Etiology and Epidemiology

Babesiosis in dogs is most commonly associated with Babesia canis and B. gibsoni, protozoans that parasitize red blood cells (RBCs), leading to progressive anemia. B. canis has worldwide distribution including Africa, Asia, Australia, Europe, Central America, South America, Japan, and the United States. Three subspecies of B. canis have been proposed to be separate species. B. canis rossi is transmitted by Haemaphysalis leachi and is the most pathogenic; B. canis canis is transmitted by Dermacentor reticulatus and is moderately pathogenic; B. canis vogeli is the least pathogenic and is transmitted by Rhipicephalus sanguineus (brown dog tick). Babesia canis vogeli is the most common B. canis subspecies infecting dogs in the United States. B. gibsoni infects dogs in the United States, Japan, Sri Lanka, Korea, Malaysia, northern and eastern Africa, Australia, and southern Europe. B. gibsoni strains, of which there are at least three (Asia, California, and Theileria annae, a B. gibsoni-like organism common in dogs in northern Spain), vary genetically (Garcia 2006). Rhipicephalus sanguineus is a proposed vector for B. gibsoni in the United States. Presence of B. gibsoni DNA in dog blood has been associated with a history of a dog bite, especially by an American Pit Bull Terrier, suggesting that fighting is a route of transmission. Babesia spp. infections were detected in 29 states and Ontario (Birkenheuer et al., 2005). Other novel Babesia spp. that genetically vary considerably from other B. canis or B. gibsoni isolates have been described in the United States; however, the prevalence rates for these infections is unknown (Kocan et al., 2001; Meinkoth et al., 2002; Birkenheuer et al. 2004a). None of the Babesia spp. that infect cats (B. cati [India], B. felis [Africa, southern Asia, Europe], B. herpailuri [South America, Africa], B. canis presentii [Israel]) have been recognized in the United States. Babesia spp. can also be transmitted by blood transfusions.

After infection with pathogenic strains of B. canis or B. gibsoni, the incubation period varies from several days to several weeks. The degree of parasitemia varies by the organism studied but can be detected transiently in some dogs as soon as day 1 (Boozer & Macintire, 2003). The organisms replicate intracellularly in RBCs, resulting in intravascular or extravascular hemolytic anemia. Immune-mediated reactions against the parasites or altered self-antigens worsen the hemolytic anemia and commonly result in a positive direct Coombs test. Activation of macrophages leads to fever and hepatosplenomegaly. Severe hypoxia occurs because of rapid breakdown of RBCs. Disseminated intravascular coagulation occurs in some infected dogs during acute infection. The severity of disease depends on the species and strain of Babesia and the host’s immune status; chronic, subclinical infection can be common with some. Administration of glucocorticoids or splenectomy may activate chronic disease. Presence of coinfections, such as Bartonella spp., may increase the pathogenic potential (Kordick et al., 1999; Tuttle et al., 2003).

Clinical Features

In the United States, subclinical Babesia spp. infections are most common. Peracute or acute Babesia spp. infections result in anemia and fever, leading to pale mucous membranes, tachycardia, tachypnea, depression, anorexia, and weakness. Icterus, petechiae, and hepatosplenomegaly are present in some dogs depending on the stage of infection and the presence of disseminated intravascular coagulation. Severe anemia, disseminated intravascular coagulation, metabolic acidosis, and renal disease are most common during acute infection and are generally most severe with B. canis rossi infections in South Africa. The main differential diagnosis for acute babesiosis is primary immune-mediated hemolytic anemia. Chronically infected dogs commonly have weight loss and anorexia. Ascites, gastrointestinal signs, CNS disease, edema, and clinical evidence of cardiopulmonary disease occur in some dogs with atypical infection.

Diagnosis

Regenerative anemia, hyperbilirubinemia, bilirubinuria, hemoglobinuria, thrombocytopenia, metabolic acidosis, azotemia, polyclonal gammopathy, and renal casts are common in dogs infected with pathogenic Babesia spp. Presence of the organism in RBCs detected by Wright’s or Giemsa stains on thin blood smears (see Chapter 92) can be used to support the diagnosis, but parasitemia can be intermittent, giving falsely negative results; capillary blood is the preferred source for blood smear evaluation. B. canis is typically found as paired, piriform bodies measuring 2.4 × 5.0 μm. B. gibsoni is typically found as single or paired annular bodies measuring 1.0 × 3.2 μm. Serologic and polymerase chain reaction (PCR) assays are also available to help aid in the diagnosis. Indirect fluorescent antibody tests for B. canis and B. gibsoni are available commercially. However, serologic cross-reactivity can exist between B. canis and B. gibsoni, so antibody test results cannot be used to determine the infective species definitively. Demonstration of increasing titers over 2 to 3 weeks is consistent with recent or active infection. No standardization between laboratories exists, so suggested positive cutoff titers vary. False-negative serologic test results can occur in some dogs, particularly those with peracute disease or concurrent immunosuppression. A titer above 1 : 320 is suggested as diagnostic for B. gibsoni, but not all infected dogs achieve this titer magnitude (Birkenheuer et al., 1999). Many dogs are seropositive but clinically normal, so serology alone cannot be used to make a definitive diagnosis of clinical babesiosis. Positive results in PCR assays performed on blood prove current infection but positive results do not always correlate with clinical illness.

Treatment

Supportive care, including blood transfusions, sodium bicarbonate therapy for acidosis, and fluid therapy, should be administered as indicated. A number of drugs, including diminazene aceturate, phenamidine, pentamidine isethionate, parvaquone, atovaquone, and niridazole, have also been used in an attempt to treat different Babesia spp. infections. In the United States, if clinical disease associated with B. canis is suspected, imidocarb diproprionate may be effective when administered (5 to 6.6 mg/kg SC or intramuscularly [IM]) twice, 14 days apart or (7.5 mg/kg, SC or IM) once. Adverse effects include transient salivation, diarrhea, dyspnea, lacrimation, and depression. Imidocarb is not as effective for the treatment of B. gibsoni infection. In the United States, if clinical disease associated with B. gibsoni is suspected, azithromycin (10 mg/kg PO q24h for a minimum of 10 days) or clindamycin hydrochloride (12.5 mg/kg PO q12h for at least 10 days) may lessen clinical disease if other drugs are not available. Azithromycin (as described) and atovaquone (13.3 mg/kg PO q8h for at least 10 days) is currently recommended for the treatment B. gibsoni infections, but this combination does not always result in elimination of infection (Birkenheuer et al., 2004b; Jefferies et al., 2007). Because no drugs are known to eliminate infection consistently, treatment of healthy, seropositive dogs is unlikely to be of benefit.

Zoonotic Aspects and Prevention

No evidence currently exists to suggest that Babesia spp. infecting dogs and cats can cause human disease. However, some Babesia spp. infections of people are genetically similar to those infecting dogs, and the organism should be considered important vector-borne diseases of people. Ticks should be controlled if possible. If controlling ticks is difficult in a B. canis–infected kennel, one dose of imidocarb at 7.5 mg/kg IM may eliminate the carrier state. Minimal cross-protection exists between species; a dog that has recovered from babesiosis may still become ill if infected with another species. Thus tick control must be maintained in endemic areas. Administration of immunosuppressive drugs and splenectomy should be avoided in previously infected dogs. Dogs used as blood donors should be assessed for infection by PCR or serologic screening and positive dogs excluded from the program. Dog bites should be avoided. A B. canis vaccine is available in Europe. For blood donor programs, high-risk breeds (Greyhound, American Pit Bull Terrier) or dogs from endemic areas should be screened for Babesia spp. infection by serology or PCR assays, and positive dogs should be excluded from the program (Wardrop et al., 2005).

Etiology and Epidemiology

Cytauxzoon felis is a protozoal disease of cats in the southeastern, mid-Atlantic, and south-central United States that is usually fatal. Large-scale prevalence rates have not been performed, but one study of 961 cats in Florida, North Carolina, and Tennessee showed a prevalence rate of 0.3% (Haber et al., 2007). Isolates from domestic cats have been genetically similar between studies (Birkenheuer et al., 2006b). Bobcats are usually subclinically affected and may therefore be the natural host of the organism. The organism can be passed experimentally from infected bobcats to domestic cats by Dermacentor variabilis (American dog tick); clinical illness occurs after an incubation period of 5 to 20 days. After infection, schizonts and macroschizonts form in mononuclear phagocytes. The infected macrophages line the lumen of veins throughout the body. Merozoites released from the infected macrophages infect erythrocytes. Clinical disease results from obstruction of blood flow through tissues by the mononuclear infiltrates and from hemolytic anemia. Domestic cats occasionally survive infection, suggesting that variants that are less virulent to cats also exist (Walker et al., 1995; Meinkoth et al., 2000; Haber et al., 2007).

Clinical Features

Most cases of cytauxzoonosis are in cats allowed to go outdoors. Fever, anorexia, dyspnea, depression, icterus, pale mucous membranes, and death are the most common clinical findings. A primary differential diagnosis is mycoplasmosis. Ticks are rarely identified on affected cats.

Diagnosis

Regenerative anemia, pancytopenia, and neutrophilic leukocytosis are the most common hematologic findings; thrombocytopenia occurs in some cats. Hemoglobinemia, hemoglobinuria, hyperbilirubinemia, and bilirubinuria are uncommon. Antemortem diagnosis is based on demonstration of the erythrocytic phase on thin blood smears (Fig. 99-1) stained with Wright’s or Giemsa stains (see Chapter 92). Infected macrophages can be detected cytologically in bone marrow, spleen, liver, or lymph node aspirates. The organism is easily identified on histopathologic evaluation of most organs. Serologic testing is not commercially available. PCR can be used to amplify organism DNA from blood.

FIG 99-1 Cytauxzoon felis in the red blood cells of a cat.

(Courtesy Dr. Terry M. Curtis, Gainesville, FL.)

Treatment

Supportive care includes fluid therapy and blood transfusion administered as indicated. Diminazene (five cats) (2.0 mg/kg IM twice, 7 days apart) or imidocarb (one cat) (2 mg/kg IM twice, 14 days apart) was used in cats that survived infection (Greene et al., 1999). Historically, parvaquone (10 to 30 mg/kg IM or SC q24h) administered for 2 to 3 days, buparvaquone (10 mg/kg IM or SC q24h) administered for 2 to 3 days, or thiacetarsemide (0.1 mg/kg IV q12h) administered for 2 days have been attempted. Diminazene, parvaquone, and buparvaquone are not routinely available; thiacetarsemide is toxic for cats and should not be used in this species. If no other drug is available, enrofloxacin at 5.0 mg/kg PO or SC q12h for 7 to 10 days could be attempted. Azithromycin and atovaquone as used for B. gibsoni may be effective.

Zoonotic Aspects and Prevention

Cytauxzoon felis is not known to be zoonotic. The disease can only be prevented by avoiding exposure. Ticks should be controlled, and cats in endemic areas should be housed during periods of peak tick activity.

Etiology and Epidemiology

Hepatozoonosis in dogs is caused by the protozoal agents Hepatozoon canis and H. americanum. In North America H. americanum predominates, is transmitted by Amblyomma maculatum (Gulf Coast tick), and is most common in the Texas Gulf Coast, Mississippi, Alabama, Georgia, Florida, Louisiana, and Oklahoma. In Africa, southern Europe, and Asia, H. canis predominates and is transmitted by Rhipicephalus sanguineus (brown dog tick). A Hepatozoon species is occasionally found in the blood of cats in Europe. Clinical disease associations are currently unclear, but the cats are commonly coinfected with feline leukemia virus or feline immunodeficiency virus. Vertebrate hosts develop macrogametes and microgametes in neutrophils and monocytes. The tick ingests the organism during a blood meal and oocysts develop. After a dog ingests an infected tick, sporozoites are released and infect mononuclear phagocytes and endothelial cells of the spleen, liver, muscle, lungs, and bone marrow and ultimately form cysts containing macromeronts and micromeronts. Micromeronts develop into micromerozoites, which infect leukocytes and develop into gamonts. Tissue phases induce pyogranulomatous inflammation, resulting in clinical disease. Glomerulonephritis or amyloidosis may occur as a result of chronic inflammation and immune complex disease. Infected dogs can serve as a source of infection for ticks for months to years (Ewing et al., 2003).

Clinical Features

H. americanum can be a primary pathogen, resulting in clinical illness without concurrent immune deficiency. Clinically affected dogs have been in all age groups, but disease is most commonly recognized in puppies. Fever, weight loss, and severe hyperesthesia over the paraspinal regions are common findings. Anorexia, pale mucous membranes from anemia, depression, oculonasal discharge, and bloody diarrhea occur in some dogs. Clinical signs can be intermittent and recurrent.

Diagnosis

Neutrophilic leukocytosis (20,000 to 200,000 cells/μL) with a left shift is the most common hematologic finding. Thrombocytopenia is unusual unless coinfection with Ehrlichia canis or Anaplasma spp. occurs. Normocytic, normochromic, nonregenerative anemia is common and is likely from chronic inflammation. Increased activity of alkaline phosphatase but not creatine kinase occurs in H. americanum–infected dogs. Hypoalbuminemia, hypoglycemia and, rarely, polyclonal gammopathy occur in some dogs. Periosteal reactions from the inflammatory reaction directed at tissue phases in muscle can occur in any bone except the skull, are most common in young dogs, do not occur in every case, and are not pathognomonic for hepatozoonosis. Definitive diagnosis is based on identification of gamonts in neutrophils or monocytes in Giemsa or Leishman’s stained blood smears or by demonstration of the organism in muscle biopsy sections. PCR assays may be used to aid in the diagnosis in the future.

Treatment

No therapeutic regimen has been shown to eliminate H. canis or H. americanum infection from tissues. However, clinical disease resolves rapidly with several drug protocols. For treatment of H. americanum, the combination of trimethoprim-sulfadiazine (15 mg/kg PO q12h), pyrimethamine (0.25 mg/kg PO q24h), and clindamycin (10 mg/kg PO q8h) for 14 days is highly successful in the acute stage (Macintire et al., 2001). Use of decoquinate (10 to 20 mg/kg q12h) with food lessens the likelihood of recurrence of clinical disease and prolongs survival time. Imidocarb dipropionate (5 to 6 mg/kg IM or SC) administered once or twice 14 days apart is the drug of choice for treatment of H. canis and may also be effective for H. americanum. Administration of nonsteroidal antiinflammatory agents may lessen discomfort for some dogs.

Zoonotic Aspects and Prevention

No evidence exists for zoonotic transfer of H. americanum or H. canis from infected dogs to people. Tick control is the best form of prevention. Glucocorticoid administration should be avoided because it may exacerbate clinical disease.

Etiology and Epidemiology

Leishmania spp. are flagellates that cause cutaneous, mucocutaneous, and visceral diseases in dogs, human beings, and other mammals. Rodents and dogs are primary reservoirs of Leishmania spp., people and cats are probably incidental hosts, and sandflies are the vector in most endemic regions other than the United States. Leishmaniasis was considered unimportant in the United States until recently, with cases only reported occasionally. In 1999 L. donovani infection was confirmed in multiple dogs in a Foxhound kennel in New York state (Gaskin et al., 2002). Further investigation of more than 12,000 foxhounds and other canids documented L. donovani infection in 18 states and two Canadian provinces (Duprey et al., 2006) (Fig. 99-2). Infection of canids other than Foxhounds appears to be uncommon. In other countries flagellated promastigotes develop in the sandfly and are injected into the vertebrate host when the sandfly feeds. Promastigotes are engulfed by macrophages and disseminate through the body. After an incubation period of 1 month to 7 years, amastigotes (nonflagellate) form and cutaneous lesions develop; sandflies are infected during feeding. In Foxhounds in the United States transmission appeared to be primarily from dog to dog (Duprey et al., 2006). Transmission by fighting, shared needles, blood transfusions, breeding, and congenital transmission can occur (Duprey et al., 2006; de Freitas et al., 2006). The intracellular organism induces extreme immune responses; polyclonal gammopathies (and occasionally monoclonal); proliferation of macrophages, histiocytes, and lymphocytes in lymphoreticular organs; and immune complex formation resulting in glomerulonephritis and polyarthritis are common.

FIG 99-2 Distribution of hunt clubs with confirmed cases of visceral leishmaniasis, United States and Canada. States in which hunt clubs or kennels had 1 or more dogs infected with Leishmania infantum are shaded. Leishmania-positive Foxhounds were also found in Nova Scotia and Ontario.

(Reprinted from Duprey ZH et al: Canine visceral leishmaniasis, United States and Canada, 2000-2003, Emerg Infect Dis 12:440, 2006.)

Clinical Features

Dogs generally develop visceral leishmaniasis. A subclinical phase of infection may persist for months or years. Weight loss in the face of a normal to increased appetite, polyuria, polydipsia, muscle wasting, depression, vomiting, diarrhea, cough, petechiae, ecchymosis, epistaxis, sneezing, and melena are common presenting complaints. Splenomegaly, lymphadenopathy, facial alopecia, fever, rhinitis, dermatitis, increased lung sounds, icterus, swollen painful joints, uveitis, and conjunctivitis are commonly identified on physical examination. Cutaneous lesions are characterized by hyperkeratosis, scaling, thickening, mucocutaneous ulcers, and intradermal nodules on the muzzle, pinnae, ears, and foot pads. Bone lesions are detected in some dogs. Most dogs die or are euthanized as a consequence of chronic kidney disease. Cats are usually subclinically infected; one cat in Texas had cutaneous nodules on the pinna.

Diagnosis

The principal clinicopathologic abnormalities include hyperglobulinemia, hypoalbuminemia, proteinuria, increased liver enzyme activities, thrombocytopenia, azotemia, lymphopenia, and leukocytosis with left shift. The hyperglobulinemia is usually polyclonal, but an IgG monoclonal gammopathy was reported in a dog (Font et al., 1994). Neutrophilic polyarthritis occurs in some dogs as a manifestation of a type III hypersensitivity reaction. Demonstration of amastigotes (2.5 to 5.0 μm ×1.5 to 2.0 μm) in lymph node aspirates, bone marrow aspirates, or skin imprints stained with Wright’s or Giemsa stain gives a definitive diagnosis (Fig. 99-3). The organism can also be identified by histopathologic or immunoperoxidase evaluation of skin or organ biopsy, culture, inoculation of hamsters, or PCR. Antibodies against Leishmania can be detected in serum; IgG titers develop 14 to 28 days after infection and decline 45 to 80 days after treatment. Serologic cross-reactivity occurs between Trypanosoma cruzi and Leishmania. Because dogs are unlikely to eliminate infection spontaneously, most true-positive antibody test dogs are currently infected. PCR can be performed on ethylenediamine tetraacetic acid anticoagulated blood, bone marrow, or lymph node aspirates. Real-time PCR assays can be used to monitor response to therapy (Francino et al., 2006).

FIG 99-3 Impression smear of a lymph node of a Leishmania spp.–infected dog showing intracellular amastigotes.

(Courtesy Dr. Arturo Font, Barcelona, Spain.)

Treatment

Although clinical signs of disease often improve with drug administration, the prognosis for visceral leishmaniasis in dogs is variable; most cases are recurrent. No drug or drug combination has been used to clear Leishmania from the body successfully. The combination of antimony and allopurinol (15 mg/kg PO q12h) was superior to treatment with either drug alone (Denerolle et al., 1999), but even long-term therapy does not always eliminate infection (Manna et al., 2007). Because antimony drugs are not available in the United States, infected dogs should be started on allopurinol therapy initially. In one study, marbofloxacin was effective in vitro and may be considered for the treatment of infected dogs if other drugs are not available (Vouldoukis et al., 2006). Liposomal or lipid-emulsified amphotericin B at varying doses (0.8 to 3.3 mg/kg IV for varying numbers of treatments) has been prescribed with good clinical results, but recurrences can still occur (Oliva et al., 1995; Cortadella et al., 2003). Dogs with chronic kidney disease have a poor prognosis, but a recent study showed administration of allopurinol to be beneficial (Plevraki et al., 2006).

Zoonotic Aspects and Prevention

The primary zoonotic risk for canine leishmaniasis is from dogs acting as a reservoir host for the organism. Direct contact with amastigotes in draining lesions is unlikely to result in human infection. None of the 185 persons with potential exposure to infected Foxhounds had evidence of infection (Duprey et al., 2006). Avoidance of infected sandflies is the only means of prevention. If in endemic areas, house animals during night hours and control breeding places of sandflies. Use of 10% imidacloprid/50% permethrin may lessen transmission in sandfly-endemic areas (Otranto et al., 2007). A vaccine is available for use with dogs in some countries (Dantas-Torres, 2006). For blood donor programs, high-risk breeds (e.g., Foxhounds) or dogs from endemic areas should be screened for Leishmania spp. infection by serology or PCR assays, and positive dogs should be excluded from the program (Wardrop et al., 2005).

Etiology and Epidemiology

Neospora caninum is a coccidian previously confused with T. gondii because of similar morphology. The sexual cycle is completed in the gastrointestinal tract of dogs and results in the passage of oocysts in feces. Oocyst shedding can continue for several months in some dogs (McGarry et al., 2003). Sporozoites develop in oocysts within 24 hours of passage. Tachyzoites (rapidly dividing stage) and tissue cysts containing hundreds of bradyzoites (slowly dividing stage) are the other two life stages. Dogs are infected by ingestion of bradyzoites but not tachyzoites. Infection has been documented after ingestion of infected bovine placental tissue. Dogs can become infected from ingesting intermediate hosts such as white-tailed deer (Gondim et al., 2004). Thus free-roaming dogs may be at increased risk of infection. Transplacental infection has been well documented; dams that give birth to infected offspring can repeat transplacental infection during subsequent pregnancies. Because repeated transplacental infections occur, puppies from a bitch who previously birthed infected puppies are at an increased risk. Canine neosporosis has been reported in many countries around the world. Seroprevalence of infection has varied from 0% to 100% depending on the country and lifestyle of the dog (Dubey et al., 2007a). The pathogenesis of the disease is primarily related to the intracellular replication of tachyzoites. Although organism replication occurs in many tissues, including the lungs, in dogs clinical illness is primarily neuromuscular.

Encephalomyelitis and myositis develop in experimentally infected kittens and seropositive, naturally exposed cats have been detected (Bresciani et al., 2007), but clinical disease in naturally infected cats has not been reported. N. caninum seropositive, nondomestic felids also have been reported (Spencer et al., 2003).

Administration of glucocorticoids may activate bradyzoites in tissue cysts, resulting in clinical illness.

Clinical Features

Ascending paralysis with hyperextension of the hindlimbs in congenitally infected puppies is the most common clinical manifestation of the disease. Muscle atrophy occurs in many cases. Polymyositis and multifocal CNS disease can occur alone or in combination. Clinical signs can be evident soon after birth or may be delayed for several weeks. Neonatal death is common. Although disease tends to be most severe in congenitally infected puppies, dogs as old as 15 years have been clinically affected. In one dog presented primarily for respiratory disease, cough was the principal sign. Myocarditis, dysphagia, ulcerative dermatitis, pneumonia, and hepatitis occur in some dogs. Whether clinical disease in older dogs is from acute, primary infection or exacerbation of chronic infection is unknown. Administration of glucocorticoids may activate bradyzoites in tissue cysts, resulting in clinical illness. Disease is caused by intracellular replication of Neospora caninum tachyzoites. Infection of CNS structures causes mononuclear cell infiltrates, which suggests an immune-mediated component to the pathogenesis of disease. Intact tissue cysts in neural structures are generally not associated with inflammation, but ruptured tissue cysts induce inflammation. Untreated disease generally results in death.

Diagnosis

Hematologic and biochemical findings are nonspecific. Myositis commonly results in increased creatine kinase and aspartate aminotransferase activities. CSF abnormalities include increased protein concentration (20 to 50 mg/dL) and a mild, mixed inflammatory cell pleocytosis (10 to 50 cells/μL) consisting of monocytes, lymphocytes, neutrophils and, rarely, eosinophils. Interstitial and alveolar patterns can be noted on thoracic radiographs.

Definitive diagnosis is based on demonstration of the organism in CSF or tissues. Tachyzoites are rarely identified on cytologic examination of CSF, imprints of dermatologic lesions, and bronchoalveolar lavage. Mixed inflammation with neutrophils, lymphocytes, eosinophils, plasma cells, macrophages, and tachyzoites was noted on transthoracic aspirate of one dog with lung disease. Neospora caninum tissue cysts have a wall thicker than 1 μm; T. gondii tissue cysts have a wall thinner than 1 μm (Fig. 99-4). Oocysts can be detected in feces by microscopic examination after flotation or by PCR. The organism can be differentiated from T. gondii by electron microscopy, immunohistochemistry, and PCR. A multiplex PCR assay that detects both T. gondii and N. caninum for use with tissues or CSF has been reported (Schatzerg et al., 2003)

FIG 99-4 Neospora caninum cyst filled with bradyzoites in canine central nervous system tissue.

A presumptive diagnosis of neosporosis can be made by combining appropriate clinical signs of disease and positive serology or presence of antibodies in CSF with the exclusion of other etiologies inducing similar clinical syndromes, particularly T. gondii. Serologic cross-reactivity between T. gondii and N. caninum exist in some assays (Silva et al., 2007). IgG antibody titers of at least 1 : 200 have been detected in most dogs with clinical neosporosis; minimal serologic cross-reactivity occurs with T. gondii at titers of 1 : 50 or higher when using the immunofluorescent assay test.

Treatment

Although many dogs with neosporosis die, some have survived after treatment with trimethoprim-sulfadiazine combined with pyrimethamine; sequential treatment with clindamycin hydrochloride, trimethoprim-sulfadiazine, and pyrimethamine; or clindamycin alone. Administration of trimethoprim-sulfadiazine (15 mg/kg PO q12h) with pyrimethamine (1 mg/kg PO q24h) for 4 weeks or clindamycin (10 mg/kg PO q8h) for 4 weeks was recommended for the treatment of canine neosporosis. In one recent study of naturally infected beagle puppies, administration of clindamycin alone (75 mg/puppy at 9 weeks of age, PO, q12h [dose doubled at 13 weeks] for 6 months) lessened clinical signs of disease but did not eliminate the infection (Dubey et al., 2007b). Treatment of clinically affected dogs should be initiated before the development of extensor rigidity, if possible. The prognosis for dogs presented with severe neurologic involvement is grave.

Zoonotic Aspects and Prevention

Neospora caninum antibodies have been detected in people, but in one study no link was found to repeated abortion (Petersen et al., 1999). In addition, the organism has not been isolated from human tissues (Dubey et al., 2007a), so the zoonotic potential is still unproven. An epidemiologic link has been shown between dogs and cattle; efforts should be made to lessen dog fecal contamination of livestock feed, and dogs should not be allowed to ingest bovine placentas. Consuming raw meat is a risk factor for dogs and should be avoided (Reichel et al., 2007). Hunting behavior of dogs should be restricted if possible. Bitches that whelp clinically affected puppies should not be bred. Glucocorticoids should not be administered to seropositive animals, if possible, because a potential exists for activation of infection.

Etiology and Epidemiology

Toxoplasma gondii is one of the most prevalent parasites infecting warm-blooded vertebrates. Only cats complete the coccidian life cycle and pass environmentally resistant oocysts in feces. Sporozoites develop in oocysts after 1 to 5 days of exposure to oxygen and appropriate environmental temperature and humidity. Tachyzoites disseminate in blood or lymph during active infection and replicate rapidly intracellularly until the cell is destroyed. Bradyzoites are the slowly dividing, persistent tissue stage that form in the extraintestinal tissues of infected hosts as immune responses attenuate tachyzoite replication. Tissue cysts form readily in the CNS, muscles, and visceral organs. Bradyzoites may persist in tissues for the life of the host.

Infection of warm-blooded vertebrates occurs after ingestion of any of the three life stages of the organism or transplacentally. Most cats are not coprophagic and so are infected most commonly by ingesting T. gondii bradyzoites during carnivorous feeding; oocysts are shed in feces from 3 to 21 days. Sporulated oocysts can survive in the environment for months to years and are resistant to most disinfectants (Fig. 99-5). Results of a recent study confirm that the T. gondii oocyst shedding prepatent period is stage dependent (ingestion of bradyzoites has a shorted prepatent period than ingestion of sporozoites) and not dose dependent (Dubey, 2006). In addition, transmission of T. gondii is most efficient when cats consume tissue cysts (carnivorism) and when intermediate hosts consume oocysts (fecal-oral transmission). T. gondii infection of rodents changes the behavior of the prey species, making it less averse to cats, potentially increasing the likelihood the definitive host (felid) will become infected and potentiate the sexual phase of the organism (Vyas et al., 2007). Approximately 30% to 40% of cats and people in the United States are seropositive and are presumed to be infected. In a recent study of clinically ill cats, T. gondii antibodies were detected in 31.6% of the 12,628 cats tested (Vollaire et al., 2005).

FIG 99-5 Unstained Toxoplasma gondii unsporulated oocysts. The oocysts are 10 × 12 μm.

Clinical Features

Approximately 10% to 20% of experimentally inoculated cats develop self-limiting, small-bowel diarrhea for 1 to 2 weeks after primary oral inoculation with T. gondii tissue cysts; this is presumed to be from enteroepithelial replication of the organism. However, detection of T. gondii oocysts in feces is rarely reported in studies of naturally exposed cats with diarrhea. T. gondii enteroepithelial stages were found in intestinal tissues from two cats with inflammatory bowel disease. Positive response to anti-Toxoplasma drugs in these two cats suggests that toxoplasmosis may occasionally induce inflammatory bowel disease. Eosinophilic fibrosing gastritis was recently described in a T. gondii–infected cat (McConnell et al., 2007).

Fatal extraintestinal toxoplasmosis can develop from overwhelming intracellular replication of tachyzoites after primary infection; hepatic, pulmonary, CNS, and pancreatic tissues are commonly involved. Kittens infected by the transplacental or transmammary routes develop the most severe signs of extraintestinal toxoplasmosis and generally die of pulmonary or hepatic disease. Common clinical findings in cats with disseminated toxoplasmosis include depression, anorexia, and fever followed by hypothermia, peritoneal effusion, icterus, and dyspnea. If a host with chronic toxoplasmosis is immunosuppressed, bradyzoites in tissue cysts can replicate rapidly and disseminate again as tachyzoites; this is common in people with acquired immunodeficiency syndrome (AIDS). Disseminated toxoplasmosis has been documented in cats concurrently infected with feline leukemia, feline immunodeficiency, or feline infectious peritonitis viruses as well as after cyclosporine administration for skin disease or after renal transplantation (Bernstein et al., 1999; Barrs et al., 2006).

Sublethal, chronic toxoplasmosis occurs in some cats. T. gondii infection should be on the differential diagnosis list for cats with anterior or posterior uveitis, cutaneous lesions, fever, muscle hyperesthesia, myocarditis with arrhythmias, weight loss, anorexia, seizures, ataxia, icterus, diarrhea, or pancreatitis (Fig. 99-6). Based on results of T. gondii–specific aqueous humor antibody and PCR studies, toxoplasmosis appears to be a common infectious cause of uveitis in cats. Kittens infected transplacentally or lactationally commonly develop ocular disease. Immune complex formation and deposition in tissues and delayed hypersensitivity reactions may be involved in chronic, sublethal clinical toxoplasmosis. Because none of the anti-Toxoplasma drugs totally clear the body of the organism, recurrence of disease is common.

FIG 99-6 Punctate chorioretinitis caused by Toxoplasma gondii in an experimentally inoculated cat.

Diagnosis

Cats with clinical toxoplasmosis can have a variety of clinicopathologic and radiographic abnormalities, but none documents the disease. Nonregenerative anemia, neutrophilic leukocytosis, lymphocytosis, monocytosis, neutropenia, eosinophilia, proteinuria, and bilirubinuria as well as increases in serum protein and bilirubin concentrations, creatinine kinase, alanine aminotransferase, alkaline phosphatase, and lipase activities occur in some cats. Pulmonary toxoplasmosis most commonly causes diffuse interstitial to alveolar patterns or pleural effusion. Mass lesions may be detected on computed tomography or magnetic resonance imaging examinations. CSF protein concentrations and cell counts are often higher than normal. The predominant white blood cells in CSF are small mononuclear cells, but neutrophils also are commonly found.

The antemortem definitive diagnosis of feline toxoplasmosis can be made if the organism is demonstrated; however, this is uncommon, particularly in association with sublethal disease. Bradyzoites or tachyzoites are rarely detected in tissues, effusions, bronchoalveolar lavage fluids, aqueous humor, or CSF. Detection of 10 × 12 μm oocysts in feces in cats with diarrhea suggests toxoplasmosis but is not definitive because Besnoitia and Hammondia infections of cats produce morphologically similar oocysts.

T. gondii–specific antibodies (IgM, IgG, IgA), antigens, and immune complexes can be detected in the serum of normal cats as well as in those with clinical signs of disease, so antemortem diagnosis of clinical toxoplasmosis is impossible based on these tests alone. Of the serum tests, IgM correlates the best with clinical feline toxoplasmosis because this antibody class is rarely detected in serum of healthy cats. The antemortem diagnosis of clinical toxoplasmosis can be tentatively based on the combination of the following:

Some cats with clinical toxoplasmosis will have reached their maximal IgG titer or have undergone antibody class shift from IgM to IgG by the time they are serologically evaluated, so the failure to document an increasing IgG titer or a positive IgM titer does not exclude the diagnosis of clinical toxoplasmosis. Because some healthy cats have extremely high serum antibody titers and some clinically ill cats have low serum antibody titers, the magnitude of titer is relatively unimportant in the clinical diagnosis of toxoplasmosis. Because the organism cannot be cleared from the body, most cats will be antibody positive for life, so repeating serum antibody titers after the clinical disease has resolved is not necessary.

The combination of aqueous humor or CSF T. gondii–specific antibody detection and organism DNA detection by PCR is the most accurate way to diagnose ocular or CNS toxoplasmosis (e.g., Diagnostic Laboratory, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins). Whereas T. gondii–specific IgA, IgG, and organism DNA can be detected in aqueous humor and CSF of both normal and clinically ill cats, T. gondii–specific IgM has only been detected in the aqueous humor or CSF of clinically ill cats and therefore may be the best indicator of clinical disease. Because T. gondii DNA can be detected in the blood of healthy cats, positive PCR results do not correlate to clinical disease (Burney et al., 1999).

Treatment

Supportive care should be instituted as needed. Clindamycin hydrochloride (10 to 12 mg/kg PO q12h) administered for 4 weeks or a trimethoprim-sulfonamide combination (15 mg/kg PO q12h) administered for 4 weeks has been used most frequently by the author for the treatment of clinical feline toxoplasmosis. Azithromycin (10.0 mg/kg PO q24h) has been used successfully in a limited number of cats, but the optimal duration of therapy is unknown. Pyrimethamine combined with sulfa drugs is effective for the treatment of human toxoplasmosis but commonly results in toxicity in cats. Cats with systemic clinical signs of toxoplasmosis, such as fever or muscle pain combined with uveitis, should be treated with anti-Toxoplasma drugs in combination with topical, oral, or parenteral corticosteroids to avoid secondary lens luxations and glaucoma. T. gondii–seropositive cats with uveitis that are otherwise normal can be treated with topical glucocorticoids alone unless the uveitis is recurrent or persistent. In these situations, administration of a drug with anti–T. gondii activity may be beneficial.

Clinical signs not involving the eyes or the CNS usually resolve within the first 2 to 3 days of clindamycin or trimethoprim-sulfonamide administration; ocular and CNS toxoplasmosis responds more slowly to therapy. If fever or muscle hyperesthesia does not decrease after 3 days of treatment, other causes should be considered. Recurrence of clinical signs may be more common in cats treated for less than 4 weeks. No evidence suggests that any drug can totally clear the body of the organism, so recurrences are common and infected cats will always be seropositive. The prognosis is poor for cats with hepatic or pulmonary disease caused by organism replication, particularly in those that are immunocompromised.

Zoonotic Aspects and Prevention

T. gondii is a major zoonosis. Primary infection of mothers during gestation can lead to clinical toxoplasmosis in the fetus; stillbirth, CNS disease, and ocular disease are common clinical manifestations. Primary infection in immunocompetent individuals results in self-limiting fever, malaise, and lymphadenopathy. As T-helper cell counts decline, approximately 10% of people with AIDS develop toxoplasmic encephalitis from activation of bradyzoites in tissue cysts.

People most commonly acquire toxoplasmosis transplacentally or by ingesting sporulated oocysts or tissue cysts. To prevent toxoplasmosis, avoid eating undercooked meats or ingesting sporulated oocysts (Box 99-1). In a recent study of 6282 meat samples from 698 retail meat stores, T. gondii was detected by bioassay in cats in none of the beef or chicken samples tested and only a small number of pork samples (Dubey et al., 2005). Although owning a pet cat was epidemiologically associated with acquiring toxoplasmosis in one study of pregnant women, touching individual cats is probably not a common way to acquire toxoplasmosis for the following reasons:

However, because some cats will repeat oocyst shedding when exposed a second time, feces should always be handled carefully. If a fecal sample from a cat is shown to contain oocysts measuring 10 × 12 μm, the organism is assumed to be T. gondii. The feces should be collected daily until the oocyst shedding period is complete; administration of clindamycin (25 to 50 mg/kg PO divided q12h) or sulfonamides (100 mg/kg PO divided q12h) can reduce levels of oocyst shedding.

Because human beings are not commonly infected with T. gondii from contact with individual cats, testing healthy cats for toxoplasmosis is not recommended. Fecal examination is an adequate procedure to determine when cats are actively shedding oocysts but cannot predict when a cat has shed oocysts in the past. No serologic assay accurately predicts when a cat shed T. gondii oocysts in the past, and most cats that are shedding oocysts are seronegative. Most seropositive cats have completed the oocyst shedding period and are unlikely to repeat shedding; most seronegative cats would shed the organism if infected. If owners are concerned that they may have toxoplasmosis, they should see their physician for testing.

Etiology and Epidemiology

Dogs do not produce T. gondii oocysts like cats, but they can mechanically transmit oocysts after ingesting feline feces. The tissue phases of T. gondii infection occur in dogs and can induce clinical disease. Approximately 20% of dogs in the United States are seropositive for T. gondii antibodies. Before 1988 many dogs diagnosed with toxoplasmosis based on histologic evaluation were truly infected with Neospora caninum (see Neosporosis section).

Clinical Features

Respiratory, gastrointestinal, or neuromuscular infection resulting in fever, vomiting, diarrhea, dyspnea, and icterus occurs most commonly in dogs with generalized toxoplasmosis. Generalized toxoplasmosis is most common in immunosuppressed dogs, such as those with canine distemper virus infection or those receiving cyclosporine to prevent rejection of a transplanted kidney. Neurologic signs depend on the location of the primary lesions and include ataxia, seizures, tremors, cranial nerve deficits, paresis, and paralysis. Dogs with myositis present with weakness, stiff gait, or muscle wasting. Rapid progression to tetraparesis and paralysis with lower motor neuron dysfunction can occur. Some dogs with suspected neuromuscular toxoplasmosis probably had neosporosis. Myocardial infection resulting in ventricular arrhythmias occurs in some infected dogs. Dyspnea, vomiting, or diarrhea occurs in dogs with polysystemic disease. Retinitis, anterior uveitis, iridocyclitis, and optic neuritis occur in some dogs with toxoplasmosis, but they are less common than in cats. Cutaneous disease has also been detected (Webb et al., 2005).

Diagnosis

As in cats, hematologic, biochemical, urinalysis, and radiographic abnormalities are not specific. Increased protein concentrations and mixed inflammatory cell infiltrates occur in dogs with CNS toxoplasmosis.

Demonstration of the organism associated with inflammation in tissues or exudates can lead to a definitive diagnosis. More commonly an antemortem diagnosis is based on the combination of appropriate clinical signs, exclusion of other likely etiologies, positive serum antibody tests, exclusion of N. caninum infection by serologic testing, and response to an anti-Toxoplasma drug. Interpretation of serum, aqueous humor, and CSF antibody and PCR test results is as discussed for toxoplasmosis in cats.

Therapy

Clindamycin hydrochloride (10-12 mg/kg PO q12h) has been used most frequently for treatment of canine toxoplasmosis by the author. Trimethoprim-sulfa (15 mg/kg PO q12h) is an alternative protocol. Treatment should be continued for a minimum of 4 weeks. If uveitis occurs, topical glucocorticoid treatment should also be used.

Zoonotic Aspects and Prevention

Dogs do not complete the enteroepithelial phase of T. gondii but can mechanically transmit oocysts after ingesting feline feces. Like all other warm-blooded vertebrates, dogs are infected by the ingestion of sporulated oocysts or tissue cysts. Toxoplasmosis in dogs can be prevented by not allowing dogs to be coprophagic and to feed only cooked meat and meat byproducts.

Etiology and Epidemiology

Trypanosoma cruzi is a flagellate that infects many mammals and causes American trypanosomiasis. The disease is diagnosed primarily in South America, but several cases have been detected in dogs of North America. Infected reservoir mammals (dogs, cats, raccoons, opossums, armadillos) and vectors (reduviid [kissing] bugs) are found in the United States, but infection in dogs or people is rare; this may relate to differences in vector behavior and sanitation standards in the United States. In one study in Texas the number of serologically positive dogs increased between 1987 and 1996 (Meurs et al., 1998). Foxhounds infected with Leishmania spp. were recently shown to be coinfected with T. cruzi (Duprey et al., 2006) (Fig. 99-7). The organism has three life stages: trypomastigotes (flagellated stage found free in blood), amastigotes (nonflagellated intracellular form), and epimastigotes (flagellated form found in the vector). When infected kissing bugs defecate during feeding, epimastigotes enter the vertebrate host, infect macrophages and myocytes, and transform into amastigotes. Amastigotes divide by binary fission until the host cell ruptures, releasing trypomastigotes into the circulation. The vector is then infected by ingesting trypomastigotes during a blood meal. Transmission can also occur transplacentally by vector ingestion, blood transfusion, or ingestion of infected tissues or milk. Peak parasitemia occurs 2 to 3 weeks after infection, causing acute disease. Disease in dogs is primarily a cardiomyopathy that develops from parasite-induced damage to myocardial cells or immune-mediated reactions.

FIG 99-7 Distribution of hunt clubs with Trypanosoma cruzi–positive hounds, United States and Canada. States in which hunt clubs or kennels had 1 or more dogs infected with T. cruzi are shaded. A T. cruzi–positive hunt club was also found in Ontario.

(Reprinted from Duprey ZH et al: Canine visceral leishmaniasis, United States and Canada, 2000-2003, Emerg Infect Dis 12:440, 2006.)

Clinical Features

Exercise intolerance and weakness are nonspecific presenting complaints that relate to myocarditis or heart failure during acute infection. Generalized lymphadenopathy, pale mucous membranes, tachycardia, pulse deficits, hepatomegaly, and abdominal distension can be detected on physical examination. Anorexia, diarrhea, and neurologic signs occasionally occur. Dogs that survive acute infection can present for evaluation of chronic dilative cardiomyopathy. In one study of 11 dogs with chronic infection, right-sided cardiac disease, conduction disturbances, ventricular arrhythmias, and supraventricular arrhythmias were most common (Meurs et al., 1998).

Diagnosis

Common clinicopathologic abnormalities include lymphocytosis and increased activities of liver enzymes and creatine kinase. Thoracic radiographic, abdominal radiographic, and echocardiographic findings are consistent with cardiac disease and failure but are not specific for trypanosomiasis. The primary electrocardiographic findings are ventricular premature contractions, heart block, and T-wave inversion. Definitive diagnosis is based on organism demonstration. Trypomastigotes (one flagellum, 15 to 20 μm long) can be identified during acute disease on thick blood film (see Chapter 92) or buffy coat smears stained with Giemsa or Wright’s stain. The organism is sometimes detected in lymph node aspirates or abdominal effusions. Histopathologic evaluation of cardiac tissue may reveal amastigotes (1.5 to 4.0 μm). Trypomastigotes can also be cultured from blood or grown by bioassay in mice. PCR assays can also be used to prove infection (Nabity et al., 2006).

Treatment

Nifurtimox has been prescribed most frequently for Chagas disease but are toxic and not routinely available in the United States. In a recent study of allopurinol for the treatment of T. cruzi infection in an experimentally infected mouse model, a positive response was noted. Thus treating clinically affected dogs with allopurinol as described for Leishmania may be prudent. Glucocorticoid therapy may improve survival of infected dogs. Therapy for arrhythmias or heart failure should be instituted as needed. Most dogs that survive acute infection develop dilative cardiomyopathy. Survival time in 11 dogs ranged from 0 to 60 months (Meurs et al., 1998).

Zoonotic Aspects and Prevention

Infected dogs can serve as a reservoir of T. cruzi for vectors, and blood from infected dogs can be infectious to human beings. Vector control is the primary means of prevention. In one recent study use of deltamethrin-treated collars reduced Triatoma infestans feeding success on dogs (Reithinger et al., 2005). Dogs should be kept from other reservoir hosts, such as opossums, and should not be fed raw meat. Potential blood donors from endemic areas should be serologically screened. For blood donor programs, high-risk breeds (e.g., Foxhounds) or dogs from endemic areas should be screened for T. cruzi infection by serology or PCR assays, and positive dogs should be excluded from the program (Wardrop et al., 2005).