Chapter 24 Diseases associated with Rickettsiales
Etiology Anaplasma marginale in cattle and wild ruminants, and A. ovis in sheep and goats. A. centrale causes mild anaplasmosis in cattle
Epidemiology Common in tropical and sub-tropical regions; sporadic in temperate regions. Carrier animals are the source of infection. Disease transmitted by ticks, mechanically by tabanid vectors, iatrogenically, and transplacentally. Disease can be endemic in tick areas or sporadic in interface regions between endemic and free areas
Clinical findings In cattle death or severe debility, emaciation, anemia and jaundice are the major clinical signs. The disease is usually subclinical in sheep and goats
Clinical pathology Anemia, demonstration of organism in red cells by microscopy, fluorescent stains or PCR, serology
Necropsy findings Anemia and attendant findings. Demonstration of organism
Diagnostic confirmation Detection of the organism in blood smears, positive serology, PCR, and in some circumstances positive transmission tests. The sensitivity, in a group of animals, can be increased by using parallel blood smears and serological tests
Treatment Clinical cases treated with tetracycline or imidocarb. Blood transfusion. Carrier state cannot be eliminated by treatment with tetracycline
Control Tetracycline provides temporary or prolonged protection in face of an outbreak. Vaccination with killed A. marginale vaccine or live A. centrale vaccine used in endemic areas as is weekly dipping in acaricides. In non-endemic areas, serological identification of carriers and culling or treatment of reactors. Prevention of iatrogenic transmission
Anaplasma spp. are obligate intra-erythrocytic parasites belonging to the order Rickettsiales and infecting ruminants. Analyses of 16S rRNA, groESL, and surface proteins have resulted in reclassification of the order Rickettsiales. The genus Anaplasma, of which A. marginale is the type species, now also includes A. bovis, A. platys, and A. phagocytophilum, which were previously known as Ehrlichia bovis, E. platys, and E. phagocytophila.1
Anaplasma marginale is the causative agent of anaplasmosis in cattle and wild ruminants, and A. ovis in sheep and goats. A. centrale is closely related to A. marginale and causes mild anaplasmosis in cattle. It was originally isolated in Africa but has been introduced as an immunizing agent in Australia, South America and Asia. There are antigenic variants among isolates of A. marginale, the six major surface protein antigens being antigenically polymorphic.1,2
Anaplasmosis in cattle is common on all six continents, being present in South Africa, Australia, Asia, the former USSR, South America and the United States. It is transmitted by a diverse group of biological and mechanical vectors. Infection in cattle is endemic in tropical and subtropical areas that support large populations of these vectors. Infection occurs more sporadically in temperate climate areas.
In the United States, and in other countries, the disease has occurred beyond the boundaries of tick-infested areas and the area distribution in Europe has been advancing northward in recent years with sporadic cases in France, Switzerland, the Netherlands, Hungary, and Austria.3 Anaplasmosis of sheep and goats has a distribution similar to that of cattle.
In the USA anaplasmosis is enzootic throughout the southern Atlantic states, the Gulf Coast states and many of the midwestern and western states.3,4 Disease occurs sporadically in the northern states and Canada.
In Australia infection is closely related to the distribution of Boophilus microplus, which is restricted to the northern areas. Seropositivity is negligible in cattle south of the tick line but above the tick line increases from south to north. Differences in enzootic and epizootic areas in South America and South Africa are also largely related to tick distribution and climate.
In most countries there is wide geographical variation in seroprevalence and this variability contributes to the development of geographically stable or unstable enzootic regions.
There is concern, and some evidence, that the global warming trend will expand the boundaries and movement of host ticks.1
Cattle are susceptible to A. marginale and A. centrale and sheep to A. ovis. A. marginale will establish in sheep by experimental infection but A. ovis will not infect cattle.5 A variety of species of wild ruminants in both North America and Africa can be infected and may have significance as reservoirs for A. marginale.6,7 In the United States the black-tail deer (Odocoileus hemionus columbianus) in the West Coast region is believed a reservoir and a number of species of antelope play a similar role in South Africa. Bighorn sheep (Ovis canadensis) may be a reservoir for A. ovis in the western United States.8
The prevalence of infection in cattle in endemic areas is very high with seropositivity rates exceeding 60% and often approaching 90%. Seropositivity is much lower in regions that interface between endemic and non-endemic regions.9
The source of infection is always the blood of an infected animal. Recovery from acute infection results in persistent infection characterized by repetitive cycles of rickettsemia. Persistent carriers are the reservoir for herd infection. The level of parasitemia is often too low for detection by microscopy but can be detected by nucleic acid probe analysis.10 Transmission is biologically by ticks but can also occur transplacental and can also be effected mechanically by biting flies or blood-contaminated fomites.
Spread from animal to animal occurs chiefly by insect vectors. A variety of arthropods may act as vectors but significant natural vectors are ticks in the family Ixodidae and flies in the family Tabanidae. Of the ticks, the one-host Boophilus spp. are of major importance in tropical and sub-tropical regions and the three-host Dermacentor spp. of major importance in the western US.
The organism undergoes a complex developmental cycle in the gut cells of ticks and the final infective stage is present in the salivary gland.11 Trans-stadial transmission of the organism occurs in ticks but there is little evidence for transovarial transmission. Intrastadial transmission is significant with some species and transmission occurs as the ticks move from one host to another while they are engorging, including from cow to calf. Male D. andersoni can act as effective vectors in this manner for at least 120 days.12
There appears to be no developmental sequence of Anaplasma spp. in flying insects. Tabanids are efficient mechanical vectors and can transmit infection for 2 hours after feeding.
Over 20 species of tick have been incriminated as vectors world-wide.13 In Australia the ticks Boophilus microplus and Rhipicephalus sanguineus14 are the vectors and in South Africa it is B. microplus, B. decoloratus, and Rhipicephalus simus. In the United States Boophilus annulatus, Dermacentor andersoni, D. variabilis, Argas persicus and biting flies of tabanid species and eye gnats (Hippelates pusio)15,16 also act as vectors. The male ticks of Dermacentor albipictus (the winter tick) and D. occidentalis (the Pacific Coast tick) parasitize both deer and cattle and have been suspected as vectors.17
Anaplasmosis may also be spread mechanically by infected hypodermic needles, by castrating, spaying and dehorning instruments, and by blood transfusions and embryo transplants. The ease with which the infection is spread mechanically may vary with the virulence of the protozoan strain and this method of spread may be more important in some countries than others. Anaplasmosis may also be spread when cattle, used as donors of infected blood for immunization against babesiosis, are infected with A. marginale, the reaction occurring some 3 weeks later than that due to the babesia.
Intra-uterine infection also occurs in cattle but much less frequently in field cases than in experimental ones.18 Abortion or neonatal infection may result. In ewes intra-uterine infection appears to occur with ease in experimental cases provided the ewe is exposed during the latter two-thirds of pregnancy.19
Bos indicus, Bos taurus and their crosses have equal susceptibility to infection and show the same age susceptibility, but under field conditions Bos indicus are not as commonly affected, probably because of their relative resistance to heavy tick infestation. However, the effects of the disease on body weight and clinicopathological parameters are the same for the two races of cattle.20 Breeds with black or red coat color have a higher risk of infection than those with white coats in regions where biting flies are the insect vector.4,9 Dairy breeds may be at greater risk for iatrogenic transmission.
Clinical disease is less severe in cattle on a low plane of nutrition. Exposure of infected, clinically normal animals to devitalizing environmental influences, particularly shortage of feed, and the presence of other diseases, may result in the development of acute anaplasmosis. For example, cattle introduced into feedlots are highly susceptible and outbreaks among them are not uncommon 2–3 weeks after entry.
In temperate climates a seasonal occurrence of disease occurs in association with seasonal occurrence of the insect vectors.4,16,21,22 Winter outbreaks are likely associated with iatrogenic transmission9 or possibly the winter tick, D. albipictus.16
All cattle are susceptible to infection but age at infection is a major determinant of the severity of clinical disease. Young calves are less susceptible to infection with A. marginale than older cattle and, when infected, are less susceptible to clinical disease. The reason for, this is not understood but splenectomized calves are fully susceptible to infection, which may be more severe than in the adult. Infection between six months and three years of age has increasing risk of clinical illness and animals infected after 3 years of age are commonly affected by a peracute fatal form of the disease. The age-specific incidence of clinical disease recorded in an outbreak in the USA showed 81% of cases in cattle aged between 2–4 years with 94% of cases in cattle 3 years of age or older.4
Clinical disease is rare in enzootic areas because the infection pressure is high and cattle are infected at an age when they are age-resistant to clinical disease. The average age at which calves in enzootic areas become infected is 11 (4–24) weeks and the clinical and hematological changes in them are mild and brief. Animals in an infected environment which have become seronegative for whatever reason are fully susceptible to infection.23 Clinical disease occurs where there is introduction of susceptible animals into endemic areas or the expansion of the vector population into previously free areas or into the interface between endemic and non-endemic regions.
Case fatality rates are usually high in outbreaks but the mortality rate varies widely depending on susceptibility, and may be 50% or more in cattle introduced to enzootic areas. Case fatality rates of 29–49% are recorded in outbreaks in the USA; recovered animals are emaciated and there is a prolonged convalescence.4
Phylogenetic analysis of A. marginale geographic isolates support the existence of clades.1 Australian isolates do not appear to differ significantly in antigenicity or virulence.24 In contrast in other countries there can be significant differences between isolates in antigenic composition, the protection afforded against heterologous challenge and virulence.25-29
Recent research has demonstrated that the phenomenon of infection exclusion occurs with A. marginale. Infection of tick cells and bovine erythrocytes with one genotype of Anaplasma marginale excluded infection with other genotypes and in herds of cattle from endemic areas where many genotypes were detected only one genotype was found per animal. Further cattle inoculated with two A. marginale isolates became infected with only one isolate.28,29
Costs are from death and abortion in clinical cases, loss of production in sick and recovered animals, and costs associated with preventive measures such as tick control. There have been few recent estimates of cost, the most recent estimates in 1977 and were of a cost of $875 millions in Latin American nations.30
In developed countries with the disease exports of cattle to countries that do not have it are constrained. A major cost in developing countries is the constraint to efficient production and the limit to the introduction of susceptible cattle breeds with superior genetics.4,31
Anaplasma are obligate intra-erythrocytic bacteria. They infect mature erythrocytes by an endocytic process and reproduction occurs by binary fission to produce 2–8 infective initial bodies which leave by exocytosis to infect other erythrocytes. The number of infected erythrocytes doubles every 24–48 hours and the infection becomes patent 2–6 weeks after infection,32 the time influenced by the initial challenge dose.33 Depending upon the strain and the susceptibility of the host, from 10–90% of erythrocytes may be parasitized in the acute stage of the infection. At least 15% have to be parasitized before there is clinical disease. Parasitized erythrocytes are removed by phagocytosis in the reticular endothelial system, with release of acute-phase inflammatory reactants and the consequent development of fever. Continued erythrocyte destruction occurs resulting in the development of mild to severe anemia and icterus without hemoglobinemia and hemoglobinuria. Anaplasmosis is primarily an anemia, the degree of anemia varying with the proportion of erythrocytes which are parasitized. The first appearance of the protozoa in the blood coincides with a fall in the hematocrit and erythrocyte levels, the appearance of immature erythrocytes in blood smears and the development of fever. Acutely affected animals may die shortly after this phase is reached. The appearance of anti-erythrocyte antibodies late in the acute stage may exacerbate the anemia.32
If the animal recovers from the initial acute attack, periodic attacks of parasitic invasion of mature erythrocytes occur regularly, but with diminishing intensity. The degree of anemia varies widely in young cattle up to 3 years of age but is always severe in adults and in splenectomized animals. Cattle that survive the disease become carriers and serve as reservoirs of A. marginale because they provide a source of infective blood for both mechanical and biological transmission by ticks. They have lifelong immunity and are resistant to clinical disease on challenge exposure.
Carrier animals have cycles of parasitemia, possibly associated with the development of new antigenic variants to allow new cycles of invasion and multiplication.34 These occur at approximate 5-week intervals during which the new variants replicate and are then controlled by a variant-specific immune response.
In cattle, the incubation period varies with the challenge dose but is generally about 3–4 weeks with tick-borne infection and 2–5 weeks with the inoculation of blood. In most cases the disease is subacute, especially in young animals. Rectal temperature rises rather slowly and rarely to above 40.5°C (105°F). It may remain elevated or fluctuate with irregular periods of fever and normal temperature alternating for several days to 2 weeks. Anorexia is seldom complete. Death can occur at this stage but many survive in an emaciated condition, and their fertility is impaired. The mucous membranes are jaundiced and show marked pallor, particularly after the acute stage is passed, but there is no hemoglobinuria.
Peracute cases, with a sudden onset of high fever, anemia, icterus, severe dyspnea and death, often within 24 hours, are not uncommon in adult dairy cows. Affected animals are often hyperexcitable and tend to attack attendants just before death. Pregnant cows frequently abort. In convalescent bulls there may be depressed testicular function for several months.
In sheep and goats, infection is usually subclinical but in some cases, particularly in goats, a severe anemia may occur and a clinical picture similar to that found in cattle may be seen. Severe reactions of this type in goats are most frequent when the animals are suffering from concurrent disease. Goats may show hyperexcitability and may bite at inanimate objects. The experimental disease in lambs includes fever, constipation or diarrhea, pale, icteric conjunctivae and severe anemia 15–20 days after inoculation. The anemia is not completely resolved in 3–4 months.
Erythrocyte destruction may be so severe that the erythrocyte count is reduced to 1.5 million/μL. Immature red cells are common at this stage and their presence is considered to be a favorable sign. The small dot-like protozoa are discernible at the periphery of up to 10% of the red cells in subacute cases, but in peracute cases more than 50% of the cells may be parasitized. A. ovis are usually situated at the periphery of erythrocytes but as many as 40% of infested cells may show submarginal protozoa. Diff-Quik staining of blood smears is as accurate as Giemsa in the detection of A. marginale and can be completed in 15 seconds as compared to nearly an hour35 for Giemsa. There are no diagnostic clinical chemistry findings.
The complement fixation test is the standard test for the detection of carrier animals. It is satisfactory for use in cattle, goats and sheep but the antibody titer is highest during the active phase of the disease and sufficiently low in carrier animals to give a proportion of false negative results. False positive reactions can occur because of erythrocyte contamination of the A. marginale antigen and the presence of antibodies to erythrocytes in some cattle sera. A rapid card agglutination test, which tests serum or plasma for antibodies against A. marginale, is cheap and quick, and sufficiently accurate to be used as a herd test. Currently, in most countries, the card agglutination and complement fixation tests are routinely available.
There are a number of other tests that have been developed. A capillary tube agglutination test of comparable efficiency is available, is more economical and faster than the CF test36 and is particularly suited to testing in extensive field situations. An indirect fluorescent antibody test is also accurate37 and has a particularly suitability for testing blood which has been dried onto paper for passage through the mails. It is also an accurate test for selecting recently affected animals. A dot-ELISA with high sensitivity, specificity and predictive value is also described and could be particularly applicable to field examinations.10,38 A competitive inhibition ELISA test, with high sensitivity and specificity, has been developed that detects antibody to a major surface protein that is conserved among anaplasma species; this test can be used to detect cattle persistently infected for as long as six years.39-41 Vaccinated animals may react to all of the serological tests for periods of over one year.
The most obvious findings are emaciation, pallor of the tissues, and thin, watery blood. There is mild jaundice and the liver is enlarged and orange. The kidneys are congested and there may be myocardial hemorrhages. The spleen is enlarged with a soft pulp. The bone marrow cavity may be reddened by increased hematopoietic tissue in acute cases but there may be serous atrophy of marrow fat in chronic cases. Postmortem identification of A. marginale can be established by staining blood smears with Giemsa or direct fluorescent stains. Peripheral blood is superior to organ smears. Brain smears are unsatisfactory. The technique is applicable to fetuses suspected of being aborted as a result of infection with Anaplasma sp. Nucleic acid-based tests may be used but are rarely needed for routine diagnosis at necropsy.
Treatment is with tetracyclines. Treatment of clinical disease can be with oxytetracycline, 6–10 mg/kg BW daily for three days, or a single injection of long-acting oxytetracycline at a dose of 20 mg/kg intramuscularly. The convalescent period is long. Concurrent administration of estradiol cypionate (14.3 mg/kg BW intramuscularly) appears to improve the rate of recovery by promoting parasitemia during treatment.45 Tetracycline treatment will not eliminate infection and immunity will persist.32 Blood transfusions are indicated in animals with a PCV less than 15%. Rough handling must be avoided.
Imidocarb (3 mg/kg BW) is also an effective treatment for clinical cases and does not interfere with the development of acquired immunity to A. marginale.46
The risk for infection in the rest of the herd should be assessed and, if necessary, temporary or prolonged protection should be provided. Protection can be provided by tetracyclines, or by vaccination.
Temporary protection in the face of an exposure risk can be achieved with a single intramuscular injection at 20 mg/kg BW of long-acting tetracycline.32 The results generally are good except when the cattle are exposed to infection during the 14 days prior to the treatment. Prolonged protection can be achieved by the intramuscular injection at 20 mg/kg BW of long-acting tetracycline every 28 days or by chlortetracycline in the feed at 1.1 mg/kg BW daily.32
Elimination of infection cannot be achieved with tetracycline therapy. A trial testing the ability of oxytetracycline therapy to eliminate the carrier state examined therapy with 300 mg/mL oxtetracycline solution of administered at 30 mg/kg, by intramuscular (IM) injection for one day; the same preparation administered at 30 mg/kg, IM on day 0 and again on day 5; and a treatment with a 200 mg/mL solution of oxytetracycline administered at 22 mg/kg, intravenously (IV), q 24 h for 5 days (a treatment dose that corresponds with current Office International des Epizooties (OIE) recommendations for treatment prior to export). All treated cattle were still, PCR and cELISA positive 60 days after therapy and their carrier status was confirmed by inoculation of blood in splenectomized calves.47
Methods for the control of anaplasmosis have not changed greatly over the past several decades and consist of arthropod control with acaricides, chemotherapy for prevention and vaccination. The eradication of anaplasmosis is not a practicable procedure in most countries at the present time because of the wide range of insects which are capable of carrying the disease, the long period of infectivity of carrier animals, and, in some areas, the presence of carriers in the wild animal population. In enzootic areas some benefit is derived from the control of ticks and other vectors and weekly dipping in an acaricide is used in tropical areas to control this and other tick-borne diseases.
The introduction of the disease into herds by carrier animals should be prevented by prior serological testing. Attention should also be given to preventing iatrogenic transmission with instruments used for injections or surgical operations by disinfection after use on each animal. This is particularly important in feedlots where introduced groups are often subjected to multiple vaccinations and implantation at a time when their resistance is lowered by transport and change of feed.
Exposure-naive animals that are to be introduced into an enzootic area should be vaccinated. Some advantage can be gained when introducing animals into an enzootic area by limiting the introductions to animals of less than 2 years of age and by bringing them in when the insect population is least numerous.
This is feasible in regions that are subject to only periodic incursions of infection and that do not have endemic tick vectors. It can be achieved by serologic testing and culling of reactors or treating them as outlined above to eliminate the carrier state.
If an outbreak does occur, affected animals should be treated vigorously as described above and in-contact animals vaccinated and/or placed on a regimen of prolonged tetracycline protection. Subsequently all exposed animals should be tested serologically and the reactors treated or preferably salvaged. Prolonged treatment regimens can be used to provide protection to cattle in seasonal risk periods of transmission.
Chemotherapy for control is more commonly used in the United States than in other areas of the world. It can be of value in feedlot cattle but is not applicable to range cattle. It is expensive and carries the risk of causing selection of resistant strains.1
Vaccines for the control of anaplasmosis are either live or killed vaccines. Both types use A. marginale from infected bovine erythrocytes and while both types induce protective immunity that reduces or prevents clinical disease neither type prevents cattle from becoming persistently infected with A. marginale. Cattle that have recovered from acute infection or immunized with killed vaccines are solidly protected against challenge with the homologous strain but are only partially protected against challenge with heterologous strains.48
Most control programs in enzootic areas are based on increasing the resistance of the population by immunization. In any vaccination program particular attention should be paid to the animals at high risk, particularly animals brought in from non-enzootic areas, those in surrounding similar areas to which infection may be spread by expansion of the vector population under the influence of suitable climatic conditions, and animals within the area which are likely to be exposed to climatic or nutritional stress.
Killed A. marginale are usually in an adjuvant vehicle. The vaccine requires two doses, four weeks apart, the last dose given at least two weeks before the vector season. Subsequently, booster doses should be given two weeks before the next vector season. The vaccine does not prevent infection but does significantly reduce the severity of the disease. It does have the advantage over the other vaccines of having a relatively short post-vaccination period when animals remain positive to serological tests. The duration of the immunity is at least 5 months.
There is a risk for neonatal isoerythrolysis. This can be reduced by vaccinating only empty cows and avoiding unnecessary booster injections. When this vaccine is used in the face of an outbreak, tetracyclines can also be given to provide temporary protection during the period of development of immunity; tetracyclines do not interfere with the development of this immunity.
Preliminary reports of the efficacy of DNA vaccines are not encouraging.49
A living A. centrale vaccine is used extensively in Australia, Africa, Israel, and Latin America, but not in the United States and there is some reluctance to introduce it into areas where A. centrale does not already occur.
Living A. centrale vaccine is prepared from the blood of infected splenectomized donor calves and is stored chilled or frozen. The vaccine causes a mild, inapparent disease, but does cause severe reactions in occasional animals. It is generally safe in young cattle. A single vaccination is used in endemic areas and the immunity is reinforced by continuous challenge and considered to persist for life in tick areas. A. centrale and A. marginale share immunodominant epitopes and have similar antigenic variation in major surface proteins which may play a role in the cross immunity that occurs.50,51 It is unclear whether A. centrale infection and immunity induced by the live vaccine in cattle prevents subsequent infection with A. marginale by the infection exclusion phenomenon.28,29,51
The efficacy of this vaccine varies geographically.52 Vaccination with A. centrale reduces the severity of the reaction when infection with A. marginale occurs but does not give absolute protection. Protection against challenge in Australia is adequate in most cases, and certainly sufficiently effective enough to justify its use. In contrast the use of the same vaccine in countries other than Australia, where there are more antigenically diverse and highly virulent strains is often inadequate and better vaccines are required.52
Tetracyclines will prevent establishment of infection and immunity by the vaccine and should not be administered for 3 weeks before vaccination.
Living A. marginale has been used as a vaccine but its administration is limited to the relatively resistant age group below 1 year of age, to the winter months when vectors are sufficiently rare to avoid the chance of spread to other age groups, and to circumstances where animals which react severely can be restrained and treated adequately. The method has the serious disadvantage of creating a large population of carrier animals which may subsequently spread the disease.53
Attenuated vaccines have been attempted by irradiation of strains and passage of the organism through sheep or deer and the use of naturally low virulence isolates. While most have been received with initial enthusiasm some have proved not effective while others have been associated with adverse reactions. Some, while effective against strains in some geographic regions, give unsatisfactory protection against clinical disease in other regions.54-56
All vaccines currently must be produced in live animals, which is expensive. With non-inactivated vaccines there is a risk of transmitting blood-borne viruses. In Australia, a single calf infected with bovine leukosis virus was unsuspectingly used in the production of A. centrale vaccine. The contaminated vaccine was given to 22 627 cattle in 111 herds and resulted in a high rate of infection with bovine leukosis virus in the vaccinated cattle.14
It is highly probable that a safe subunit vaccine, containing epitopes critical to effective immunity, will be developed in the near future.
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Etiology Anaplasma phagocytophila
Epidemiology Occurs in the northern latitudes and is transmitted by Ixodes ricinus in UK and Europe and Ixodes scapularis and Ixodes pacificus in the USA. Disease in cattle and sheep primarily reported from the UK and Europe. Seasonal occurrence associated with the feeding activity of the vector. More severe disease in naive introduced animals. Increases susceptibility to other infections
Clinical findings Fever, depression, lethargy, polypnea and fall in milk production in cattle. Abortion
Clinical pathology Thrombocytopenia followed by more prolonged neutropenia and lymphocytopenia. The organism is demonstrable in the neutrophils and monocytes during each febrile period
Diagnostic confirmation Demonstration of the E. phagocytophila in leukocytes at acute stage of the disease or by serology retrospectively
Tick-borne fever (also called pasture fever in cattle) is associated with infection with Anaplasma phagocytophila, (Ehrlichia phagocytophila, Cytoecetes phagocytophila) which is an obligate intracellular parasite in the family Anaplasmataceae. Anaplasma phagocytophila and the human granulocytic ehrlichial agent (HGE) and Ehrlichia equi are morphologically identical, they have only minor variations in their 16S rRNA genes and 100% identity in their GroESL amino acid sequences, and so are now all classified as Anaplasma phagocytophila.
There are strains (variants) of A. phagocytophila that have biological and ecological differences, including variations in host pathogenicity, vectors, and geographical distribution.1,2 In sheep, different variants of A. phagocytophilum may exist simultaneously in the same sheep flock.3
Tick-borne fever is also used as a name for similar, but less well defined, diseases of ruminants that are associated with infection with related organisms such as Anaplasma (Ehrlichia) bovis. These are reported from other areas of the world such as India and Africa, and are transmitted by the ticks Rhipicephalus appendiculatus, Amblyomma variegatum and Hyalomma truncatum.4,5
This description is of tick-borne fever associated with A. phagocytophila.
Infection with A. phagocytophilum occurs in a wide range of mammalian hosts including humans, dogs, sheep, cows, horses, wild deer and rodents. The association of A. phagocytophilum with human granulocytic ehrlichiosis in the mid-1990s has led to much activity defining its geographical occurrence by serological surveys or detection in ticks by molecular methods. These studies have determined that the organism is present where the host ticks are present in Europe, Iran, North America and probably Asia.6
However, the disease tick-borne fever, as apposed to infection, occurs primarily in certain areas of the United Kingdom, Ireland, Norway, Finland, France, Germany, Spain, and Switzerland.7,8 Because ticks favor particular optimal environmental conditions, the geographic distribution of the ticks is usually restricted to a specific area (small or large) and tick-borne fever only occurs in these areas. Within these areas infection can be intense and in the endemic coastal area of Norway close to 100% of sheep grazing Ixodes infested pastures are infected.9 Tick-born fever has a seasonal occurrence in association with the feeding activity of the vector tick. Infection can be endemic in affected areas.
Sheep, cattle, goats, deer and reindeer may be infected. The disease has long been known as a disease of sheep but in recent years is being recognized as a common infection in cattle in at-risk areas.7 The incidence rate of infection is high but clinical disease may be mild and not easily observed in many areas where this disease occurs, as these area are commonly wild with little human habitation and little frequent observation of at-risk livestock. Infection, as determined by seropositivity, can occur in sheep that have had no clinical evidence of disease because of the existence of variants with low pathogenicity.2
In Europe, E. phagocytophila is transmitted by the three host tick Ixodes ricinus which requires a single blood meal at each stage of development. The tick feeds for approximately 3 weeks every year and completes its life cycle in 3 years. The larval and nymphal stages will feed on any vertebrate but the adult female will engorge and mate only on larger mammals.
A. phagoctyophila infect and multiply in the organs of ticks, in particular the salivary glands which enables the transmission to vertebrate hosts during feeding. The tick becomes infected by feeding on an infected host and there is trans-stadial but not transovarial passage of the organism. It is estimated that the majority of ticks are infected with the organism in enzootic areas10 and one study of ticks from a field site found 44% of nymphs and 32% of adults infected but no infected larval stages.11 There is a close relation tick density and the proportion of sheep and ticks infected with A. phagoctophila but it is non-linear and complex.12,13
In the USA, Ixodes scapularis has been implicated in transmission of the organism in eastern USA and Ixodes pacificus on the west coast,14 as have Ixodes persulcatus and Haemaphysalis longicornis in Asia15 but clinical disease in ruminants is not a feature in these locations.
Congenital infection of a calf has occurred following experimental infection of the dam in late pregnancy16 and the organism is also present in leukocytes in milk during the acute phase of the disease,17 but the significance of this in the epidemiology of the disease is not known.
A few as one A. phagocytophila-infected cell may be enough to transmit infection and use of a single needle between sheep in a group could possibly transmit infection.18
It has been suggested that the presence of ticks in migratory birds could spread infection of this agent to other geographic regions.19
The disease can be readily reproduced experimentally. The severity of the clinical response of sheep following experimental infection is not dose dependant and there is no dose effect on the degree of parasitemia or neutropenia.18
Calves and lambs are much more susceptible than adults although clinical disease may be less severe in very young lambs4,20 possibly due to mitigating effects of colostral antibody. Hyperimmunization of the pregnant ewe will produce high levels of colostral antibody that will protect the lamb against experimental challenge. However, in the field colostral immunity does not protect against infection and lambs born of ewes raised in endemic areas become infected. Natural infection is followed by a state of low-grade premunity due to the presence of the organism in the blood which provides partial resistance to subsequent infections, the disease manifesting itself in a less severe form. Once infected, animals probably remain carriers for life and act as reservoirs of infection for new generations of ticks.
The case fatality is very low and most reported mortality is in association with intercurrent disease. A significant indirect effect of tick-borne fever is that it increases the susceptibility of lambs to staphylococcal pyemia, staphylococcal pneumonia, septicemic and pneumonic pateurellosis,20 louping-ill and possibly other diseases. The mortality rate is negligible in cattle but may be higher in sheep.
The activity of the tick is seasonal and consequently tick-borne fever has a seasonal occurrence. The tick is active at temperatures between 7–18°C and most ticks feed in the spring, with peak activity dependent upon the latitude and elevation of the pasture but generally occurring in April and May. In some areas there is a second period of activity of a separate population of I. ricinus in the autumn during August and September. Clinical signs in cattle occur predominantly in spring, one to two weeks after they start to graze.
Human granulocytic ehrlichiosis is associated with A. phagocytophila and was first described in the USA in 1994 and in Europe in Slovenia in 1997. It presents most commonly as an undifferentiated, febrile, potentially severe illness occurring in summer or spring associated occupational or recreational activities that allow exposure to infected ticks. There is no recognized direct zoonotic risk from exposure to infected animals but there is evidence that sheep may be one of the maintenance hosts for the organism.2,6,9
The organism is also present in leukocytes in milk during the acute phase of the disease,17 but the risk to humans consuming this is not known.
A. phagocytophilum infect and replicate within neutrophils where they live in cytoplasmic vacuoles to form clusters called morulae. They are able to evade activation of the cytotoxic arsenal of the neutrophil but they do perturb neutrophil function.21 Tissue pathology is not associated with direct A. phagocytophilum-mediated injury but results from immunopathologic mechanisms associated with cytokine and chemokin production.
Fever develops in association with parasitemia and is the prominent clinical abnormality in the experimental disease. It persists for approximately 8 days, may exceed 41°C, and is accompanied by depression. While this syndrome is of limited importance in the experimental setting, the occurrence of fever, dullness, and depression of the sucking drive can be a significant influence on the viability of lambs in the cold, wet, rough-grazing areas where this disease commonly occurs and may contribute to lamb mortality.
Tick-borne fever produces profound effects on immunological defense systems. There is a significant lymphocytopenia that develops 6 days after infection and which affects all T- and B-lymphocyte subsets.22 There is also a prolonged neutropenia lasting for 2–3 weeks combined with a thrombocytopenia. Up to 70% of the neutrophils are parasitized from the onset of the parasitemia and have impaired function.10 The antibody response of infected sheep to immunogens such as tetanus toxoid is also impaired.23
Field observations and experimental challenge has shown that infected lambs are more susceptible to disease and mortality from intercurrent infections. The ability of an infection with A. phagoctophila to predispose to secondary disease varies with the strain of the organism2 which may explain why secondary complications are not observed in all flock infections with tick-borne fever. There is a clear relationship between infection with E. phagocytophila and susceptibility to infection with Staphylococcus aureus and the resultant disease, tick pyemia. This is established both by epidemiological and experimental studies.24
Concurrent infection of sheep with the agent of tick-borne fever potentiates the pathogenicity of louping-ill virus, in experimental infections, to result in more severe disease and a higher mortality. Both diseases are transmitted by I. ricinus. However, in areas where both diseases are endemic, colostral immunity will delay infection of lambs with the louping-ill virus until the second year of exposure to the vector tick while allowing infection with tick-borne fever. Simultaneous primary infection with both agents may be uncommon in nature.
Infection also facilitates invasion and systemic mycotic infection with Rhizomucor pusillus resulting in diarrhea and dysentery and a high mortality rate.25 Concurrent experimental infection of sheep with E. phagocytophila and Chlamydia psittaci results in chlamydial pneumonia26 and simultaneous infection with parainfluenza-3 (PI-3) virus potentiates the pathogenic effect of PI-3 virus.27 The immunosuppressive effect of tick-borne fever is believed to have resulted in the exacerbation of latent Brucella abortus in a naturally occurring abortion outbreak in cattle.28 Concurrent infection of tick-borne fever and Listeria monocytogenes or Pasteurella haemolytica promotes the respective septicemic disease in lambs.
Disease is generally benign but infection can produce abortion and can cause significant loss of weight in lambs and calves.
In cattle there is an incubation period of 5–9 days followed by a rise in temperature to about 40.5°C (105°F) which persists for 2–12 days and for a longer period in late pregnant cows than in lactating cows.29 The temperature falls gradually and is followed by a secondary febrile period and, in some cases, yet further episodes of pyrexia. During each febrile period there is a marked fall in milk yield, lethargy and polypnea and in experimentally produced cases, a mild cough although feed intake is not reduced. The fall in milk production can be severe and may be the first indication of infection.30 Pregnant cattle, in the last two months of pregnancy, and placed on tick-infected pastures for the first time, commonly abort and occasionally animals die suddenly. The abortions occur shortly after the systemic disease. Some calves are born alive but they are weak and die.
In sheep the syndrome is similar to that observed in cattle except that respiratory distress is not observed. However, there can be marked differences in clinical manifestation, neutropenia, antibody response, with different variants of A. phagocytophilum.2
The reaction in young lambs is quite mild and manifested only by a fever which fluctuates between 40–42°C for up to 10 days. Ewes exposed to the disease for the first time commonly experience outbreaks of abortion and affected rams are temporarily infertile.
Abortion is a major manifestation in northern Spain8 whereas in the Scandinavian countries the main consequence of infection is immunosuppression leading to secondary infections with Staph. aureus (tick pyemia) and Pasteurella hemolytica.31
At the commencement of the fever there is a severe but transient thrombocytopenia and this is followed by more prolonged neutropenia and lymphocytopenia. The anaplasmae are demonstrable in the neutrophils and monocytes during each febrile period and for a few days afterwards in cattle and for several weeks in sheep; they can be detected as intracytoplasmic inclusion bodies in Giemsa stained blood smears, or by PCR.17,33,34
Serological diagnosis is possible using counter-immunoelectrophoresis which detects IgM antibody35 or indirect immunofluorescence using cytospin preparations of blood granulocytes, which detects IgG.7,36 Antibody is at a high level at the second week after experimental infection with both tests and is detectable for 6–8 weeks with counter-immunoelectrophoresis and for at least 18 weeks with the indirect fluorescent antibody test.36 ELISA is also available for serological diagnosis.9
Transmission of the disease for diagnosis may be effected by the intravenous injection of blood taken at the height of the fever.
There are no gross changes other than splenomegaly in sheep, and histologically the only characteristic lesion is a depletion of lymphocytes from lymphoid tissue.
Multifocal leucomalacia spongy change of white matter and swelling of oligodendrocytes is found in the brain of aborted lambs probably the result of fetal anoxia.37
The best results are with tetracyclines or sulfadimidine although cattle may recover without therapy.29 In sheep, a single dose of long acting tetracyclines (20 mg/kg IM) or a 5-day course with oxytetracycline (10 mg/kg IV daily) given during the acute phase of the disease is effective in treatment but infection is not eliminated in a significant proportion of sheep.38 In goats good results are provided by single doses of oxytetracycline (10 mg/kg BW intravenously) or a potentiated sulfonamide containing trimethoprim and sulfadimidine, and sulfamethylphenazole (20, 50, and 50 mg/kg BW respectively); ampicillin is ineffective. The anaplasmae persist in treated animals which may subsequently suffer a relapse.
Control of tick-borne fever depends upon control of the tick population. The annual dipping of ewes with organophosphates or synthetic pyrethroid acaricides will help reduce tick numbers and the double dipping of lambs during the tick season will help reduce disease in the lambs but can be difficult to achieve in the terrain and with the management practices of affected areas.10 Disease is reduced if the flock can be kept off the tick-infested pastures until the lambs are 6 weeks old and if the flock is dipped prior to introduction to the pasture. In some areas it may be possible to reduce tick numbers by pasture management systems that disturb the pasture microclimate required by the tick.
The disease can be more severe when adult animals are exposed to infection for the first time and naive late pregnant cattle should not be introduced to tick-infested pastures during the tick rise periods. A single administration of 20 mg/kg of long-acting tetracycline is reported to provide protection against experimental challenge for periods up to 3 weeks.10 The prophylactic administration of long-acting tetracycline to lambs10 and to calves39 during the season of tick activity is reported to reduce mortality and improve growth rates over untreated controls.
Scott GR. Tick-borne fever in sheep. Vet Ann. 1984;24:100-106.
Brodie TA, et al. Some aspects of tick-borne disease of British sheep. Vet Rec. 1986;118:415-418.
Vaughan M. Control of tick-borne diseases in cattle. In Pract. 1988;102:79-84.
Ogden NH, et al. A review of studies on the transmission of Anaplasma phagocytophilum from sheep: Implications for the force in endemic cycles. Exp Appl Acarol. 2002;28:195-202.
Parola P. Tick-borne rickettsial diseases: emerging risks in Europe. Comp Immunol Microbiol Infect Dis. 2004;27:297-304.
1 Massung RF, et al. Emerg Infect Dis. 2002;8:467.
2 Stuen S, et al. Clin Diag Lab Immunol. 2003;10:692.
3 Stuen S, et al. J Clin Microbiol. 2002;40:3192.
4 Stuen S, et al. Res Vet Sci. 1992;52:211.
5 Kolte SW, et al. Indian Vet J. 2003;80:399.
6 Parola P. Comp Immunol Microbiol Infect Dis. 2004;27:297.
7 Pusterla N, et al. Schweiz Arch Tierhlk. 1997;139:392.
8 Garcia-Perez A, et al. Ann NY Acad Sci. 2003;990:429.
9 Stuen S, et al. Acta Vet Scand. 2001;42:347.
10 Brodie TA, et al. Vet Rec. 1986;118:415.
11 Webster KA, Mitchell GBB. Res Vet Sci. 1989;47:30.
12 Ogden NH. Exp Appl Acarology. 2002;28:195-202.
13 Ogden NH. Parasitol. 2002;124:127.
14 Barlough JE. J Clin Microbial. 1997;35:2018.
15 ChulMin K, et al. Vector-Bourne Zoonot Dis. 2003;3:17.
16 Pusterla N, et al. Vet Rec. 1997;141:101.
17 Pusterla N, et al. Clin Diag Lab Immunol. 1997;4:643.
18 Stuen S, Artursson K. Vet Rec. 2000;146:669.
19 Habalek Z. J Wildlife Dis. 2004;40:639.
20 Overas J, et al. Vet Rec. 1993;133:398.
21 Sciafe H, et al. Infect Immun. 2003;71:1995.
22 Waldehiwet Z. Res Vet Sci. 1991;51:40.
23 Larsen HJS, et al. Res Vet Sci. 1994;56:216.
24 Webster KA, Mitchell GBB. Vet Parasitol. 1989;34:129.
25 Reid HW, et al. Res Vet Sci. 1986;41:56.
26 Munro R, et al. J Comp Pathol. 1982;92:117.
27 Batungbacal MR, Scott GR. J Comp Pathol. 1982;92:415.
28 Juste RA, et al. Vet Rec. 1989;124:636.
29 Pusterla N, Braun U. J Vet Med A. 1997;44:385.
30 Cranwell MP, Gibbons JA. Res Vet Sci. 1986;119:531.
31 Stuen SI, Bergstrom K. Acta Vet Scand. 2001;42:331.
32 van Miert ASJPAM, et al. Vet Parasitol. 1984;16:225.
33 Ogden NH. Parasitol. 2002;124:127.
34 Hulinska D, et al. Acta Path Microbiol Scand. 2004;112:239.
35 Pascton EA, Scott GR. Vet Microbiol. 1989;21:133.
36 Webster KA, Mitchell GBB. Res Vet Sci. 1988;45:28.
37 Scholes SFE, Watson PJ. Vet Rec. 2004;154:32.
Etiology Ehrlichia (Cowdria) ruminantium, a rickettsial organism
Vectors Amblyomma variegatum and A. habraeum
Epidemiology Endemic disease of cattle, sheep, goats and wild ruminants in Africa and the Caribbean; high mortality in exotic animals
Pathogenesis Tick inoculation → local lymph node → blood → endothelial cells → vascular permeability → clinical signs and lesions.
Clinical signs High fever, nervous signs, diarrhea and death if acute; may be mild and inapparent
Clinical pathology Non-specific
Diagnostic confirmation Rickettsial colonies in capillary endothelium (brain preparations)
Lesions Ascites, hydrothorax, hydropericardium and severe pulmonary edema
Differential diagnosis list Anthrax, rabies, cerebral babesiosis, cerebral theileriosis, meningitis or encephalitis
Treatment Short- and long-acting tetracyclines
Control Vaccination based on infection and treatment methods, tick control and chemoprophylaxis
Ehrlichia (Cowdria) ruminantium is a Gram-negative, intracellular rickettsial organism in the tribe Ehrlichiae. It occurs in colonies or morulae with a predilection for the vascular endothelium and stains blue with Giemsa stain. The organism is coccoid, 0.2–0.5 microns in diameter. It can now be cultivated in vitro1 and it can also grow in mice. Cyclical development is believed to take place in intestinal and salivary epithelia of ticks. Although strain differences exist, all isolates possess a major antigenic protein 1 (MAP1) that is used for diagnosis. However, the antigen cross-reacts with other Ehrlichia spp., including Ehrlichia equi, the cause of equine granulocytic ehrlichiosis.
Heartwater is limited in its occurrence to sub-Saharan Africa, Madagascar and three Caribbean islands of Guadeloupe, Marie-Galante, and Antigua. It is one of the main causes of death in imported breeds of cattle, sheep and goats in endemic areas. Heartwater has been diagnosed recently in the island of Mayotte in the Indian Ocean.2
In endemic areas, morbidity and mortality rates are low, but the percentage of sera positive titers for heartwater could be as high as 100% in adults,3 depending on the abundance of tick vectors. Case mortality can be as high as 100% in peracute cases in sheep and goats and as low as 0–10% in cattle. The disease is less severe in indigenous breeds and related game animals reared in enzootic areas, some of which may become symptomless carriers. The N’Dama breed in West Africa is said to be well adapted to heartwater, partly because it can resist tick burdens under traditional farming system.4
Heartwater is transmitted by many ticks of the Amblyomma genus, especially A. variegatum (the tropical bont tick) and A. habraeum, and their geographic distribution is spreading. A single infected tick can transmit the disease to cattle. In the Caribbean, cattle egrets are suspected to spread A. variegatum between islands. Consequently, heartwater is considered a threat to the American mainland where potential vectors are present but do not harbor the disease or where the vector may be introduced and become established. Infection in ticks is transmitted trans-stadially and possibly transovarially. Vertical transmission to calves in colostral milk has also been reported.5 Several wild ruminants can be infected and become subclinical carriers and reservoirs. Ticks feeding on them can transmit the disease to domestic ruminants.6 The organism does not infect humans.
Animals at greatest risk are exotics imported into endemic areas and at times when the vector population is high, usually during the rains. Angora goats are also highly susceptible and therefore difficult to immunize by the current method of infection and treatment. Cattle and sheep recovering from the disease are immune for 6 months to 4 years but may be carriers for 8 months or longer. An age-dependent resistance has long been recognized and young animals were believed to have innate resistance. This was later shown to be due to low-grade infection of the young in colostral cells.7
Heartwater is the most important rickettsial infection of ruminants in Africa and the second most important tick-borne disease after East Coast fever. In southern Africa, it is regarded as the most important disease of ruminants. In general, heartwater is a more serious problem where A. habraeum is the vector.8 In countries or regions where there is endemic stability, losses from heartwater are minimal until new animals are introduced. On the other hand, since most losses are in exotic animals, heartwater is a major constraint to livestock improvement in sub-Saharan Africa. Furthermore, it has the potential to spread from the Caribbean to the American mainland.
The rickettsial organisms are introduced into the host in the saliva of an infected tick. They multiply in reticulo-endothelial cells of the local lymph node, rupture the cells and are released into the circulation from where they invade endothelial cells of blood vessels in all organs. Organisms can be found in phagosomes of circulating neutrophils,9,10 but are more abundant in endothelial cells. They cause increased vascular permeability, leading to edema especially in the lungs, body cavities and the brain, by mechanisms that are not understood, since infected endothelial cells show minimal cytopathic effects. In goats, renal ischemia and nephrosis have been described and irreversible kidney damage may be the cause of death in such cases.11
The incubation period is 1–3 weeks after transmission in tick saliva. Depending on the susceptibility of individual animals and the virulence of the infecting organism, the resulting disease may be peracute, acute, subacute or mild and inapparent. Peracute cases show only high fever, prostration and death with terminal convulsions in 1–2 days. Acute cases are more common and have a course of about 6 days. A sudden febrile reaction is followed by inappetence, listlessness and rapid breathing followed by the classical nervous syndrome which is characteristic of heartwater. It comprises ataxia, chewing movements, twitching of the eyelids, circling, aggression, apparent blindness, recumbency, convulsions and death. Profuse, fetid diarrhea is frequent. Subacute cases are less severe but may terminate in death in 2 weeks or the animal may gradually recover. The mild form is often subclinical and is seen mainly in indigenous animals and wild ruminants with high natural or induced resistance. The case mortality rate in peracute cases is 100%, in acute cases 50–90% and in calves below 4 weeks of age it is 5–10%; most animals recover in mild cases.
Hematological changes in heartwater are not specific but there may be thrombocytopenia, neutropenia, eosinopenia and lymphocytosis. Confirmatory diagnosis is based on identifying the rickettsia in capillary endothelial cells using a Giemsa-stained squash preparation of brain tissue at postmortem. The rickettsiae occur as blue to reddish-purple colonies or morulae of five to several hundred coccoid organisms (0.2–0.5 microns in diameter) in the cytoplasm of the cells. An immunohistochemical staining technique has also been described.12 Injection of blood into sheep may also be used as a diagnostic procedure. The available serological test is an indirect fluorescent antibody test used for surveys but the close antigenic relationship with other Ehrlichia spp. often leads to false positives. An ELISA based on recombinant MAP1 protein of C. ruminantium was reported to be more sensitive.13 In general, clinical detection of heartwater is not always easy because all serological assays so far available have poor sensitivity or specificity.14 A polymerase chain reaction assay has therefore been suggested as the method of choice for detection of E. ruminantium infection.
Standard lesions are ascites, hydrothorax and hydropericardium. Pulmonary edema is often severe, accompanied by copious froth in the tracheobronchial airways. There may be subserosal hemorrhages in most cavities. Lymph nodes are swollen and wet and the spleen is markedly enlarged. In goats with nephrosis, the kidneys will be soft. Although hemorrhages have been described in the brain, it often has no remarkable gross lesions but microscopically, there is perivascular mononuclear infiltration and edema along with presence of rickettsial colonies in capillary endothelial cells. Foci of malacia may be present. Tissues for histopathology should include brain, lungs, lymph nodes, spleen and kidneys.
• In endemic areas, heartwater should be suspected in susceptible animals infected with Amblyomma and having a fever of unknown origin, especially when accompanied by nervous signs. The clinical and pathological findings are not specific and the diagnosis must be based on detection of rickettsial organisms.
• The peracute form should be differentiated from anthrax and the acute form from rabies, sporadic bovine encephalomyelitis, tetanus, cerebral forms of theileriosis, babesiosis, trypanosomosis, meningitis, listeric or other encephalitis, hypomagnesemia and poisoning with strychnine, lead and organophosphates. Appropriate laboratory tests are utilized to eliminate these differentials.
Field cases of heartwater are difficult to treat successfully because available drugs are effective only in early febrile stages before neurological signs develop. In the early stages, short-acting tetracyclines at 10–20 mg/kg BW and long-acting forms at reduced doses are effective. Sulphonamides can also be used in the early stages but are less effective. Hyperimmune serum is said to be of no curative value. Supportive therapy to reduce either the pulmonary edema or the neurologic signs or to stabilize membranes in general are being investigated but with little success.
Chemoprophylaxis involves administration of tetracyclines or subcutaneous implantation of doxycycline in susceptible animals when they are introduced into an endemic area. Results are not always predictable.
Past efforts to control heartwater were based on intensive acaricide treatment in endemic areas. It involved frequent use of acaricides (plunge dipping) up to 52 times a year. This has now been shown to be environmentally unfriendly, economically unsustainable, and would invariably lead to animals that remained always susceptible.15 For example, it was observed in Zimbabwe that large farms applying acaricides very frequently (more than 30 times per annum) had higher morbidity and mortality than farms applying acaricides less frequently.16 What is advocated today is integrated control based on the establishment of endemic stability by vaccination or natural challenge. Vaccination is based on infection and treatment regimen that was first developed more than 50 years ago.17 It involves an intravenous injection of virulent organisms in cryopreserved sheep blood, followed by treatment with tetracyclines at the first indication of fever. The exposure of calves and lambs up to 3 weeks of age, without treatment, is considered optimal for the development of resistance but kids may still be susceptible. Vaccination may lead to some deaths, the immunity may wane in absence of reinfection, and animals may become carriers.1,18 More recently, cattle were successfully immunized for up to 10 months with a killed vaccine from a lysate of E. ruminantium formulated in Freund’s adjuvant.19 In another study, the use of inactivated vaccines from cell-cultured E. ruminantium combined with an adjuvant led to a reduction in mortality from heartwater in cattle, sheep and goats exposed to field challenges in Botswana, Zambia, Zimbabwe, and South Africa.20 Experimental studies using DNA recombinant vaccines so far have met with only limited success.17,21
For tick control, flumenthrin 1% pour-on at 45 days interval was found to provide effective protection of Friesian/Zebu crossbred cattle against important ticks, but it must be applied correctly at the recommended dose.22 Pure Zebu and N’Dama cattle would probably require less frequent applications. Flumenthrin pour-on is gradually replacing plunge dipping for the control of ticks and tickborne diseases in general. Other than routine surveillance, there are no special biosecurity concerns with heartwater, since transmission requires presence of the vector.
Bigalke RD. Heartwater: past present and future. Onderstepoort J Vet Res. 1987;54:163-546.
Scott GR. Cowdriosis. In: Sewell MMH, Brocklesby DW, editors. Handbook on animal diseases in the tropics. 4th edn. London: Baillière Tindall; 1990:234-237.
Bezuidenhout JD, et al. Heartwater. Coetzer JAW, Thomson GR, Tustin RC, editors. Infectious diseases of livestock with special reference to Southern Africa, vol 1. Cape Town: Oxford University Press. 1994:351-370.
OIE Manual of diagnostic tests and vaccines for terrestrial animals. http://www.oie.int/eng/normes/mmanual/A_00046.htm. Chapter 2.27
1 Yunker CE. Onderstepoort J Vet Res. 1996;63:159.
2 Camus E, et al. Rev Elev Med Vet Pays Trop. 1998;51:282.
3 Gueye A, et al. Rev Elev Med Vet Pays Trop. 1993;46:449.
4 Knopf L, et al. Prev Vet Med. 2002;53:21.
5 Deem SL, et al. Vet Parasitol. 1996;61:133.
6 Peter TF, et al. Trends in Parasit. 2002;18:214.
7 Deem SL, et al. Vet Parasitol. 1996;61:119.
8 Mahan SM, et al. Epidemiol Infect. 1995;115:345.
9 Brown CC, et al. Am J Vet Res. 1990;51:1476.
10 Logan LL, et al. Onderstepoort J Vet Res. 1987;54:197.
11 Gueye A, et al. Rev Elev Med Vet Pays Trop. 1984;37:268.
12 Jardine JE, et al. Onderstepoort J Vet Res. 1995;62:277.
13 van-Vliet AH, et al. Ann NY Acad Sci. 1996;791:35.
14 Simbi BH, et al. Onderstepoort J Vet Res. 2003;70:231.
15 Meltzer MI, et al. Trop Anim Health Prod. 1995;27:129.
16 Chamboko T, et al. Prev Vet Med. 1999;39:191.
17 Collins NE, et al. Vaccines for OIE list A and emerging diseases. Proceedings of a symposium, Ames, Iowa, USA, September 2002. 2003; p. 121.
18 Andrew HR, Norval RA. Vet Parasitol. 1989;34:261.
19 Totte P, et al. Infect Immun. 1997;65:236.
20 Mahan SM, et al. Vet Parasit. 2001;97:295.
Anaplasma phagocytophila causes disease of horses, humans, dogs, cattle, cats, and other mammalian species, that is characterized in horses by fever, depression, limb edema, icterus and ataxia. The disease is described here with emphasis on that occurring in horses. See ‘Tick-bone fever’ for a description of the disease in other species.
The disease in horses is associated with the same agent that causes human granulocytic ehrlichiosis (HGE).1 The organisms Ehrlichia equi, E. phagocytophila, and the HGE agent are now classified as Anaplasma phagocytophila.1 The variety of species affected and geographical distribution of the disease suggests strains of Anaplasma phagocytophila of varying pathogenicity and host specificity. For example, while A. phagocytophila is a recognized cause of tick-borne fever in cattle, sheep, and goats in Europe, the disease has not been recognized in the United States despite the widespread occurrence of this organism in North America. A. phagocytophila infects some domestic and wild ruminants, including deer, without inducing clinical signs.1 The HGE agent causes tick-borne disease in horses identical to that associated with E. equi and horses infected with HGE are resistant to subsequent challenge with E. equi.2,3 Anaplasma phagocytophila is an obligate intracellular bacterium that replicates in cells derived from the bone marrow (granulocytes).
Anaplasma phagocytophila also causes disease in humans, dogs, domestic ruminants, and cats.1,4 As mentioned above, the geographic distribution of A. phagocytophila does not match the distribution of disease in all species, suggesting that strains of differing pathogenicity for various species occur. Indeed, strains of A. phagocytophila that cause disease in dogs and horses in southern Sweden differ slightly in their genetic composition from isolates derived from North America.5 Similarly, nucleotide sequences of strains of A. phagocytophila from the west coast of the United States differ from those of strains originating from the east coast.6
The disease in horses occurs in the Americas (the United States including California, Washington, Oregon, Minnesota, Wisconsin, and the southeastern and the northeastern states, and Brazil), France, Italy, Switzerland, Sweden, Germany, and the United Kingdom.4,7-9
The prevalence of horses with serum antibodies to E. equi (A. phagocytophila) in endemic areas of California is 10%, compared with 3% in areas where the disease is uncommon.10 On farms where the disease occurs frequently, 50% of horses have serum antibodies to E. equi (A. phagocytophila).10 Approximately 18% of horses in areas of the upper Midwest of the United State in which Ixodes sp. ticks are endemic have antibodies to E. equi (A. phagocytophila) whereas 4 % of horses in areas in which the tick does not occur are seropositive.11 A survey of 563 horses in Lazio region of Italy (near Rome), where the disease in horses occurs, revealed a seroprevalence of 0.3%.12
There is extensive evidence of exposure of dogs to A. phagocytophila and of disease consistent with granulocytic ehrlichiosis. 47% of dogs in endemic areas in California have antibodies to E. equi (A. phagocytophila) and some show clinical signs consistent with the disease.13,14
A total of 0.2% of 2725 serum samples from cattle in California had detectable antibodies to A. phagocytophila.15 Forty-three to 96% of deer and moose, respectively, in Norway are seropositive to E. equi (A. phagocytophila), indicating the widespread extent of exposure of these species to the organism.16
A. phagocytophila is maintained by infection in wild cervids (such as the white tailed deer in North Eastern United States) and small mammals such as the white footed mouse or dusky-footed wood rat.1,4 Infection of horses, dogs, and humans occurs through the bite of A. phagocytophila-infected ticks.4 E. equi (A. phagocytophila) is detectable using a polymerase chain reaction test in I. pacificus that have fed on E. equi infected horses17 and infected I. pacificus can cause granulocytic ehrlichiosis in previously unexposed horses.18
The organism is transmitted by hard ticks that are members of the Ixodes persulcatus complex, which includes Ixodes pacificus, Ixodes scapularis, and Ixodes ricinus.1,4 Transstadial, but not transovarial, transmission occurs. The tick vectors of A. phagocytophila pass through four stages in their life-cycle:4 egg, larva, nymph, and adult. Maturation from larva to nymph and from nymph to adult, and egg laying, all require the ingestion of a blood meal. As transovarial transmission of infection does not occur, larvae or uninfected nymphs become infected by feeding on an infected mammal. The engorged and infected immature tick then dismounts and matures to the next life stage away from a mammalian host. When the immature tick reaches the nymph or adult stage, it again seeks a mammalian host. Transmission of the infection from the tick to a mammal occurs through feeding of an infected nymph or adult on a susceptible host.
Horses that have not been exposed to A. phagocytophila are susceptible to infection and disease. There is a marked seasonality of the disease in California with most cases occurring in late autumn, winter, and spring.19
Anaplasma phagocytophila infects domestic ruminants in Europe where it causes tick-borne fever.20 However, cattle infected with HGE agent or A. phagocytophila isolated from horses with the disease in the United States do not develop clinical signs of disease nor do they have detectable A. phagocytophila morulae in blood, although they do seroconvert.15 This study, and the low seroprevalence of antibodies to A. phagocytophila in cattle in California indicate the organism rarely, if ever, causes disease in cattle in these geographic regions.
Infection in horses is followed by a solid immunity and recovered animals are resistant to the disease for at least 20 months although it is suggested that reinfection and disease can occur.21 Serum antibodies persist for at least 300 days after infection in some horses but decrease to low levels in most horses by 200 days after infection.21-23
As discussed above, transmission is through the bite of an infected tick. Transmission through use of blood contaminated veterinary equipment or by blood transfusion is possible, the later being used to induced disease in experimental challenges. Perinatal transmission of A. phagocytophila is reported in humans.
The case fatality rate is low, approximately 4%, and deaths of horses with uncomplicated disease are rare.19
There is no evidence that infection spreads directly from infected horses or dogs to humans. However, dogs have been suggested to be sentinel animals in that humans in areas in which dogs have a high prevalence of antibodies in serum to A. phagocytophila might be at increased risk of infection from bites of infected ticks.14
The pathogenesis of the disease is poorly understood. Following experimental infection, horses have organism detectable by PCR beginning 5 days after infection, with development of fever and depression 7–8 days after infection. Inclusions in granulocytes are detectable beginning 9 days after infection, at which time there is edema of the limbs.24 The disease in horses is associated with rapid changeover of expressed p44 genes such that there is marked antigenic variation in the major surface protein, p44, during infection in an animal.25 The rapid changeover of expression of p44 is attributed to development of specific antibody to the hypervariable region of p44.25 Infection in sheep results in immune suppression secondary to granulocytic and lymphocytic leukopenia, impaired antibody production, reduced lymphocyte response to mitogens, and a decreased oxidative burst activity of neutrophils.26 The prominent clinical sign of edema is likely related to the vasculitis that is characteristic of the disease.
The incubation period for the spontaneous disease is less than 2 weeks. Subclinical infections are believed to be common, based on the number of horses with serologic evidence of infection but no history of disease.
Clinically there is high fever of 40–42°C (104–107°F) followed by mucosal pallor, jaundice, anorexia, depression, increased respiratory movement, incoordination and reluctance to move and, after 3–4 days, edema and heat of the extremities.24 There may be petechial hemorrhages on mucosal membranes. The edema persists for 7–10 days and clinical signs resolve in 14 days. Clinical disease is more severe in horses over 3 years of age and is minor in young horses.19 Severely affected horses can have signs consistent with neurologic disease, including ataxia, defects in conscious proprioception, and recumbency.23
Arrhythmias may occur during the acute phase of the disease. Chronic infection and disease is not recognized.
Hematological examination may reveal a mild anemia and leukopenia. Thrombocytopenia is common in the acute stage of the disease. There are no consistent serum biochemical abnormalities.
Positive identification of the disease is made on the presence of inclusion bodies (morulae) in the cytoplasm of neutrophils and eosinophils. Careful and protracted microscopic examination of a blood smear, stained with Giemsa, may be necessary to identify the inclusions (morulae). The inclusions are apparent as pleomorphic, blue-gray color bodies, often in a spoke wheel formation, in the cytoplasm of granulocytes.19 The number of infected cells may be quite small and examination of a buffy coat preparation may increase the sensitivity of the test.
Diagnosis is achieved through use of a polymerase chain reaction test to identify Anaplasma phagocytophila DNA in blood samples of infected horses and by demonstration of an increase in antibody titer detected by indirect fluorescent antibody staining.10 However, antibody titers are low to undetectable in approximately 44% of horses with at the onset of clinical signs, and reach a maximum within one month of infection.23,24 An ELISA that detects antibodies against the p44 surface antigen of A. phagocytophila is suitable for use in dogs and horses.27
At necropsy there are petechiae and edema of the legs and at histological examination there is a vasculitis. There are often inflammatory lesions in the brain, heart and kidneys.
Differential diagnoses include: equine infectious anemia, which has a much more protracted course and does not respond to treatment; purpura hemorrhagica, which is often associated with infectious upper respiratory tract disease; liver disease, viral encephalitis; equine herpesvirus 1 myeloencephalopathy, rabies, botulism, and equine viral arteritis.
The specific treatment is oxytetracycline (7 mg/kg BW IV, every 24 hours) for approximately 5–7 days. Penicillin, streptomycin and chloromycetin are not effective. The response to treatment with oxytetracycline is rapid and the fever is reduced or eliminated in 12–24 hours, and signs of the disease resolve within 5–7 days in most horses. Inclusion bodies are difficult to find 24 hours after beginning treatment. Without treatment the disease is usually self-limiting to 2–3 weeks.
1 Dumler JS, et al. Int J Syst Evol Microbiol. 2001;51:2145.
2 Madigan JE, et al. J Infect Dis. 1995;172:1141.
3 Pusterla N, et al. J Vet Med B. 2002;49:484.
4 McQuiston JH, et al. J Am Vet Med Assoc. 2003;223:1750.
5 Bjoersdorff A, et al. Clin Diag Lab Immunol. 2002;9:341.
6 Chae J, et al. J Clin Microbiol. 2000;38:1364.
7 Shaw S, et al. Vet Rec. 2001;149:127.
8 Bermann F, et al. Vet Rec. 2002;150:787.
9 Magnarelli LA, et al. J Am Vet Med Assoc. 2000;217:1045.
10 Madigan JE, et al. J Am Vet Med Assoc. 1990;196:1962.
11 Bullock PM, et al. J Vet Int Med. 2000;14:252.
12 Scarpulla M, et al. Ann NY Acad Sci. 2003;990:259.
13 Magnarelli LA, et al. J Am Vet Med Assoc. 1997;211:1134.
14 Foley JE, et al. Am J Vet Res. 2001;62:1599.
15 Pusterla N, et al. J Am Vet Med Assoc. 2001;218:1160.
16 Stuen S, et al. J Wildlife Dis. 2002;38:1.
17 Barlough JE, et al. Vet Parasitol. 1996;63:319.
18 Richter PJ, et al. J Med Entomol. 1996;33:1.
19 Madigan TE, Gribble D. J Am Vet Med Assoc. 1987;190:445.
20 Woldehiwet Z. Vet Res Commun. 1983;6:163.
21 Van Andel AE, et al. J Am Vet Med Assoc. 1998;212:1910.
22 Nyindo MBA, et al. Am J Vet Res. 1978;39:15.
23 Artursson K, et al. Equine Vet J. 1999;31:473.
24 Franzen P, et al. J Vet Intern Med. 2005;19:232.
25 Wang XQ, et al. Inf Immunity. 2004;72:6852.
Etiology Neorickettsia risticii, a rickettsia. Infection occurs by ingestion of aquatic insects (caddis flies) infected by the organism
Epidemiology An infectious, but not contagious, sporadic disease of horses in North and South American and parts of Europe. Localized epidemics may occur. Disease is most common near large rivers, but can occur elsewhere
Clinical signs Fever and diarrhea with colic and laminitis in severe cases. Abortion is a sequela of clinical disease in mares
Lesions No gross lesions, except for laminitis. Histologic evidence of typhlitis and colitis
Diagnostic confirmation Demonstration of N. risticii by PCR or cultivation, in blood or feces of sick horses. More commonly the presence of a high antibody titer in horses with appropriate clinical signs is considered diagnostic
Treatment Oxytetracycline (6.6 mg/kg, IV every 24 hours), fluids, and supportive care. Prophylaxis for laminitis
The causative agent is Neorickettsia risticii, formerly named Ehrlichia risticii, a small Gram-negative coccus that is closely related to the agents of human ehrlichiosis (E. sennetsu) and salmon poisoning of dogs (Neorickettsia helminthoeca).1
The disease is infectious, but not contagious, and usually has a sporadic occurrence. Localized epidemics may occur.
Equine neorickettsiosis is recorded in the United States, Canada, Europe, Uruguay and southern Brazil.2 While it might have wider occurrence, evidence of infection based on the commonly used indirect fluorescent antibody test should be interpreted with caution because of the high rate of false positive results.3 The highest prevalence of disease is near large rivers, apparently related to the infection of horses by ingestion of infected aquatic insects, although the disease can occur elsewhere.4
Clinical disease is sporadic and seasonal with the predominance of cases occurring during the summer and the autumn periods in areas with cool to cold winters.5 In warmer areas, such as Florida and Texas, cases occur year round. The prevalence of horses in the mid-west and east coast of the United States with antibodies to N. risticii varies with geographical region, but can be as high as 86% of horses tested, although the overall rate appears to be closer to 25%.4 The prevalence of horses with serological evidence of exposure is much less in California.3 There is a marked seasonal variation in the prevalence of seropositive horses, with the highest prevalence being in the summer months (July and August) and the lowest prevalence being in the winter.6
Clinical disease is believed to be uncommon in horses under 1 year of age, although peracute disease can occur in foals,7 and there is no age difference in prevalence of disease in adult horses.5 Similarly, there is no evidence that breed and sex influence susceptibility to disease.5 The risk for disease is greater in horses housed on premises with a history of previous infection or those that have other livestock.5
The clinical attack rate varies considerably, but estimates range between 0.44 and 19 cases per year per 1000 horses at risk.5,8 During epidemics, the clinical attack rate may be as high as 20–50% of horses on affected farms.9 Case fatality rates as high as 30% were initially reported for horses with clinical disease but with the subsequent improved recognition and early treatment the case fatality rate is currently closer to 7%.
The risk of horses being seropositive in some areas is related to breed (Thoroughbreds are three times more likely to be seropositive than are non-Thoroughbreds and non-Standardbreds), sex (females are 2.7 times more likely to be exposed than are stallions and geldings), and age (increasing risk up to 12 years of age).10 Horses that have had clinical signs compatible with neorickettsiosis are more likely to be seropositive than are horses with no such history.11
The disease is infectious but not contagious.5 Horses develop infection and disease after ingestion of aquatic insects including caddis flies (Dicosmoecus gilvipes).12,13 The disease can be transmitted experimentally to horses by parenteral administration of N. risticii or blood from infected horses.14 Studies of a tick (Dermacentor variabilis), black flies (Simulium spp.), fleas, flies (Tabanus spp., Hybomitra spp., Stomoxys spp., Haematobia spp.), and mosquitoes, have failed to demonstrate transmission of infection.
The complete life cycle of Neorickettsia risticii has not been elucidated, but it is known that the organism infects trematodes stages (cercariae and xiphidiocercariae) found in freshwater snails (Juga sp. in California and Elimia sp. in Ohio).15,16 Neorickettsia risticii infects metacercariae found in adults and juveniles of aquatic insects including caddis flies (Trichoptera), mayflies (Ephemeroptera), damselflies (Odonata, Zygoptera), dragonflies (Odonata, Anisoptera), and stoneflies (Plecoptera).17 N. risticii DNA has been detected in trematodes (Lecithodendriidae) infecting bats and swallows.18,19 N. risticii DNA is present in eggs of the trematodes (Acanthatrium oregonense) found in bats demonstrating vertical (adult to egg) transmission of infection in trematodes.19 Furthermore, N. risticii DNA was detected in the blood, liver or spleen of bats infected with the trematode, suggesting that N. risticii can also be transmitted horizontally from trematode to bat.19 These results indicate that the trematode A. oregonense is a natural reservoir and probably a vector of N. risticii. This information suggests that insectivorous bats and birds are the definitive hosts of trematodes that maintain the natural reservoir of N. risticii.18 Briefly, it appears that horses are accidentally infected by N. risticii that normally cycles between trematode life stages in bats, fresh water snails, and aquatic insects.
Infected horses develop a sterile immunity and so are unlikely to be a source of subsequent infection.
Infection is followed by monocyte-associated bacteremia and the organism is present in monocytes, macrophages and the glandular epithelial cells of the intestinal tract. The number of N. risticii in blood is greatest before the development of clinical signs, which in experimentally infected horses and ponies occurs approximately 19 days after infection by ingestion of infected aquatic insects.13 The prominent clinical sign of diarrhea is due to colitis and typhlitis and is associated with an neorickettsia-induced disruption of sodium and chloride absorption by the large colon.20 Fluid and electrolyte losses associated with the diarrhea cause dehydration, hyponatremia and acidosis. Transplacental infection with N. risticii occurs and causes abortion.21
The classic manifestation of N. risticii infection in horses is fever, depression, anorexia, diarrhea, colic, and laminitis. However, infection can result in a variety of clinical abnormalities ranging from inapparent infection, through transient fever and depression, to the severe signs described above. Equine neorickettsiosis should be considered in any horse living in an endemic area that demonstrates fever and depression.
In naturally occurring cases of severe clinical disease there is typically an acute onset with depression, anorexia, tachycardia, congested mucous membranes and fever up to 107°F. There are decreased intestinal sounds on abdominal auscultation in the early stages of the syndrome and subsequently tinkling sounds prior to the onset of diarrhea which usually occurs 24–72 hours later. The severity of the diarrhea varies but it is usually profuse and projectile. It persists for up to 10 days and there may be sufficient fluid loss to result in severe and rapid dehydration and hypovolemic shock. Colic is a presenting sign in some horses and may be mild or present as an acute abdomen. Laminitis occurs in up to 40% of horses and is usually apparent within 3 days of initial signs of disease. There can also be subcutaneous edema in the ventral abdomen and limbs. Less severe clinical manifestations of infection include the occurrence of fever and anorexia without other signs, or the occurrence of mild colic or subcutaneous edema.
Abortion occurs as a result of N. risticii infection and, in experimental infections, occurs 65–111 days after infection of the dam. The dams that aborted all became clinically ill after infection, but clinical signs of disease had resolved at the time they aborted.21 Abortion was presaged by ventral edema and enlargement of the udder, and placenta was retained in some cases.21
Hematological examination usually reveals leukopenia (<5000 leukocytes per μl) with neutropenia and a marked left shift, mild thrombocytopenia, and hemoconcentration (hematocrit 50–60%, 0.5–0.6 L/L). Serum biochemical analysis often reveals hyponatremia, hyperkalemia, hypochloremia, metabolic acidosis and azotemia. Peritoneal fluid is usually normal.
Diagnostic confirmation is achieved by demonstration of N. risticii in blood or feces, or serological evidence of infection, in horses with clinical signs compatible with the disease. Routine diagnosis is based on demonstration of a high serum antibody titer on the indirect immunofluorescent antibody (IFA) test.11 Most horses with disease due to N. risticii have titers ≤1:80 at the onset of clinical signs while horses with titers ≥1:40 probably do not have the disease.11 The presence of a high titer at the time of onset of clinical signs is a result of the 8–12 day incubation period during which there is a high level neorickettsemia and the production of a strong IgM antibody response.22 The IgM antibody response wanes rapidly and may be undetectable by 60 days after infection, although a prominent IgG response occurs.22 Therefore, by the time clinical signs are apparent the horse has a high titer that might decline, making the use of acute and convalescent (2 weeks after clinical signs resolve) serum titers potentially misleading. A rising titer in samples collected several days apart soon after the onset of disease is indicative of the disease, but a declining titer does not rule it out. The IFA test performed in some laboratories has a high rate of false-positive reactions.3
Detection of the organism in white blood cells by microscopic examination of stained blood smears is usually not possible because of the low level of infection of blood monocytes. The organism can be cultivated but this is time consuming and expensive. However, a test that detects the presence of N. risticii nucleic acid by the polymerase chain reaction has a sensitivity similar to that of blood culture in experimental infection.23 Similarly, N. risticii can be detected by a polymerase chain reaction test in feces of horses with disease.24
The gross changes in horses dying of EME usually include subcutaneous edema of the ventral body wall and a very fluid consistency to the contents of the large bowel. Congestion, hemorrhage and mucosal erosions can occur throughout the alimentary tract but are concentrated in the cecum and colon. The mesenteric lymph nodes are often swollen and edematous. There may be lesions of laminitis. Histologic examination confirms the alimentary mucosal erosion and ulceration, which is accompanied by an infiltrate of a mixed population of leukocytes within the lamina propria and submucosa. The causative organisms can be demonstrated in tissue sections using Steiner’s silver stain. Detection using electron microscopic or PCR techniques are other options.
Fetuses that are aborted as a result of N. risticii infection of the dam have histologic evidence of enterocolitis, periportal hepatitis and lymphoid hyperplasia with necrosis of mesenteric lymph nodes.21
The parasite can be demonstrated in cecum, colon, and mesenteric lymph node by a polymerase reaction test or electron microscopy. Formalin-fixed tissue for light microscopy should include cecum, colon, liver and mesenteric lymph node.
The main differentials are as follows (Table 5.14):
The specific treatment of equine neorickettsiosis is oxytetracycline (6.6 mg/kg BW IV every 24 hours for 5 days) and horses treated early in the disease respond well. Given the effectiveness of oxytetracycline in the treatment of the disease and the lack of clear evidence that oxytetracycline at the recommended dose induces or exacerbates diarrhea, this drug should be administered to all horses that live in an endemic area and that develop signs consistent with equine neorickettsiosis. Treatment does not interfere with the development of immunity.25 Other antibiotics that have been used include combinations of a sulfonamide and trimethoprim or rifampin and erythromycin. Doxycycline has been used, but intravenous administration is associated with cardiovascular abnormalities and sudden death.26
Treatment of horses with acute diarrhea is discussed under Acute diarrheas of horses. Prophylaxis for laminitis may be useful.
Control centers on vaccination although it is of unproved efficacy in field situations. The apparent lack of efficacy of vaccination in the United States might be due to the inclusion of only one strain of N. risticii in the vaccine. There is evidence of the presence of a number of strains of the organism and the vaccine might not confer immunity to all these strains.
Infection is followed by the development of a neutralizing antibody response that is associated with clearance of N. risticii and the presence of a sterile immunity which persists for at least 20 months.27,28 An inactivated whole cell adjuvanted vaccine is available and vaccinated animals have resistance to experimental challenge.25 However, protection from vaccination is not complete and wanes within 6 months. In an area with a low attack rate of the disease (0.44–1.7 horses/1000 per year) the risk of neorickettsiosi in horses vaccinated once per year is almost identical (odds ratio = 0.93) to that of unvaccinated horses, and there is no difference in the severity of the disease in vaccinated and unvaccinated horses.29 Furthermore, it is more economical not to vaccinated horses in areas with a low attack rate.30 In areas with a high attack rate it may be appropriate to provide an initial vaccination of two doses 3 weeks apart with revaccination at 4-month intervals during the disease season.25
1 Dumler JS, et al. Int J Syst Evol Microbiol. 2001;51:2145.
2 Dutra F, et al. J Vet Diag Invest. 2001;13:433.
3 Madigan JE, et al. J Am Vet Med Assoc. 1995;207:1448.
4 Atwill ER, et al. Equine Vet J. 1994;26:143.
5 Goetz TE, et al. Am J Vet Res. 1989;50:1936.
6 John GA. Equine Vet Sci. 1989;9:250.
7 Perry BD, et al. Prev Vet Med. 1986;4:69.
8 Atwill ER, et al. Prev Vet Med. 1995;23:41.
9 Palmer JE, et al. J Am Vet Med Assoc. 1986;189:197.
10 Atwill ER, et al. Am J Vet Res. 1992;53:1931.
11 Farrar WP, et al. Vet Microbiol. 1993;34:345.
12 Madigan JE, et al. Equine Vet J. 2000;32:275.
13 Mott J, et al. J Clin Microbiol. 2002;40:690.
14 Pusterla N, et al. J Clin Microbiol. 2000;38:1293.
15 Barlough JE, et al. Vet Parasitol. 1997;68:367.
16 Kanter MJ, et al. J Clin Microbiol. 2000;38:3349.
17 Chae J, et al. J Med Entomol. 2000;37:619.
18 Pusterla N, et al. J Helminthol. 2003;77:335.
19 Gibson KE, et al. Environ Microbiol. 2005;7:203.
20 Rikihisa Y, et al. Res Vet Sci. 1992;52:353.
21 Long MT, et al. Am J Vet Res. 1995;56:1307.
22 Pretzman C, et al. J Clin Microbiol. 1987;25:31.
23 Mott J, et al. J Clin Microbiol. 1997;35:2215.
24 Biswas B, et al. J Clin Microbiol. 1994;32:2147.
25 Palmer JE. Cornell Vet. 1989;79:201.
26 Riond JL, et al. Equine Vet J. 1992;24:41.
27 Palmer JE, et al. Am J Vet Res. 1990;51:763.
28 Rikihisa Y, et al. Vet Microbiol. 1993;36:139.
Epidemiology High seroprevalence in ruminants. Latent infection with recrudescence and excretion at parturition. Infection by direct contact and inhalation. Important zoonotic disease
Clinical findings Infection in ruminants common but clinical disease is rare and presents mainly as abortion in sheep
Necropsy findings Placentitis. Organisms demonstrable in trophoblast cells by fluorescent antibody
Diagnostic confirmation Fluorescent antibody staining and PCR. Serology for herd infection status
Control Isolation of aborting ruminants. Vaccination possible
Q fever is a zoonosis associated with Coxiella burnetii which is an obligate intracellular parasite classified within the family Rickettsiaceae and which can be divided into six genomic groups on the basis of restriction fragment length polymorphism. Unlike other Rickettsiae members C. burnetii it is quite resistant to environmental influences and is not dependant upon arthropod vectors for transmission. C. burnetii displays two antigenic phases, phase 1 and phase 11. Phase 1 organisms are more infectious.
The organism has worldwide distribution although a large serological survey argues that it is not present in New Zealand.1
C. burnetii cycles in a wide variety of wildlife species and their ectoparasites. The infection also cycles in domestic animals; cattle, sheep and goats are the main livestock reservoirs of infection for humans.2,3 Rates of infection in farm animals vary considerably between locations, between countries and with time as there appears to be cycles of infection within regions.
In cattle, prevalence figures range from 6–82% of cattle and 23–96% of herds seropositive depending upon location and country.4-7 Seropositivity rates in sheep and goats are similar but also vary according to year and region.8-11 There is little information on management or other factors that might influence this variation in prevalence but one study found a significantly higher prevalence in housed cattle compared to cattle at pasture kept at pasture.7
Infection and transmission is by direct contact and by inhalation. Infection of non-pregnant animals is clinically silent and is followed by latent infection until pregnancy when there is recrudescence with infection in the intestine, uterus, placenta and udder and excretion from these sites at parturition. The organism is present in high concentration in the placenta and fetal fluids, and subsequent vaginal fluids, is also excreted in urine and is present in the feces of sheep from 11–18 days post partum.2,12 Infection can result in abortion, stillbirths or poorly viable lambs but commonly the neonates of infected, excreting, ewes are born clinically normal. Abortion usually does not occur at successive pregnancies but there can be recrudescence of infection and excretion at these pregnancies, especially the one immediately following.13,14
Goats also excrete the organism in vaginal discharges for up to 2 weeks, and it is present in goat milk for up to 52 days after kidding and also in feces.15 Maximum shedding in cattle also occurs at parturition and for the following two weeks but cattle excrete the organism in the milk for at least several months and up to 2 years2 and infection is common in bulk tank milk.
As with sheep, infection in goats can be accompanied by abortion but abortion in cattle is rare although it is recorded.9,16
There is a significant contamination of the environment of infected animals at the time of parturition and this is probably a major period for transmission of the disease within herds and flocks.17
The organism is present in the semen of seropositive bulls and venereal transmission is suspected.18
C. burnetii is very resistant to physical and chemical influences and can survive in the environment and soil for several months. It can resist common chemical disinfectants but is susceptible to sodium hypochlorite, 1:100 lysol solution and formalin fumigation provided a high humidity is maintained.19
There is strain variation in the organism and differences in plasmid sequence types have been correlated with differences in the type of disease occurring in humans. The organism is highly infectious and it is estimated that the infective dose for humans approximates one organism.20
In humans infection is primarily by inhalation. Sources of infection include such diverse materials as soil, air-borne dust, wool, bedding and other materials contaminated by urine, feces or birth products of animals. The potential for human infection from these sources is substantial; for example, ovine manure used as a garden fertilizer has been incriminated as a source.21
Sheep have traditionally been incriminated as the major reservoir of infection for humans but the trend for urban populations to locate in close proximity to large dairy herds suggests that cattle could become an increasingly significant reservoir.
The organism is present in the milk of infected cattle, sheep and goats. A significant proportion of seropositive cattle excrete the organism in milk and periods and duration of excretion are variable but may persist at least 2 years.2 C. burnetii is destroyed by pasteurization but there is a risk for the farm family that consumes raw milk and a particular concern for the occurrence of the organism in raw sheep and goats’ milk.
Rates of seropositivity in humans vary markedly between surveys but there is a higher rate of seropositivity in people (farm workers, veterinarians, livestock dealers, dairy plant and slaughter house workers, shearers, etc.) that are associated with domestic animals and their products and with farm environments.8,22-24 Several incidents of infection in humans have been linked to exposure to parturient sheep and goats.2,8,10,25
The infection in humans is also commonly asymptomatic but can result in disease characterized by fever, general malaise, headache, and less commonly, pneumonitis, hepatitis and meningoencephalitis. Endocarditis and hepatitis are manifestations of chronic disease. Immunocompromised persons are at greater risk for disease. There is a concern that the prevalence of infection in farm animals is increasing and spreading geographically and that there is a subsequent greater risk for infection in humans.10,14,26
C. burnetii is considered a potential agent for bioterrorism because of its survival in the environment, the ease with which it can transmit by aerosol and windborne means and the very low infectious dose.
Infection of ruminants can occur at any age and is usually clinically inapparent. In the experimental disease in cattle, anorexia is the only consistent clinical finding. C. burnetii is a cause of abortion in sheep and goats. Abortion occurs during the latter part of the lambing period in the flock and in the latter period of pregnancy in individual ewes. The dam shows no signs of impending abortion. In cattle the organism is a rare cause of abortion. Correlations between herd level seroprevalence and herd fertility are equivocal.5,6,27
There are a number of serological tests available including complement fixation, micro agglutination, ELISA and indirect immunofluorescence.28
The immunofluorescence assay is used as the sero-reference test for the serodiagnosis of Q fever. It can detect antibody to phase variants and can provide epidemiological information as phase 11 antibody is associated with recent and acute infections and phase 11 antibody with chronic infections.14
There are seldom gross lesions in aborted fetuses, but foci of necrosis and inflammation are occasionally seen in the liver, lung, and kidney microscopically. The placenta from aborting animals is usually thickened and a purulent exudate or large, red-brown foci of necrosis are typically seen in the thickened intercotyledonary areas. Microscopically, large numbers of necrotic neutrophils are usually visible on the chorionic surface and swollen trophoblasts filled with the organisms can also be found in well-preserved specimens. Examination of placental impression smears stained with Gimenez, Koster’s, or other appropriate techniques provides a means of rapid diagnosis. However, care must be taken to avoid confusing Coxiella-infected trophoblasts with cells containing Chlamydophila organisms. Coxiellosis can be confirmed fluorescent antibody staining of fresh tissue or immunohistochemical staining of formalin-fixed samples. In most laboratories, culture is not attempted due to the zoonotic potential of this agent.
• Bacteriology – chilled placenta (CYTO, FAT)
• Histology – fixed placenta, lung, liver, kidney (LM, IHC).
Note the zoonotic potential of this agent when handling carcasses and submitting specimens.
• The diagnosis of the disease in farm animals, other than abortion, suspected as associated with this agent is difficult and relies upon the detection of the organism
• A positive serological test in an animal or herd is indicative of infection at some time but does not indicate an association with the problem at hand
• PCR or PCR-ELISA has been used for detection of the organism in milk.29
Aborting animals should be isolated for 3 weeks and aborted and placental-contaminated material burnt. Ideally, manure should be composted for 6 months before application to fields. Feed areas should be raised to keep them free from contamination with feces and urine.
While Q fever has significant implications for human health, it is not significantly important enough to have generated national or regional control strategies based on control in the animal population.
Milk and milk products should be pasteurized. Veterinarians dealing with herds that provide raw milk should insure that these herds are seronegative for C. burnetii.
Vaccine trials with killed vaccines in animals show a good and persistent antibody response and suggest that vaccination can limit the excretion of the organism.30 However, there is little economic incentive for a vaccination program involving livestock and livestock vaccines are not available in most countries.
Vaccination of humans has reduced infection rates in high risk groups and has been used in the appropriate circumstances in Australia.31 As defined by recognized infections, personnel in research facilities that contain sheep are at greatest risk for infection and management guidelines for the control of ‘Q’ fever in research facilities containing agricultural animals are available.32
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Marrie TJ. Q fever: A review. Can Vet J. 1990;31:555-563.
Marrie TJ. Q Fever. Boca Raton, Florida: CRC Press, 1990.
Woldehiwet Z. Q fever (coxiellosis): epidemiology and pathogenesis. Res Vet Sci. 2004;77:93-100.
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Bovine petechial fever is associated with Ehrlichia ondiri and occurs in Kenya, and possibly Tanzania, in cattle grazing thick scrub land or indigenous forest areas at 1500–3000 m altitudes. Characteristically, disease occurs in cattle that break out from fenced pastures and graze the adjacent forest or bushland areas, or when they are grazed on these areas at the end of the dry season.1
Bovine petechial fever occurs in cattle that have been recently introduced to these areas and indigenous cattle appear to acquire resistance. Epidemics occur in cattle imported to infected areas, last 1–2 months to involve 60–80% of the group with significant losses. Dairy cattle have significant drop in milk yield which persists for several weeks.1
Infection can be experimentally transmitted to cattle, sheep, goats, wildebeest and impala but natural disease is seen only in cattle.1,2 Bushbuck (Tragelaphus scriptus) are suspected as the reservoir for infection but the vector is not known although epidemiological findings suggest a tick vector of restricted distribution.3
The disease in cattle is characterized by high fever and the occurrence of petechial hemorrhages in mucous membranes for periods up to 10 days; epistaxis, melena, and hyphema occur in more severely affected animals. Pregnant animals may abort and there is a fall in milk production in lactating animals. Anemia may be severe enough to result in death 3–4 weeks after infection.1 There is a profound lymphocytopenia by the second day of infection followed by leukopenia and thrombocytopenia. The organism is demonstrable in granulocytes and monocytes during the febrile period but cannot be cultured.
Tetracyclines are effective in treatment. Recovered animals may be latently infected and are immune to reinfection for at least 2 years.