Cytauxzoonosis

Cytauxzoonosis is a tick-borne, potentially fatal protozoal disease of domestic and wild felids caused by the apicomplexan hemoprotozoan Cytauxzoon felis. The disease is characterized by an acute, severe febrile syndrome with profound lethargy, anorexia, anemia, icterus, and often death, particularly in domestic cats. The organism has a complex life cycle involving a tick vector (primarily Amblyomma americanum, the lone star tick) and a reservoir host, the bobcat (Lynx rufus), in which infection is typically subclinical. In domestic cats, the disease progresses rapidly from initial infection to a severe parasitemia with intraerythrocytic piroplasms and a unique schizogenous phase in macrophages, leading to widespread vascular occlusion and multi-organ failure. The disease is endemic in the south-central and southeastern United States, but cases have been reported in other regions. Early recognition and aggressive treatment are critical for survival, as the disease historically had a very high mortality rate, though newer therapeutic protocols have improved outcomes.

The causative agent is Cytauxzoon felis, a protozoan parasite belonging to the phylum Apicomplexa, order Piroplasmida, family Theileriidae. It is closely related to Theileria and Babesia species. The life cycle involves a tick vector, primarily Amblyomma americanum (lone star tick), but other ticks such as Dermacentor variabilis and Amblyomma maculatum may also be competent vectors. The reservoir host is the wild bobcat (Lynx rufus), which typically maintains a chronic, subclinical infection. Transmission to domestic cats occurs when an infected tick feeds on a susceptible cat. The sporozoites are inoculated into the host during tick feeding. In the domestic cat, the parasite undergoes two distinct phases: a schizogenous phase in macrophages and a subsequent erythrocytic phase. The schizogenous phase involves the development of large schizonts within macrophages, which can cause mechanical obstruction of blood vessels in various organs, leading to ischemia and necrosis. The erythrocytic phase involves the release of merozoites that invade red blood cells, forming characteristic piroplasms. The molecular mechanisms of pathogenicity include cytoadherence, immune-mediated hemolysis, and disseminated intravascular coagulation (DIC). The parasite's ability to evade the host immune response and its rapid replication contribute to the high virulence in domestic cats.

Cytauxzoonosis is primarily a disease of domestic cats (Felis catus) in the United States, with the highest prevalence in the south-central and southeastern states, including Missouri, Arkansas, Oklahoma, Texas, Georgia, and Florida. The disease is also reported in other regions, including the mid-Atlantic and Midwest, and cases have been documented in South America and Europe. The disease is seasonal, with most cases occurring in spring and summer, correlating with peak tick activity. The primary reservoir is the bobcat (Lynx rufus), which has a high seroprevalence and serves as an asymptomatic carrier. Domestic cats are considered accidental hosts, and infection is often fatal without treatment. There is no breed or sex predisposition, but outdoor cats with access to tick habitats are at higher risk. The disease is more common in young to middle-aged cats, likely due to increased outdoor exposure. The incidence of cytauxzoonosis has been increasing in recent years, possibly due to changes in tick distribution and habitat encroachment. In endemic areas, the prevalence of infection in domestic cats is low, but the case fatality rate is high, historically exceeding 90% without treatment. With newer therapeutic protocols, survival rates have improved to 60-70%.

The pathophysiology of cytauxzoonosis involves two distinct phases: the schizogenous phase and the erythrocytic phase. After inoculation of sporozoites by an infected tick, the parasites invade macrophages in the host's tissues, where they undergo schizogony, producing large schizonts containing numerous merozoites. These schizonts cause marked hypertrophy of infected macrophages, which can occlude small blood vessels in multiple organs, including the lungs, liver, spleen, lymph nodes, and brain. This vascular occlusion leads to ischemia, infarction, and tissue necrosis, contributing to multi-organ dysfunction. The schizogenous phase is often associated with severe clinical signs and is a major cause of mortality. Eventually, the schizonts rupture, releasing merozoites that invade erythrocytes, initiating the erythrocytic phase. In the erythrocytic phase, the parasites appear as small, ring-shaped piroplasms within red blood cells. This phase is associated with hemolytic anemia, which is primarily extravascular, as infected erythrocytes are phagocytosed by the mononuclear phagocyte system. The anemia is often regenerative, but in severe cases, it can be non-regenerative due to bone marrow suppression. The release of pro-inflammatory cytokines, such as tumor necrosis factor-alpha and interleukins, contributes to systemic inflammatory response syndrome (SIRS), fever, and shock. Disseminated intravascular coagulation (DIC) may occur due to endothelial damage and activation of the coagulation cascade, leading to consumption of clotting factors and thrombocytopenia. The combination of vascular occlusion, hemolysis, and DIC results in multi-organ failure, which is often fatal.

The primary predisposing factor for cytauxzoonosis is exposure to infected ticks, particularly Amblyomma americanum. Cats that are allowed outdoors in endemic areas, especially those in rural or wooded environments, are at increased risk. The disease is more common in the spring and summer months when ticks are most active. Age is a predisposing factor, with young to middle-aged cats being more commonly affected, likely due to increased outdoor activity. There is no known breed or sex predisposition. Immunosuppression, whether due to concurrent viral infections (e.g., feline leukemia virus, feline immunodeficiency virus) or drug-induced, may increase susceptibility or severity of disease. However, cytauxzoonosis can occur in immunocompetent cats. Environmental factors, such as the presence of bobcat populations and suitable tick habitats, are important. Additionally, cats that are not on regular tick prevention are at higher risk. Stress and concurrent illnesses may also exacerbate the clinical course.

The clinical signs of cytauxzoonosis typically appear 5-14 days after tick exposure. The disease can be peracute, acute, or chronic. Peracute cases may present with sudden death without premonitory signs. Acute cases are most common and are characterized by a sudden onset of fever (often >104°F/40°C), lethargy, depression, anorexia, and weakness. As the disease progresses, cats may develop pale or icteric mucous membranes, tachypnea, dyspnea, and signs of shock. Physical examination may reveal splenomegaly, hepatomegaly, and lymphadenopathy. Neurological signs, such as seizures or ataxia, may occur due to cerebral vascular occlusion. In the terminal stages, hypothermia, bradycardia, and coma may develop. Chronic or subclinical infections are rare in domestic cats but have been reported, with cats showing mild or no clinical signs. The clinical signs are largely due to the schizogenous phase, which causes vascular occlusion and tissue ischemia, and the erythrocytic phase, which causes hemolytic anemia. The disease progresses rapidly, and without treatment, death often occurs within 1-2 weeks of onset.

Differential diagnoses for cytauxzoonosis include other tick-borne diseases, hemolytic anemias, and systemic infections. Key differentials include: 1) Feline infectious anemia (Mycoplasma haemofelis): causes hemolytic anemia with regenerative response, but lacks the schizogenous phase and is often less severe; 2) Babesiosis (Babesia felis): also causes intraerythrocytic parasites, but the organisms are larger and the disease is less common; 3) Feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV) infection: can cause immunosuppression and secondary infections, but clinical signs are more chronic; 4) Sepsis due to bacterial infections: may present with fever, lethargy, and shock, but blood cultures and response to antibiotics help differentiate; 5) Immune-mediated hemolytic anemia (IMHA): characterized by a positive Coombs test and spherocytosis, but no organisms on blood smear; 6) Hepatic lipidosis: causes icterus and anorexia, but lacks fever and parasitemia; 7) Toxoplasmosis: can cause fever, lethargy, and organ dysfunction, but is diagnosed by serology and PCR; 8) Feline panleukopenia: causes severe leukopenia and gastrointestinal signs, but no hemolytic anemia. Definitive diagnosis of cytauxzoonosis is made by identification of Cytauxzoon felis organisms on blood smear or by PCR.

The diagnostic algorithm for cytauxzoonosis begins with a thorough history and physical examination, with emphasis on tick exposure and outdoor access. If cytauxzoonosis is suspected, a complete blood count (CBC) and blood smear evaluation should be performed immediately. The presence of characteristic intraerythrocytic piroplasms (small, round, signet-ring or oval structures) on a Wright-Giemsa stained blood smear is highly suggestive of cytauxzoonosis. However, parasitemia may be low in early infection, so multiple smears may be needed. If blood smear is negative but clinical suspicion is high, polymerase chain reaction (PCR) testing on whole blood is the gold standard for diagnosis, as it is highly sensitive and specific. PCR can also be used to confirm infection in recovered carriers. Additional diagnostic tests include serum biochemistry profile, which may show elevated liver enzymes, bilirubin, and azotemia, and urinalysis, which may reveal bilirubinuria and proteinuria. Coagulation profiles may be indicated if DIC is suspected. In cases where the diagnosis is uncertain, bone marrow aspiration or lymph node biopsy may reveal schizonts in macrophages. Imaging, such as thoracic radiographs and abdominal ultrasound, may be performed to assess organ involvement but is not diagnostic. Early diagnosis is critical, as treatment is most effective when initiated early in the course of disease.

Hematology: The most characteristic finding is the presence of Cytauxzoon felis piroplasms within erythrocytes on a blood smear. These are small (1-2 μm), round, signet-ring or oval structures, often with a pale blue cytoplasm and a dark blue nucleus. Anemia is common and may be regenerative or non-regenerative, depending on the stage of disease. The anemia is typically normocytic, normochromic, and may be accompanied by anisocytosis and polychromasia. Leukopenia is often present, particularly in the early stages, due to bone marrow suppression and sequestration of leukocytes in inflamed tissues. Thrombocytopenia is common and may be severe, contributing to bleeding tendencies. Serum biochemistry: Common abnormalities include hyperbilirubinemia (due to hemolysis and hepatic dysfunction), elevated liver enzymes (ALT, AST, ALP), azotemia (due to renal ischemia), and hyperglycemia or hypoglycemia (due to stress or sepsis). Electrolyte imbalances, such as hyponatremia and hyperkalemia, may occur. Urinalysis: Bilirubinuria and proteinuria are common. Blood gas analysis may reveal metabolic acidosis due to lactic acidosis from tissue hypoxia. Specific biomarkers: Acute phase proteins, such as serum amyloid A and C-reactive protein, may be elevated. Coagulation profiles may show prolonged PT and aPTT, decreased fibrinogen, and elevated D-dimers, indicating DIC. Serology: Antibody testing is not widely available and is not useful for acute diagnosis. PCR: Polymerase chain reaction is highly sensitive and specific for detecting Cytauxzoon felis DNA in blood, and is the preferred confirmatory test.

Imaging findings in cytauxzoonosis are non-specific but may support the diagnosis and assess complications. Thoracic radiographs may reveal interstitial or alveolar patterns due to pulmonary edema or hemorrhage, and cardiomegaly may be present if there is heart failure. Abdominal ultrasound may show hepatosplenomegaly, with a mottled or hypoechoic appearance due to infarction and necrosis. Lymphadenopathy may be detected. In cases with neurological signs, computed tomography (CT) or magnetic resonance imaging (MRI) of the brain may reveal ischemic lesions or hemorrhage. Echocardiography may be indicated if there is evidence of cardiac dysfunction. However, imaging is not diagnostic for cytauxzoonosis and should be used in conjunction with laboratory testing.

Cytology: Fine needle aspirates of lymph nodes, spleen, or bone marrow may reveal macrophages containing schizonts of Cytauxzoon felis. These schizonts are large (up to 20 μm) and contain numerous merozoites, giving a 'raspberry' or 'mulberry' appearance. The presence of schizonts is pathognomonic for cytauxzoonosis. Histopathology: On post-mortem examination, tissues such as lung, liver, spleen, and brain may show vascular occlusion by hypertrophied macrophages containing schizonts, with associated ischemic necrosis and hemorrhage. The erythrocytic phase may be seen in blood vessels. Special stains, such as Giemsa or Wright's stain, are used to highlight the organisms. Immunohistochemistry using specific antibodies against Cytauxzoon felis can confirm the diagnosis.

Treatment of cytauxzoonosis has evolved significantly. Historically, the disease was almost always fatal, but with aggressive therapy, survival rates have improved. The current recommended treatment protocol includes: 1) Antiprotozoal therapy: Atovaquone (15 mg/kg orally, three times daily) combined with azithromycin (10 mg/kg orally, once daily) for 10 days. This combination has been shown to be effective and is the treatment of choice. Alternative drugs include imidocarb dipropionate (2-5 mg/kg intramuscularly, repeated in 7-14 days), but it is less effective and has more side effects. 2) Supportive care: Intravenous fluid therapy with crystalloids (e.g., lactated Ringer's solution) at maintenance rates (60-100 ml/kg/day) to correct dehydration and maintain perfusion. Colloids (e.g., hetastarch) may be needed for hypoalbuminemia. 3) Blood transfusion: Packed red blood cells or whole blood may be required for severe anemia (PCV <15%). 4) Antithrombotic therapy: Low molecular weight heparin (e.g., dalteparin 100-150 IU/kg subcutaneously every 8-12 hours) or unfractionated heparin (200-300 IU/kg subcutaneously every 8 hours) may be used to prevent DIC. 5) Nutritional support: Assisted feeding via nasoesophageal or esophagostomy tube if anorexic. 6) Nursing care: Keep the cat warm, clean, and stress-free. 7) Monitoring: Serial CBC, biochemistry, and coagulation profiles to assess response and complications. Treatment should be initiated as early as possible, as delayed treatment is associated with poorer outcomes.

The prognosis for cytauxzoonosis is guarded to poor, but has improved with modern treatment. Historically, mortality was >90%, but with atovaquone and azithromycin, survival rates of 60-70% have been reported. Prognostic indicators include: 1) Early diagnosis and treatment: Cats treated within the first few days of clinical signs have a better prognosis. 2) Severity of clinical signs: Cats presenting with severe lethargy, dyspnea, or neurological signs have a poorer prognosis. 3) Degree of anemia and thrombocytopenia: Severe anemia (PCV <15%) and thrombocytopenia (<50,000/μL) are negative prognostic indicators. 4) Presence of DIC: Coagulopathy is associated with a worse outcome. 5) Response to treatment: Cats that show improvement within 48-72 hours of starting treatment have a better prognosis. 6) Age and overall health: Young, otherwise healthy cats may have a better prognosis. Cats that survive the acute phase may become chronic carriers and are at risk for recrudescence, especially if immunosuppressed.

Follow-up care for cats that survive cytauxzoonosis is essential. Recheck examinations should be performed at 1, 2, and 4 weeks after discharge, then monthly for 3 months, and then every 3-6 months. At each visit, a CBC and blood smear should be performed to monitor for parasitemia and anemia. PCR testing may be repeated to confirm clearance of the parasite, but cats may remain PCR-positive for months. Serum biochemistry should be monitored to assess organ function, especially liver and kidney. Tick prevention is crucial to prevent re-infection; use of topical or oral acaricides (e.g., fipronil, selamectin, fluralaner) is recommended year-round. Owners should be educated about the risk of cytauxzoonosis and the importance of keeping cats indoors or limiting outdoor exposure during peak tick season. If the cat is a chronic carrier, it should not be used as a blood donor. Long-term monitoring for recrudescence is important, especially if the cat becomes ill or immunosuppressed.

Pearls: 1) Cytauxzoonosis should be on the differential list for any febrile, lethargic cat with a history of tick exposure in endemic areas. 2) Blood smear examination is quick and can be diagnostic; look for small, signet-ring piroplasms in erythrocytes. 3) The combination of atovaquone and azithromycin is the treatment of choice and has significantly improved survival. 4) Early aggressive supportive care, including IV fluids and blood transfusions, is critical. 5) Cats that survive may become chronic carriers; monitor for recrudescence. Pitfalls: 1) Do not wait for PCR results to start treatment if clinical suspicion is high; treatment should be initiated immediately. 2) Do not rely solely on blood smear; parasitemia may be low in early infection, so PCR is needed for confirmation. 3) Avoid using imidocarb as first-line therapy due to its lower efficacy and potential side effects. 4) Do not overlook the possibility of DIC; monitor coagulation parameters and consider anticoagulant therapy. 5) Do not assume that a negative blood smear rules out cytauxzoonosis; repeat smears or use PCR. 6) Do not forget to address tick prevention in endemic areas to prevent future infections.

Based on Plumb's Veterinary Drug Handbook and current literature, the following drug protocols are recommended for cytauxzoonosis: 1) Atovaquone: 15 mg/kg orally every 8 hours for 10 days. It is a hydroxynaphthoquinone that inhibits electron transport in the parasite. It is well absorbed with food; administer with a fatty meal to enhance absorption. Side effects include diarrhea and vomiting. 2) Azithromycin: 10 mg/kg orally every 24 hours for 10 days. It is a macrolide antibiotic that has synergistic antiprotozoal activity with atovaquone. It may cause gastrointestinal upset. 3) Imidocarb dipropionate: 2-5 mg/kg intramuscularly, repeated in 7-14 days. It is a diamidine that is less effective than atovaquone/azithromycin and may cause cholinergic signs (salivation, vomiting, diarrhea) which can be mitigated with atropine (0.04 mg/kg SC). 4) Supportive drugs: IV fluids (Lactated Ringer's solution) at 60-100 ml/kg/day; adjust based on hydration and perfusion. Blood transfusion: administer if PCV <15% or clinical signs of anemia. Anticoagulants: Dalteparin (100-150 IU/kg SC q8-12h) or unfractionated heparin (200-300 IU/kg SC q8h) for DIC. Antiemetics: Maropitant (1 mg/kg SC q24h) or ondansetron (0.5-1 mg/kg IV q12h) if vomiting. Nutritional support: Provide high-quality, highly palatable diet; consider feeding tube if anorexic. Analgesics: Buprenorphine (0.01-0.02 mg/kg IV/SC q8-12h) for pain. Note: Dosages should be adjusted for renal or hepatic impairment; monitor for drug interactions, especially with other hepatotoxic drugs.

Key studies and consensus guidelines: 1) A landmark study by Cohn et al. (2011) evaluated the efficacy of atovaquone and azithromycin in 80 cats with cytauxzoonosis, reporting a survival rate of 60% compared to 25% with imidocarb. 2) A study by Birkenheuer et al. (2006) described the use of PCR for diagnosis and monitoring of Cytauxzoon felis infection. 3) The ACVIM consensus statement on tick-borne infectious diseases (2018) includes recommendations for diagnosis and treatment of cytauxzoonosis. 4) A retrospective study by Reichard et al. (2009) identified Amblyomma americanum as the primary vector and highlighted the importance of tick prevention. 5) A study by Sherrill et al. (2015) investigated the use of low molecular weight heparin in cats with cytauxzoonosis, showing potential benefit in preventing DIC. 6) A recent study by Wang et al. (2020) evaluated the long-term outcome of cats surviving cytauxzoonosis, finding that some cats remain PCR-positive for months but do not show clinical signs. These studies support the current treatment protocol and emphasize the importance of early diagnosis and aggressive supportive care.