a risk in xenotransplantation.1
The cause is a cardiovirus (family Picornaviridae) that is primarily a pathogen of rodents but which has the ability to produce disease in domestic animals, wild and zoo animals (particularly elephants), primates, and artiodactyls and man. It is found worldwide but its seriousness as a pathogen varies from probably inconsequential in the USA to important in Belgium.2
It was first described as a cause of neonatal mortality in 1975.3 When the disease was first described, pigs from 3–6 weeks were affected with myocarditis and encephalitis.4 It is now known that the virus may cause reproductive failure in gilts and sows characterized by stillbirths and mummified fetuses.5 The prevalence of inapparent infection in the swine population is high.
Outbreaks of the disease or serological evidence of the virus have been reported from the United States,6-8 Canada,9 Australia,4,10 Italy,11 Greece,12 and many other countries including central America and now Venezuela.13 Most work appears to have been carried out in Belgium14,15 where there were major outbreaks in 1995–96 due to a new virus.16 Serological studies of pigs in the United Kingdom, where the disease has not yet been recorded, found approximately 30% possess antibody to the virus. It is extraordinary that there never has been an outbreak that could be described clinically, pathologically or histologically as EMCV considering the close proximity of the UK to Belgium where the disease is widely reported. First reported in 1991 in Belgium, between 1995 and 1996 the disease was diagnosed 154 times in Belgium16 either as a cause of myocardial failure with sudden death in finishing pigs and suckling piglets or as a cause of reproductive failure in sows. Their experience suggests that each isolate is specific for one age category and that the spread of the virus is limited. This recent finding suggests that rodents do play a part in the transmission of the virus but that pig to pig transmission is equally important as a source of infection.
In Iowa,17 infection is widespread in swine herds; the true prevalence of infection in breeding stock is estimated at 13.8% and 8.5% in finishing animals.8 About 90% of the herds surveyed in Iowa had one or more seropositive animals and seroprevalence increased with age. In Italy, most herds and 70% of pigs are seropositive for the virus. Clinical disease has been observed in very young suckling pigs to grower pigs up to 4 months of age but not in adults. It may occur as a sporadic disease or as an outbreak involving several litters of pigs, or pigs within a group. In outbreaks in Greece, in one herd of 100 breeding sows, 200 pigs aged 8–16 weeks of age died from the disease within 2 months.12 Population mortality in a group of young pigs is variable but it may approach 50% in younger pigs. Transmission is usually believed to be oral and spread amongst pigs is said to be limited18 although because of the presence of virus clones there may be the occasional large outbreak as well.
The role of rodents, especially the genus Rattus, always supposed to be the main reservoir of the virus for domestic pigs has been suspected but not documented as the source of spread of the infection to pigs.19 No pig to pig transmission has been shown and the pig is probably not a risk to man. Serological surveys of free-living animal species in Iowa in the United States have failed to find evidence of infection in these species and it is suggested that swine themselves are the main reservoir of infection. In an Australian outbreak, a plague of rats in the piggery may have been the source of the virus.4 The virus is relatively resistant to heat and chemical influences and a wide variety of pH but is sensitive to desiccation. Outbreaks are frequently associated with rodent plagues in the piggery or area, or with rodent infestation of feed stores. An epidemic in Australia was associated with a plague of mice which were present in all piggeries reporting the disease.20
The virus is now considered a major cause of reproductive failure in swine herds.21 The virus has been recovered from fetuses, antibodies to the virus have been demonstrated in fetal fluids, and histological lesions supporting a diagnosis of the viral infection have been observed.21
The economic losses associated with reproductive and neonatal losses associated with the virus have been estimated at US$100 per inventoried sow.21 Investigations of outbreaks on two Minnesota (United States) swine farms indicated that the monthly averages for the numbers of piglets born dead per litter reached 4.6 and 3.6, the pre-weaning mortalities 50% and 31%, and the farrowing rates 52% and 63%, respectively.
Isolates of the virus from different countries have different clinical characteristics and differences in pathogenicity, molecular and antigenic properties.22 A possible pneumotropic strain was identified in Quebec, Canada and this caused interstitial pneumonia.9 Strain differences between isolates are manifested in differences in virulence. The Belgian isolate is classified as a reproductive strain and the Greek isolate as a myocardial strain. Both strains are able to cause reproductive failure in sows in gestation and to cause myocardial lesions in piglets but a difference in virulence between both isolates is evident.22
The effects of different experimental doses and ages in experimental infections of pigs are described in a paper from Greece.23 The pathogenesis of these Greek viruses has been described24 and in most cases there is a viremia with the lymphoid tissues containing the virus and they are probably the main source of the virus replication. Inoculation of a suspension of heart, spleen and lymph node tissues from affected pigs can result in sudden deaths of experimental pigs within 3 days.12 The highest titers of the virus are found in the areas of damaged heart muscle. The virus can cause fetal death if the pregnant sow is infected in late pregnancy. The experimental inoculation of the virus into pregnant sows at 46–50 d of gestation results in transplacental infection and fetal deaths.21 On the other hand experimental infections of 4–6 week old conventional pigs with a USA isolate produced no overt clinical disease.25
Rarely is there clinical disease as most cases are seen as sudden death without clinical signs. Sub-clinical infection is the normal event and particularly in older or adult animals but even here, occasionally, death may occur.
The clinical course in young and growing pigs is short and manifested by inappetence, depression, trembling, incoordination and dyspnea. It has been described as being associated with respiratory disease in the USA.26 There may be cyanosis of the extremities. Most frequently, pigs are found dead or die suddenly while feeding or when excited. Death appears to result from cardiac failure and clinical signs referable to encephalitis are rare. The reproductive form of the disease is characterized clinically by inappetence and fever, possibly to 41°C followed by farrowing at 109–111 days of gestation in affected sows.6 There are numbers of mid to term abortions. The numbers increase for stillborn piglets, mummified fetuses and weak piglets, which are more susceptible to crushing and starvation and other common neonatal diseases. The course of the outbreak will usually last several weeks and possibly as long as 2–3 months with continuing reproductive failure with persistence of the virus.23,27 Animals with cardiac failure should be killed humanely because the heart damage does not resolve.
Neutralizing antibodies to the virus are present in sows and healthy in-contact pigs of affected farms.12 In outbreaks of reproductive failure, specific antibody to the virus can be found in both fetal and neonatal sera collected from abnormal litters.6 The hemagglutination inhibition and AGID tests are comparable for the detection of antibodies to encephalomyocarditis virus in fetal thoracic fluids.22 A microtiter serum-neutralization test is a relatively specific and sensitive test for the diagnosis.28 Antibodies of above eight are suspicious, and titers ≤16 are positive. Serum-neutralizing antibodies persist for several months and it is necessary to examine paired samples. A nucleic-acid probe can detect the presence of the virus in infected cell lysates.29 Enzymes such as serum creatine kinase-MS and lactic dehydrogenase isoenzyme are also elevated.30
At necropsy, there is reddening of the skin, excess peritoneal, pleural and pericardial fluid – frequently with fibrinous strands and edema of the omentum and mesentery. Sometimes there is pulmonary edema and liver enlargement. Characteristically, the heart appears pale and soft and there is diffuse or focal myocardial pallor involving the ventricles and associated with myocardial necrosis.12 These may appear as distinct white foci or streaks31 from 2–15 mm in diameter and these are most commonly on the right ventricular epicardium. Histologically, there is diffuse or focal myocarditis, with infiltration by histiocytes, lymphocytes, plasma cells and degeneration of cardiac muscle cells.12 The virus can be identified in the cytoplasm of cardiac muscle cells32 and virus particles are also seen in the protrusions from the cell surface of the Purkinje fibers and endothelial cells of the capillaries and intranuclearly in the cardiac muscle fibers. In chronic cases these have healed in the only way possible as fibrous plaques. In acute cases virus may be isolated from the heart muscle and also from the brain, spleen and other tissues. Neutralizing antibody becomes detectable 5–7 days after infection.
The predominant histopathological lesion in stillborn fetuses is myocarditis consisting of myocyte degeneration and necrosis with focal or diffuse mononuclear cell infiltration. In nursing piglets with the disease, histologically there are lesions of multifocal interstitial pneumonia, myocarditis, and mild multifocal non-suppurative meningoencephalitis.14 The immunohistochemistry is usually positive in the nuclei of the cardiac muscle cells, Purkinje cells, the endothelial cells of the capillaries and in the macrophages.33
• Serological samples for neutralizing antibodies34 which is widely available and is specific or HI antibodies may be helpful
• The virus can be isolated from stillborn pigs.35 It can also be demonstrated by PCR, RT-PCR and one step PCR.36 The RT-PCR can be followed by genetic typing using sequence analysis and this is useful in molecular epidemiology.37 It has also been demonstrated by in situ hybridization (ISH)38
• Histopathology on heart muscle also useful with immunohistochemistry to follow to confirm. In the neonate, the brain histology may show a non-suppurative meningo-encephalitis which can be confirmed by immunohistochemistry.
The diagnosis is from the history, clinical signs, gross and microscopic pathology and from isolation of the virus or demonstration of the antigen by immunohistochemistry. In some cases it may be necessary to consider vitamin E deficiency (Mulberry heart disease). The reproductive form of the infection may need to be differentiated from porcine parvovirus infection.
The disease must be differentiated from bowel edema and mulberry heart disease in growing pigs and the per-acute bacterial diseases in suckling pigs. The myocardial lesions in suckling pigs have similarities to those produced by foot and mouth disease virus in this age group of pigs – the so-called ‘Tiger Heart’.
There is no treatment and the control of the disease currently rests with rodent control and eradication in the piggery. There is now an inactivated vaccine available, which is an oil adjuvanted vaccine developed to protect elephants. It has been shown to work in mice and pigs39 and is believed to produce high antibody titers in both domestic and wild animals.
Acland HM. Encephalomyocarditis virus. In: Pensaert M, editor. Virus infections of porcines. Edinburgh: Elsevier, 1989.
Billinis C, et al. A comparative study of the pathogenic properties and transmissibility of a Greek and Belgian encephalomyocarditis virus (EMCV) for piglets. Vet Microbiol. 1999;70:179.
Koenen F, et al. The genetics of European EMCV. Arch Virol. 1999;144:893.
1 Brewer L, et al. Xenotransplant. 2001;8(suppl):135.
2 Koenen F, et al. Arch Virol. 1999;144:893.
3 Acland HM, Littlejohns IR. Aust Vet J. 1975;51:409.
4 Mercy AR, et al. Aust Vet J. 1988;65:355.
5 Koenen F, et al. Vet Microbiol. 1994;39:111.
6 Christianson WT, et al. Rec Vet. 1990;126:54.
7 Coulson A, Carlson J. Proc 12th Int Pig Vet Soc Cong 1992; p.103.
8 Zimmerman JJ, et al. J Am Vet Med Ass. 1991;199:1737.
9 Dea SA, et al. Arch Virol. 1991;117:121.
10 Littlejohns IR. Aust Vet J. 1984;61:93.
11 Gualandi GL, et al. Microbiology. 1989;12:129.
12 Paschaleri-Papadopoulou I, et al. Vet Rec. 1990;126:364.
13 Rolo M, et al. Fonaiap Divulga. 1998;59:23.
14 Koenen F, et al. Zentbl Vetmed B. 1999;46:217.
15 Castryck F, et al. Proc 14th Int Pig Vet Soc Cong Bologna 1992; p. 132.
16 Koenen F, et al. J Vet Med Ser B. 1999;46:217.
17 Sangar DV, et al. Vet Rec. 1977;100:240.
18 Maurice H, et al. Vet Microbiol. 2002;88:301.
19 Smith KE, et al. Can Vet J. 1992;33:645.
20 Seaman JT, et al. Aust Vet J. 1986;63:292.
21 Christianson WT, et al. Am J Vet Res. 1992;53:44.
22 Kim HS, et al. J Vet Diag Invest. 1991;3:283.
23 Billinis C, et al. Vet Microbiol. 2004;99:187.
24 Papaioannou N, et al. J Comp Path. 2003;129:161.
25 Zimmerman JJ, et al. J Vet Diag Invest. 1993;5:317.
26 Dea S, et al. J Vet Diag Invest. 1991;3:275.
27 Christianson WT, et al. Am J Vet Res. 1992;53:44.
28 Zimmerman JJ, et al. J Vet Diag Invest. 1990;2:347.
29 Meng XJ, et al. J Vet Diagn Invest. 1993;5:254.
30 Billinis C, et al. Vet Rec. 1997;140:628.
31 Littlejohns IR, Acland HM. Aust Vet J. 1975;51:416.
32 Psychas V, et al. Am J Vet Res. 2001;62:1653.
33 Vlemmas J, et al. J Comp Path. 2000;122:235.
34 Joo HS, et al. Arch Virol. 1986;100:131.
35 Littlejohns IR. Aust Vet J. 1984;61:93.
36 Kassimi LB, et al. J Virol Methods. 2002;101:197.
37 Vanderhallen H, Koenen F. J Clin Microbiol. 1998;36:3463.
This disease (PMWS) was first described in weaned pigs possessing unique macroscopic and microscopic features1 It was first recognized in Western Canada2-5 and then Europe6 and finally Asia and the United States. PDNS was originally thought to be a separate condition when it occurred as sporadic PDNS but now it is seen as an important component of PMWS.
A good definition of the disease was provided by Steve Sorden.7 For a diagnosis you must have: (i) wasting, poor growth rate, often dyspnea, and swollen inguinal lymph nodes; (ii) lymphocyte depletion and granulomatous inflammation in lymph nodes; and (iii) detection of PCV2 in the lesions in the lymph nodes. The study of PCV2 and PMWS is still in its infancy.
Porcine circovirus 2 (PCV2) is not a new agent but it is newly discovered as a pathogen. It has been found in 30 year old serum.8-10 It does not appear to be found in horses or cattle.11
PCV2 is the essential infectious cause of the condition5,12 but it is not the primary pathogen in the accepted sense of the word. Some authorities and many practitioners would still the see the cause as unknown. The reason for thinking this is that there are only three things that could have changed so that an infection which has obviously been present for a long time could suddenly become infectious. One is that the agent has itself changed but there is no evidence that this is so. The second is that the environment of the pig has changed and this is so in the sense of weaning and nutritional changes. The third thought is that the host is different and this may be so because of genetic engineering and breeding developments. Compare the growth rate and feed efficiency of pigs in the 21st century with those of the 1960s. The agent PCV2 is best viewed as a ubiquitous primary pathogen that can cause disease with the help of adequate co-factors and susceptible hosts.13 The problem is that we do not know all these factors and neither do we know what the determinants of the host susceptibility are.14
PCV2 has been found in the tissues from pigs with PDNS.15 The discussion goes on. The triggering of PMWS in herds could not be linked to coinfection with either PRRSV or PPV or to the use of specific immunostimulants such as vaccines or to particular genomic differences between the PCV2 strains.16
The Danish case study and the geographical spread in Denmark suggests that PMWS is an infectious disease. These studies also show that there may be a case for thinking that there is still an agent X out there that has not yet been discovered.17
The group of viruses known as circoviruses is an odd assortment of viruses. They are an important group of plant viruses and avian viruses including chicken anemia virus, psittacine beak and feather virus and pigeon circovirus. Initially Porcine Circovirus, now known as PCV1, was discovered in tissue culture18,19 and it is now widely distributed in pigs worldwide. It is non-pathogenic20 and has no place in the etiology of PMWS. There is only about 70–80% homology between PCV1 and PCV2.
In some cases PPV has been seen in the PMWS cases in the field. In gnotobiotes it may be essential for the full expression of the disease. Where it does occur the lesions of PMWS may be more severe.21-23 Neither PCV2 or PPV on their own were pathogenic for gnotobiotes and produce a disease which is not the same as that seen in the field.
Porcine circovirus 2 (PCV2) is classified in the Circoviridae, which are single stranded circular genomic DNA viruses. Protein and viral nucleic acid has been found in the tissues of pigs with PMWS. The fulfillment of Koch’s postulates with PCV2 in both gnotobiotic and conventional colostrum deprived pigs has been carried out.6
The genetic work so far carried out on PCV2 shows that the virus is quite stable with 94% nucleotide identity. It is a very small virus with little of its own genetic makeup coding for proteins. It has only two open reading frames (ORF1 codes a replicase protein; ORF2 encodes a 30 kDa capsid protein).24
PCV1 is non-pathogenic. The pathogenic porcine circovirus type 2 (PCV2) is implicated in the occurrence of PMWS. No pig with PMWS has been found without evidence of infection with PCV2.
PCV2 is also implicated in the causation of epizootic (acute) PDNS.15,25
It has been suggested as a cause of congenital tremor AII,26 but other authors could find no evidence for this but could neither prove or disprove it.27 It was shown (using indirect fluorescent assay, ISH and PCR) that there was an association between PCV2 in neonatal pigs and congenital tremor in pigs from 4 farms in the mid west of the USA.26
It may be a cause of neonatal cardiomyopathy. Myocarditis and abortion were seen in intra uterine infections of sows.28
It may be a cause of reproductive failure. Transplacental infection does occur in the field and stillbirths in pigs may be associated with the PCV2 infections.29 Multiple abortions were seen in a multisite system.30
It is also a significant contributor to porcine respiratory disease complex (PRDC).31-33
In the mid 1980s in the UK it was a contributor to granulomatous enteritis although this was only detected retrospectively.
Worldwide distribution now includes the original areas like Canada, the USA and widely in Europe notably France,34 Spain,35 the British Isles,36 and Ireland.37 It occurred in Taiwan in 1995,38 Japan in 199539 and Korea.40 Not very significantly in the southern hemisphere until recently when it occurred in New Zealand in 2004.41 Now been found in the Australian pig herd42 and also from South Africa43 and Brazil in 2005.44
Evidence was found of the virus as early as 1973 in Ireland8 and from 1985 in Canada45 and Belgium46 and 1986 in Switzerland when archived tissue was looked at.47
In the USA proven occurrences in the field are described as rare.28,48
As well as the countries documented above it has also occurred in Korea,49 Japan,50 Switzerland since 1986,51 and Germany since 1999.52
Wild boar have been found to have both PCV1 and PCV2 not only in the young pigs but also in the pigs of 1–2 years.53 Also found in the feral pigs in Germany.54 PCV2 occurs in wild boar in Spain with 3/656 being positive and one had PMWS.55
PRRS occurs before PCV2 in the life of the pig as it is either born with the infection or picks up the infection in the nursery. Only a few pigs have serological evidence of PPV varying from perhaps over 50% in the USA, to 25% in Korea, and 13% in Canada.
One of the first studies on the epidemiology of PCV2 was carried out in Ontario.56 Data sheets were sent to 21 practitioners covering 922 herds and they said 15 herds had evidence of PMWS. The authors investigated 15 farms of which 13 turned out to be negative for PMWS. Of the other 12 some had PCV2 but were PMWS negative. 5/25 farms had epidemic disease and 7/25 endemic disease. The other 12 had either higher than average mortality or low grade PMWS. You can have a 95–100% prevalence of infection but this is rare. Most typically PMWS affects 4–33% of pigs with 70–80% case mortality.57,58
Nobody knows where it replicates although the supposition is rapidly dividing cells but it is able to infect macrophages and express its two viral proteins.59
The virus is probably found in all secretions60 including the nasal, fecal and urinary secretions.61 It is certainly shed in feces from pigs that have no enteritis and this suggests that fecal/oral/fecal route is the common method of spread.60
It is readily transmitted from inoculated to sentinel pigs and can be found in the tissues for at least 125 days and shed in all secretions.62 It is secreted intermittently in semen63,64 and it could be that it only occurs there because it is contained within macrophages which are extruded into the semen via the local lymphoid tissues particularly the bulbourethral glands.
The slow spread of the syndrome PMWS through Sweden appears not to be related to the spread of a new contagious microbe and it has not been spread through the semen.62
Herds that vaccinate for Mycoplasma hyopneumoniae (M.hyo) had a lower mortality. In Denmark the case study showed that the use of M.hyo vaccines in PMWS positive herds reduced the mortality rate.17
It has been found in wild boar in Germany and Spain so they may potentially act as a reservoir.62
Risk factors for PMWS show that infected farms had more nursery pig diseases and lower biosecurity scores than non-infected farms.65
In a study of Duroc, Landrace and Large White pigs only the Landrace had PMWS cases which suggests that there may be a predisposition in this breed.66 In a study in Denmark, albeit only in one herd, Duroc boar progeny had a higher survival rate.17 This author also made the point that 80% of Danish stock are supplied by the same breeding company and therefore nationwide the genetics are similar but the occurrence of PMWS is still very variable.
There may be a significant litter effect.67 This litter effect may in part be explained by the sow’s humoral immune state at farrowing. Measures to reduce the sow viremia and increase the colostral antibody provision at farrowing would increase the chances of piglet survival. Most farmers in the UK are convinced that there is a considerable repeatable maternal or litter effect on PMWS mortality., however the mechanism underlying this is unknown. One possible explanation is that these pigs may have extremely low levels of vitamin E.68 It may be that castrated males are more affected.69
A recent study of 1063 piglets using PCR tests concluded that there was a significant effect of genetics on the expression of PMWS. It was not possible to say whether this effect was due to breed or particular boar lines.70
In an experiment involving the transfer of passive immunity maternal antibodies to PMWS it was shown that none of 36 piglets born to sows with PCV2 serum antibody levels >1/1250 developed PMWS. Piglets from sows with levels of 1/50 and four piglets with 1/250–1/500 also developed the disease.71
Phylogenetic analysis of the entire genomic sequence of PCV2 isolates has been determined with a >95% nucleotide homogeneity amongst all the tested strains.72,73 The strains of PCV2 from PMWS subjected to genomic analysis74 had a 99% nt sequence identity with other isolated PMWS strains had a 95% homology with associated strains from around the world. The strains from congenital tremor cases (2 PCV2 and 1PCV1) had only 72% sequence identity with each other. The PCV1 strain from the neonatal pig in the 1960s did cause congenital tremor when inoculated into the sow as an experimental infection.74
Pigs with PMWS have more PCV2 compared to the pigs on the same farm that are not clinically affected.75,76 We are not as yet able to answer the question as to whether the severity of PCV2 associated disease is related to the virulence of the strain as in a recent study 94–100% shared the same nucleotide sequence and 91–100% shared the same amino acid sequence.77
The quantitative TaqMan-based real time PCR78 allows the determination of viral load. No healthy pig had in excess of 106 PCV2 genomes per mL of serum or 500 ng of tissue sample. On the other hand all the clinically sick PMWS pigs had above 107 in both the serum and the tissues. Also the estimated viral load in the tissues of the PMWS pigs was related to the IHC findings with the LN, ileum and tonsil giving both a high viral load and high IHC staining. There is increasing evidence that the viral load in the affected pig is the crucial factor in the conversion of a latently infected animal to clinical PMWS.79 PCV2 may be associated with different tissues of the reproductive tract but it is unlikely that it is associated with the uterine stage embryo.80 It was found that 13.1% of 350 aborted fetuses46 were positive for PCV2 by PCR and the virus was detected at all stages of gestation.81
The virus that causes PDNS is the same virus that is found in the cases of sow abortion.82
The agent can persist in the dendritic cells in the absence of viral replication.83
The virus is very stable. It is not affected by heat, dryness, humidity, or common disinfectants.84
Clinically healthy pigs from PMWS affected farms developed the disease when transported and commingled with clinically affected pigs.85
The Danish view is that the higher the standard of health care the lower the losses.17 It may be that co-infection is necessary to further the replication of PCV2. The more coinfections the more PCV2 material and it may be that it is this increased viral load that is the key to overcoming the host defenses. It would certainly explain why SPF and high health herds appear to be less severely affected by PMWS although they have PCV2. It would also explain that in some cases the extra antigens provided by vaccines may also be a triggering factor as the vaccine antigens or adjuvants may cause more replication of PCV2.
There is no doubt that PCV2 is associated with PMWS. No cases of PMWS have been described without the presence of PCV2.
Infections with porcine parvovirus (PPV) may be an important factor in the pathogenesis of some but not all PMWS cases.86
A study in Germany described the pathogens found in PMWS cases other than PCV2. PRRSv was found in 75%, H. parasuis in 13.4%, E. coli in 7.7%, S. suis in 7.2%, coronavirus in 7.2%, swine influenza in 4.4% and brachyspira in 4.2%.
The most common of the coinfections is M.hyo, which is a common agent in the respiratory component of PMWS. It increases the severity and duration of PCV2 induced lesions in lung and lymphoid tissues, the PCV2 replication in the tissues and the incidence of PMWS in conventional pigs.87,88
It was thought that PRRSv was the necessary sole catalytic agent for the conversion to the clinical case.89-91 In a series of Spanish cases the PRRSv was found in only 23% of cases.92 However, it seems that PRRSv infection occurs some time before PCV2 infection.76
The epidemiology of PPV suggests that the simultaneous co-infection with PPV and PCV2 is unlikely in the 6–12 -week-old period of life when pigs have PMWS.93 On the other hand94 in a study of 138 farrow to finish farms in France a strong association was suggested between PRRS and PPV in a herd with PMWS.
Simultaneous coinfection with salmonella has also been described. Here the pigs with salmonella had severe lymphoid depletion.95 Infection resulted in more severe clinical disease and an increased mortality up to 80%.
• Pneumocystis carinii has been described in PCV2 infected pigs96
• Cryptosporidium parvum has also been found in association97
• Chlamydia has been found in association with the PMWS98
• It has also been associated with pulmonary aspergillosis99
• PCV2 and PEDv infections were described in the same animal.100
Experimental studies and field observation confirm that the transfer of passive immunity maternal antibodies to PCV2 does confer protection against the development of PMWS.71
Immediately after birth the immune system of the pig is programmed to a sequence of actions that down regulate the responses of the immune system particularly in the gastrointestinal tract. In PMWS there appears to be the opposite up regulation of the immune response. Immunostimulation is not important101 so it may be a failure of down regulation. Whatever happens, and it is not yet clear, the immune system is impaired.102
Maternal antibodies can prevent PMWS but they only limit virus circulation and shedding to some extent103 and certainly not PCV2 infection.104 Active antibody production occurs during the early grower period69 at about 6–10 weeks and earlier if there is a heavy infection.105-107
It may be that conventionally raised pigs can establish protective immunity and not develop PMWS if animals at the age of 4–10 weeks are exposed to the viral proteins (virus or vaccine) but not exposed to any major immunostimulants or co-infected with any other viral infections.
Suppression of cell mediated immunity may play a part in the etiology of PMWS.108
Cytometric analysis shows that leukocyte subsets change after PCV2 infection with the increase in monocytes, reduction in T-cells (mainly CD4+), and B-lymphocytes and the presence of low density immature granulocytes. This suggests an inability to mount an effective immune response.109 Serum antibodies occur but PCV2 persists.
Serological profiles of affected and non affected herds have been described in France.94 PCV2 antibodies have a high prevalence in all countries. Convalescent sera from pigs with the clinical disease will also induce antibodies to PCV2. In the UK titers seldom exceed 1:640 but those that received serotherapy responded with serum antibody levels possibly as high as 1:32 000.
There has been a considerable controversy over the effect of vaccination on the conversion of sub clinical infection to clinical disease. The basis of this thinking was that vaccines are essentially no different to any other antigen and if other agents were important in the production of clinical PMWS why not vaccine antigens. In one study approximately 215 of the vaccinated pigs developed clinical signs and histopathological changes typical of PMWS.110,111 The vaccines particularly suggested as culprits were those for APP and more importantly M.hyo. This latter was regarded as particularly important as the use of M.hyo occurred much at the same time as the epidemic of PMWS. Vaccines may help the pathogenesis of PCV2 diseases.23,110,112,113 In an on farm trial in the UK when half the pigs were vaccinated and the other half were not there were no significant differences in overall death rates between the two groups.114 It is possible that the timing of vaccination may not be important. There are no or minimal PCV2 associated lesions when pigs are vaccinated at 2–4 weeks prior to expected PCV2 exposure.115 There is also the possibility that the adjuvants may be involved in the up regulation of the immune response. All the adjuvants at an early stage of infection increased the severity of lymphoid depletion associated with PCV2. In the later stages of infection (post 35 days) the oil/water adjuvants increased the length of viremia, the amount of PCV2 in serum and tissue and the severity of lymphoid depletion.116 It has been shown that local immunostimulation with a vaccine adjuvant is not sufficient to induce PMWS in conventional pigs and also not to increase the PCV2 load.117
PRRS may complicate PMWS. PPV is often present in the affected pigs but is not essential for PMWS. However the worst clinical signs and often the most severe lesions are seen in pigs that also have PPV infection and where the serological titers for PPV in these pigs are their highest point of seroconversion.
The role of the PPV is to enhance the PCV2 replication and thereby to exceed the threshold for the development of PMWS. How this is done is not known. There is some evidence that PMWS affected pigs seroconvert to PCV2 earlier than non affected pigs and this may suggest that they are infected earlier.118 Both are non-enveloped DNA viruses and both probably use the DNA synthesis and protein synthesis of the host cell. Where joint infections occur the number of cells that are positive for PPV is much less than those that are positive for PCV2. It may facilitate high levels of replication of PCV2 during the preclinical phase of co-infection by promoting monocyte proliferation. It is possible that both viruses have similar tissue tropisms.119 The primary target in vivo is probably not the monocytic series of cells,120 although other authors suggest that the target cell changes from cardiomyocyte, hepatocyte, and macrophages of the fetal pig to just the macrophage/monocyte series in the neonatal pig121 and possibly the nephrogenic zone of the kidney.
It is possible that the PPV may induce immunosuppression or macrophage activation. Death in these experimental infections may be due to liver failure.
Destruction of thymic lymphocytes has a central role in the pathogenesis of PMWS infections. Pigs with PMWS have altered cytokine responses to mitogens and other antigens.122
PCV2 in PMWS causes a reduction or loss in the T and B-cells, increased numbers of macrophages and partial loss and redistribution of antigen presenting cells throughout all the lymphoid tissues compared with control cases.123
The loss of B-cells is the earliest characteristic. There is also a loss of T-cells and in particular the memory T-cells although naive T-helper, cytotoxic T-lymphocytes, natural killer cells and mature granulocytes were also depleted but not the monocytes. There appears to be no direct PCV2 effect on lymphocytes and how this lymphopenia is caused is unknown. It does not cause apoptosis either124,125 Positive PCV2 macrophages can in fact phagocytose apoptotic cells.126 The PCV2 remains detectable at all stages of the infection. This suggests that it somehow does not trigger the degradation systems of macrophages and dendritic cells and the virus remains silent. Perhaps a novel thought is that the virus does not replicate in cells but can replicate free in the serum. PCV2 does not appear to harm macrophages but persistent PCV2 infection is a problem for dendritic cells. They are the most potent antigen presenting cells and are also mobile and may therefore spread through the body system. They are particularly adept at capturing antigens and migrating to lymph nodes and lymphoid tissues where they act directly with B and T-lymphocytes. The dendritic cells accumulate large quantities of PCV2 antigen by endocytosis not replication and retain this for several days if not longer. All that happens is that there appears to be no transmission of messages to the T-lymphocytes and therefore no further triggering of immune or cellular responses occurs.
The PCV2 may contain a sequence that has a marked inhibitory effect on interferon α production by porcine leukocytes.127
Co-infected pigs always have more severe microscopic lesions than pigs infected with PCV2 alone.
It may be an early or inappropriate immune stimulation which is the most likely event that triggers the PMWS in the PCV2 infected pigs. In situ hybridization shows that PPV can be visualized in the same animals that have PCV2128 but on the other hand PCV2 can induce PMWS lesions in weaned pigs in the absence of PPV.129,130
Fetuses inoculated with PCV2 in the first two-thirds of the gestation are likely to be resorbed or die with severe heart congestion whilst fetuses infected in late gestation are minimally infected.
The PCV2 capsular protein is first detected in the cytoplasm of infected cells in a perinuclear position. After 18 hours the proteins are detected in the nucleus. Later protein is found in both the nucleus and cytoplasm of the infected cells. Sometimes immense amounts of viral protein and viral DNA are produced.131
The antigen is widely distributed. The cell most usually infected is the macrophage/histiocyte and also the dendritic cells of lymphoid tissues. Kupffer cells in the liver show considerable positivity. Virus can also be identified in cardiac myofibers, isolated myofibers in the intestinal tract and also hepatocytes. Lymphocytes do not contain virus.
PCV2 increased the levels of IL-10 and IFN-γ mRNA levels in the thymus and tonsils and were thought to be indicative of a T-cell immunosuppression.132 There were no differences in the IL-6 levels between clinically and subclinically affected animals.133 However the IFN-γ levels were lower in the PMWS animals which suggests a down regulation.134
In a similar study IL-1 β, IL-2 and IL-6 were expressed to a higher degree in PMWS pigs whereas IL-4 and IFN-γ were reduced.135 In experiments with PCV2 and PPV there were no changes in the cellular response but in a joint experiment CD2+CD4+ cells decreased significantly in 21 and 35 day post-infection samples compare with 10DPI. There was also a strong influx of NK cells into the LN and peripheral blood monocytes.136 There is a collapse over time in both T and B-cell populations. All T-cells are affected but the memory activated T-helper cells may be the worst affected.137 The PMWS affected pig is depleted of CD8+ and CD4+ cells (both T and B-cells).
In the field, secondary infections are the most common presentation.90 The chief of these is often Glasser’s disease (H. parasuis infection).
Secondary infections with opportunistic organisms are common.122 Co-infection with PPV, PRRSv, or mycoplasma will trigger PMWS.21 Since these different pathogens have different targets and pathogenicity, a common aggravating factor can indeed be found in the immune stimulation.
A marked granulomatous inflammation takes place in the lymph nodes and this spreads to the parenchymatous organs. This is manifested as LN hyperplasia and macrophage infiltration of the cortex and medulla with syncytia and inclusions.22 The virus load generated is enormous and the organs are all loaded with virus. There is an extremely dysfunctional immune system.122
There is no really reliable method to produce PMWS in all experimental pigs with the characteristic clinical signs, gross lesions and microscopic lesions. PCV2 infections can be produced easily but PMWS is much more difficult.
In all experimental models of PMWS, replication of PCV2 is enhanced and concentrations of PCV2 are higher in animals that develop PMWS. It may simply be that these animals have more dividing cells to provide a replication site than those that do not develop PMWS.
One of the most interesting experiments was the clinical disease produced using a virus that had been present in the Swedish population since 1993 without causing problems but when injected into pigs did produce PMWS.138
PMWS has been completely reproduced with typical PMWS clinical signs, gross lesions, and microscopic lesions by use of CD/CD pigs with only PCV2.62,139 They also produced the disease with dual inoculation with both PPV and PCV2.21-23
In another study with PCV2 and keyhole limpet hemocyanin (KLH) emulsified in Freund’s incomplete adjuvant a total PMWS and enhanced replication of PCV2 was seen only in pigs receiving both PCV2 and the KLH.
Adjuvant and antigen resulted in immunostimulation that enhanced PCV2 replication.101 The PCV2 is present in lesions in proportion to the severity of the PMWS and it can be said that the PCV2 plays an important part in the occurrence of PMWS. Only in some pigs will co-infection evolve into PMWS between 6–10 weeks of age.140
The only piglet infection group which developed clinical evidence of PMWS were those piglets which received combined inocula as either infected lymph node homogenates or dually infected pigs with both PCV2 and PPV propagated in PK-15 cell lines.141 PCV2 was isolated or antigen recovered (IHC or ISH) from tissues in all the pigs that were given the dual inocula. An unusual dose dependent relationship has been identified. Low titer PPV/PCV2 produced a milder but still active form of the disease. A 100-fold higher titer produced fulminant and fatal disease. The liver is the primary target organ followed by the kidneys, gastro-intestinal tract and pulmonary tissues.
PRRS together with PCV2 will also produce PMWS.142 Again the PRRS seems to increase the replication of PCV2 and thereby exceed the threshold for PMWS.143,144
A Swedish virus isolated before PMWS was described in Sweden was given to the workers in N. Ireland and given to pigs. These pigs went on to show PMWS.145
Experimental infection of fetuses produced mummified, stillborn or weak piglets in 13 pigs and 24 were normal.146 This shows that PCV2 can infect late term fetuses.146 It was shown that PCV2 spreads amongst fetuses in utero. Four of six sows inoculated with PCV2 aborted 7–21 days post-infection. Three of four litters had 85–100% of stillborn piglets.146
If PCV2 is inoculated into gnotobiotes, CD/CD pigs or conventional pigs then you get typical microscopic lesions but few to moderate gross lesions whilst the clinical disease is mild or absent.21-23129 A novel study in germfree pigs confirmed PCV2 as the sole essential infectious cause of PMWS, fulfilling Koch’s postulates.147
Experimental infections in gnotobiotic pigs have been described.119,140 They reproduced PCV2 infections but most animals failed to reproduce any pathological changes and/or clinical signs similar to those observed in PMWS in the field. Most of the animals had the virus or viral antigens in the tissues such as trachea, lung, liver, kidney, pancreas, lymph nodes, spleen, thymus, ileum, colon, cecum, salivary gland heart, brain, and testis. This experiment did not fulfil Koch’s postulates. Pigs with clinical signs consistent with PMWS were only seen in pigs with PCV2/PPV co-infected animals, PPV vaccinated and non-vaccinated. Mild to severe lymphoid depletion was seen and lymphoplasmocytic interstitial nephritis and hepatitis.
An infectious molecular clone has been developed and produced PMWS148 and two chimeric infectious DNA clones also were effective149 in producing PMWS.
It should always be remembered that there are at least 25 other causes of failure to thrive and loss of weight and wasting. These should be examined first. A wasting pig does not always equal PMWS.
Diagnosis of PMWS is made on the basis is made on the basis of three criteria: clinical signs consistent with PMWS; secondly histopathologic changes; and detection of PCV2 in the microscopic lesions.
The initial clinical findings were described in Canada2,3 and a full account of the clinical side of PMWS in Spain has also been described.150 It was often found in high health herds in Canada an observation that has also been made throughout Europe. All sizes of herd have been affected.151 Things usually return to normal in the period of 10–20 months. The cynics would suggest that that is because the susceptible population of pigs has died out over this period.
Sick pigs may exist alongside fit and healthy pigs. The major sign is not thriving.
There is a progressive weight loss. The ribs of the pigs are usually visible often with a severe abdominal distension. There may also be signs of heart failure with pericardial effusions and myocardial dysfunction.
There may be evidence of respiratory disease in the form of cough, dyspnea, and or tachypnea. There may be slight fever, pallor, diarrhea and less commonly there is icterus or jaundice.
The most characteristic clinical feature is the presence of swollen inguinal lymph nodes which in Europe is the first time that this has been described as a clinical sign. This may not be so in North America where some of the strains of PRRSv have been capable of causing this gross lesion.
Unless there is a susceptible coinfection there is no response to antibiotics.
Mortality is variable, but usually low, 5–15%7,151 although the case mortality rate may reach 80–90%.151 65% of the deaths are males and 34% females. The morbidity is 10–30%.
The peak of PMWS mortality typically coincides with seroconversion to PCV2. Sub clinical infection is the most common presentation. It can be very mild and transient.152 Often the records of production may be the only method of detecting that there is a problem of PMWS or PCV2 infection. Sometimes this is only appreciated by a higher return of condemnations from the slaughterhouse. There is a reduced average daily weight gain, increased time to slaughter, more severe lung lesions, and higher amount of antigen.
There was an association between the levels of infectious PCV2 and/or PCV2 DNA load and the severity of the clinical signs as described for PMWS.153
The occurrence of PRDC is a major manifestation and both PCV2 and M.hyo are common in respiratory disease. In growing and finishing pigs PCV2 associated PRDC is characterized by slow growth, prolonged growth, and dyspnea that is refractory to antibiotic therapy. There is also a marked increase in mortality from single and multiple concurrent bacterial infections.31,154-156
There are four criteria for a diagnosis of PCV2 associated PRDC: (i) respiratory signs; (ii) microscopic characteristic lung lesions; (iii) IHC or other demonstration of PCV2 in the lesion; and (iv) the absence of characteristic microscopic lesions of PMWS in lymphoid tissues.
PCV2 associated PRDC should be differentiated from PMWS clinically and histopathologically.
The condition of Porcine Necrotizing Pneumonia (PNP) is a result of coinfection with PRRSv and PCV2.25
In the past there have been wasting problems in pigs with loss of weight and watery diarrhea in which there is a granulomatous inflammation in the intestines with characteristic lesions with giant cells and possibly inclusions affecting the Peyer’s patches.157
The common differential diagnosis of PMWS is PIA.158
Reproductive problems associated with PCV2 infection are uncommon.159 In a recent survey in Spain it was shown that PRRSv was an important cause of reproductive failure but that the PCV2 was not a common cause of a problem.160 Elevated abortion, stillbirths, and fetal mummification are a feature. Midgestation abortion, mummified fetuses, and early embryonic death were also observed in a recent Korean study.142 In utero infection may produce fetal deaths and abortions.48,161-163 Abortion is typically sporadic and characterized by increased numbers of mummified fetuses. Multiple PCV2 associated abortion and reproductive failure in a multisite production system has been described.164 Transplacental infection with PCV2 was associated with reproductive failure in a gilt.165
Reproductive failure associated with PCV2 is uncommon in the USA166 and the UK. There are no characteristic lesions in the PCV2 affected fetuses. PCV2 is associated with reproductive failure at all stages of gestation.167 The replication kinetics of these reproductive strains are different from those from pigs with PMWS or PDNS.168
The early changes in PMWS are anemia. This may be related to the sub acute/chronic nephritis, gastric ulceration or M. suis infections.
Monocytic hyperchromic regenerative anemia is the most common type with anisocytosis and polychromasia and later monocytic normochromic non-regenerative anemia. Significant differences (lower levels) were observed in RBC counts and hemoglobin levels, between healthy and naturally and experimentally infected pigs.169 Most of the pigs seem to have M. suis (Eperythrozoon suis).
Neutrophilia and lymphopenia were recorded. Animals which will subsequently become PMWS cases show a lymphopenia,102,126,170,171 very early on in their history,172 and certainly before they show clinical signs. Many of the pigs showed increased levels of blood urea and creatinine with persistent leucopenia or sporadic leukocytosis both with neutrophilia and lymphopenia.173 Wasting pigs with high blood urea, low albumin, and hemoglobin with lymphopenia provides further evidence of PMWS infection.
The risk of dying is related to the increasing titers against PCV2 from weaning until 4 weeks after weaning.174 Pigs suffering from PDNS usually show high blood levels of creatinine and urea.
The carcass is wasted. The skin is pale and rarely jaundiced except in the USA and Canada where it may be more common. The relationship between PCV2 and hepatitis E has probably not been examined. The abdomen is distended. The lesions were classically described in Canada.2,3,175 The most common manifestation of PCV2 infection is often pneumonia. Analysis of the submissions to the diagnostic laboratory at Iowa State University shows that the PCV2 antigen is often associated with the characteristic lung lesions.177,178 The second most common manifestation of PCV2 infection (in the records of the Iowa diagnostic laboratory) is PMWS.
The third manifestation is the systemic infection and PCV2 associated enteric infection is relatively uncommon but where it does occur often resembles regional ileitis.
The examination of the carcass generally reveals a lymphadenopathy in the form of enlarged lymph nodes.179 Lymphoid organs are always affected. These gross lesions occur before other gross and histopathological lesions occur. The subcutaneous lymph nodes and in particular the superficial inguinals are the most severely affected. Other swollen lymph nodes include the tracheobronchial and mesenteric lymph nodes which is a reflection of the antigenic strain that the respiratory and alimentary systems are under all of the time in PCV2 affected pigs. The lymph nodes in cross section appear homogeneous, often pale and in many cases also edematous.
The lungs are swollen, often rubbery and may be edematous and these lungs do not collapse.
The other early change is the presence of fluid in the body cavities (peritonitis, pleurisy, pericarditis). These occasionally have fibrin tags which may or may not be associated with the presence of H. parasuis which is the most commonly isolated secondary bacterium from these cases.
Stomach ulcers (pars esophagea) are a common feature and in many of these there is also an edema of the gastric wall. There may also be cecal edema.
Sometimes the liver is atrophic or yellow. The spleen may be enlarged.
The kidneys may be very swollen but may have petechiae or even white stripes. Reproductive pathology is uncommon. Occasionally, we can find lymphohistiocytic myocarditis and/or myocardial fibrosis.160 One of the manifestations is PDNS where there is both PRRSv and PCV2 in the same tissues.180
The overall histological changes in PMWS are granulomatous inflammation with inflammatory cell exudates. The lesions form angiocentric granulomatous formations and often include eosinophils, neutrophils, and lymphocytes. There are often multinucleate giant cells but not always. The granulomas may coalesce and then the LN appears as a solid mass of cells. The follicular architecture is lost and the germinal centers become obscured. Paracortical T-cell areas become less cellular and eventually there is a marked lymphocyte depletion. Often there are massive inclusions which may be distinct intracytoplasmic inclusions or intranuclear basophilic inclusions. These inclusions are not sites of active viral protein and DNA synthesis but are aggregations of ingested virions.
These occur in the lymph nodes2 and also in lymphocytic aggregates such as BALT or GALT (Peyer’s patches). Lymphoid depletion and histiocytic infiltration are highly specific for the disease.7 Necrotizing vasculitis is also a feature in some instances.181
The PCV2 antigen is associated with characteristic lung lesions, with necrotizing and ulcerative laryngitis, ulcerative bronchiolitis, bronchiolitis obliterans, fibroplasias of the lamina propria, and granulomatous inflammation of the alveolar septae. For this reason the tracheobronchial LN often show the most characteristic lesions of PMWS probably as these are the ones most affected by the continual inhalation of antigens. In many pigs the lung lesions are mild and consist only of an interstitial pneumonia.
Hepatic histiocytic infiltration is a feature and sometimes necrotizing hepatitis.21
The kidney has lesions which are very similar to some of the lesions described in mycotoxicosis which are essentially a pyelitis with a multifocal interstitial plasmacytic cellular infiltration extending through the cortex with focal lymphoid aggregates forming at the corticomedullary junction. Myocarditis is a common feature particularly in the younger pig.
Diffuse hepatic necrosis has been described associated with PCV2 infection in a piglet.182 Immunohistochemistry reveals the presence of antigen in the cytoplasm of macrophages and dendritic cells but noT-lymphocytes.120 No replicating proteins have been found in the macrophages.183 This suggests that the virus is being endocytosed by the macrophages and not produced by them. Infectious PCV2 remains associated with these cells for several days, possibly even weeks, without impairing their functions or losing its own infectivity. The IHC characterization of the LN reaction in pigs has been described.184 In the initial and intermediate stages there is an absence of follicles and depletion of lymphocytes. There is also a reduction of interfollicular dendritic cells and interdigitating cells and a reduction/absence of B-cells and in particular CD4+ T-cells.
After the evaluation of histopathology and IHC and/or ISH and the conclusion is that the pig has only mild lesions then it is possible to say that the pig may have (a) subclinical PCV2; (b) early PCV2 leading subsequently to PMWS; or (c) are recovering from PCV2 infection. If there are moderate to severe lesions then there is no problem it is ether PMWS or PRDC or granulomatous enteritis or any combination thereof.
• Blood – The virus may be present in the serum before or after the virus can replicate in the tissues so therefore blood may be useful and PCR is more sensitive than ISH185
• Tissues – Microscopy and antigen detection. The best tissues are lymphoid organs particularly tonsil, lymph nodes especially the inguinal, mesenteric and tracheobronchial as these will also give you comment on the general systemic state, lung and alimentary tract. Also ileum (which has Peyer’s patches), and spleen. Other tissues may have lesions (kidney, liver, and lung).
Techniques such as virus isolation, neutralization tests and indirect immunofluorescence have been used primarily by research workers and are not normally part of the diagnostic methods used. Virus isolation has also been described.5
PCV1 and PCV2 can be distinguished by PCR and monoclonal antibody techniques. Several serological tests have been developed for PCV28,69 and these showed that nearly all pigs become seropositive during the growing phases. Since most pigs are seropositive there is little point in carrying out serology.
A PCV2 specific antigen capture ELISA has been described.186 The double ISH technique has been used for the demonstration of both PCV1 and PCV2 in tissues.187,188
The original PCR was developed189 and superseded by a multiplex PCR that will detect both PCV1 and PCV.190,191 One that can be used for boar semen192 was then developed. A positive PCR result confirms the presence of PCV2 only193 and does make a diagnosis of PMWS.194 When the PCR detects PCV2 in lesions in histologically identified lesions then it is positive for PMWS.195 Quantitative PCR is a molecular technique that can allow you to assess the viral load.73,142
Immunohistochemistry was shown to be useful for the detection of PCV2 in tissues.12,40,196 IHC was shown to be better than ISH.197
The only treatment is for secondary bacterial diseases. These require an accurate diagnosis and probably bacterial sensitivity testing for effective results. The results of antibiotic therapy are not good.
Serotherapy has received publicity as an effective treatment but the effects are very variable. It will work for some batches but not others and is not without risk of other diseases been transmitted as well as the prospect of dirty conditions causing outbreaks of clostridial disease.198
The use of aspirin for the sows may be of use150 and the administration of corticosteroids may reduce stress and therefore help in the reduction of death losses.
The first attempt at control was the 20-point plan of Madec199 who said that you had to get 16/20 points adopted by the farmer to get the plan to work. In most cases certainly in the UK it was difficult to persuade the farmer to do 4–5.200 The core of the plan was to use all in/all out production, feed pigs with multivitamins, minimize stress, and handle pigs with extra care. In some cases the high mortality rates were still observed even after implementing good management practices.201 In many ways the plan was no different to those proposed 20 years earlier for the control of respiratory disease or alimentary disease. Many of the UK farmers simply tried to limit pig to pig contact, to remove stress from the system, to introduce better hygiene and improve nutrition. Batch farrowing helped to allow all in/all out production, and others tried partial or total depopulation. One of the major controls is to reduce stress.67
A considerable effort was put into the provision of an improved diet. Many increased the amounts of vitamin E and selenium believing that there was an element of antioxidant deficiency in the rations when the pigs were being bombarded with toxic oxygen radicals. Many farmers were also convinced that part of the problem related to a high wheat content in the diet and tried to reduce it and replace it with barley. This may be one of the key differences between the low prevalence of PMWS in the USA (where maize is used) and the high occurrence of PMWS in Europe where diets are used in which wheat and barley figure highly. Reduction of wheat was often associated with a reduction in the level of PMWS.
Many producers said that ad libitum feeding was associated with an increase in the problem. In many ways the methods of control were summarized by a study on a farm in East Anglia in the United Kingdom.202 He described the control as requiring attention to six main features: (i) keep the population closed; (ii) not mixing pigs of different ages; (iii) reducing the amount of mixing and moving between pigs of the same batch; (iv) reducing the number of moves that the piglets made; (v) clean and disinfect the buildings before the next batch; (vi) visit the pigs in situ, pull out the ill and cull immediately. One group of UK farmers said that the best way of controlling this disease was to go organic and what that means in practice is that the weaning age has to be raised to a minimum of 8 weeks.
Control of secondary infections is an essential control.31,142,144,203 The second point is the removal of the factors that may promote immune stimulation.87,88,115,204,205 The main measures in the control of the disease are as follows.150
1. Maintain the balance of health. These include the reduction of stocking density (the only measure that does reduce environmental and microbiological challenge), all in/all out management of the rooms of buildings with cleaning and disinfection and time to dry out before the next batch. No moving of animals back to clear a room. Reduction of entry of new stock to large batches i.e. 3-monthly not each month for replacement gilts. Depopulation of flat decks. Try and use a 3-site production system.
2. Avoid contagion between animals. Change weekly production to 3-weekly production. Avoid any form of fostering if possible and under no circumstances have 25% adopted piglets Avoid piglet handling and therefore stress. Increase the hygiene in the farrowing quarters. Do not use needles on more than one litter.
3. Favor the natural immune response. Make sure that the piglets have an adequate colostrums intake. Make sure that they are a good weight at weaning. Avoid stress. Avoid vaccinating piglets too early. Generate the right environmental conditions particularly for temperature and humidity and regulate with the stage of growth. Avoid overcrowding. Administer high quality feed. Avoid vaccination coinciding with the movement of pigs. Control parasites particularly if production is outside.
Segregated early weaning has limited success because (i) most pigs are weaned negative and get exposed to the PCV2 during growing;206 (ii) adult animals with antibodies are not susceptible to PCV2;207 (iii) PMWS is most common at 4–6 weeks of age or 2–3 weeks post-weaning.4
With the escalating prevalence in Denmark it has been found that rapid removal of the clinically affected animals (massive sources of virus) to hospital accommodation followed by plenty of fresh air and highly palatable food has been helpful in recovery.208
A recent paper from Denmark has described the control of PMWS simply through the use of depopulation and repopulation.209 Only one herd of the group examined was reinfected after 3 months and that was supplied by the original source of pigs to that farm.
The use of other vaccines and their role in the onset of the condition was discussed210 and recent information from Denmark suggests that in the cases in the epidemic in Denmark vaccination for M.hyo has played no part.211 In the USA there have been similar recommendations for control.212 These include the diagnosis of the specific infections that are contributing to the disease picture. If it is possible to vaccinate for these then do so (PRRS, SIV, PPV, M.hyo, APP, HPS). If there is a vaccine problem, then re evaluate. Treat the bacterial infections and consider the use of anti inflammatory drugs. Remove pigs that are not responding to treatment. Strictly adhere to all in/all out policies and the rules of pig flow. Minimize the moving and mixing of pigs. Decrease the density of stocking. Use disinfectants. Change source of pigs. By the use of segregated early weaning it is possible to produce PCV2 virus-free pigs from PCV2-positive sows.213
A tremendous amount of effort has gone into the search for an effective vaccine for PCV2 associated conditions.
Chimeric PCV1 live virus and a chimeric infectious DNA clone induce strong immune responses against PCV2 in pigs, at least experimentally, and protect pigs against PCV2 challenge.214
An inactivated PCV2 vaccine has also been described.215 Recently the use of PCV2 vaccination in France and Spain has, it is believed, been a great success.
The case definition is relatively simple.216-218 These animals are generally older than the animals affected with PMWS but the conditions usually occur in the same herd.219 There are two main features. Firstly, there is the presence of necrotizing skin lesions particularly over the hind limbs, perineum, and extending forward along the abdomen. Secondly there is a systemic necrotizing vasculitis and necrotizing and fibrinous glomerulonephritis.
The condition in terms of either gross or histological features has not been reproduced experimentally.
The probable first case of the endemic from was reported from Chile in 1976.220
It was reported in the UK as a sporadic problem221,222 and subsequently as part of the PMWS/PDNS outbreaks.223 This has been reported worldwide including recently from Hungary224 and Canada225 and the rest of the world.226
The acute form is probably associated with PCV2 infection.15 The major point is that it is indistinguishable from classical or African swine fever. The disease occurs at the end of the nursery stage and the beginning of the grower stage. It is also found occasionally in finishing pigs and replacement gilts. The skin lesions are multifocal, well-circumscribed, slightly raised, dark red, circular to irregular and 1 mm to 2 cm in diameter. They may coalesce. Other times they may heal. They are particularly severe over the hind legs, pelvis and perineum.
The lymph nodes are usually swollen as in PMWS but may also be dark red and hemorrhagic. The pigs affected are usually still eating but may be febrile and if so are usually anorectic. The lower limbs may be edematous and joints may also have excess or blood stained joint fluid. There are severe renal lesions which often include red focal lesions up to several millimeters in diameter, that correspond to congested of hemorrhagic glomeruli on the cortex of the kidney. These kidneys are often enlarged and the associated renal lymph nodes are very hemorrhagic. Many of these cases are detected in the abattoir as the kidneys are declared unfit for human consumption. The kidney failure is quite severe with a very elevated blood urea nitrogen and creatinine. Many cases also have a pneumonia and a gastric ulceration.
Histologically, the major lesion is a severe, fibrinoid, necrotizing vasculitis in the small vessels affecting the subcutis and dermis of the skin and renal pelvis and medulla. There is also a multifocal severe acute necrotizing glomerulonephritis. The lesions of the acute PDNS often mask the underlying PMWS. The target cells for the PCV2 in PDNS were cells of the macrophage/monocyte series.15 The sporadic cases which occurred before PMWS in the UK were often associated with the occurrence of P. multocida of one electrophoretic type.227,228 The cases occurring simultaneously with PMWS or after as these animals are slightly older do not show the same pattern of bacterial isolation. The condition is probably caused by an immune complex reaction occurring in the small blood vessels.
The subject has been described in detail.229
Allan GM, Ellis JA. Porcine circoviruses: a review. J Vet Diag Invest. 2000;12:3-14.
Done SH, et al. PMWS: The current European situation. Pig J. 2002;49:215-232.
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Chae C. Postweaning multisystemic wasting syndrome: a review of etiology diagnosis and pathology. Vet J. 2004;168:41-49.
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