Cytauxzoon felis

Cytauxzoon felis is a tick-borne protozoan causing rapidly progressive and often fatal disease in cats, characterized by high fever, lethargy, icterus, and pancytopenia due to schizont-laden macrophages obstructing blood flow. Diagnosis is confirmed by blood smear or PCR, and treatment requires early administration of atovaquone and azithromycin with supportive care.

Cytauxzoonosis is a life-threatening infectious disease in domestic cats caused by the protozoan parasite Cytauxzoon felis. This parasite is primarily transmitted through the bite of the lone star tick (Amblyomma americanum).

Etiology and Transmission:

Causative Agent: Cytauxzoon felis, an apicomplexan protozoan.
Vectors: Primarily transmitted by the lone star tick (Amblyomma americanum). Experimental transmission has been noted with the American dog tick (Dermacentor variabilis), though it is considered a less effective vector.
Reservoir Hosts: The natural reservoir is the bobcat (Lynx rufus), which typically harbors subclinical infections. Other wild felids, such as cougars, panthers, and tigers, have also been reported as hosts.

Pathophysiology:

After transmission via tick bite, C. felis sporozoites infect macrophages, undergoing schizogony to form schizonts. These schizonts can enlarge up to 250 micrometers in diameter and are commonly found in lymph nodes, spleen, liver, lungs, and bone marrow. They can occlude blood vessels, leading to widespread ischemia and tissue necrosis. Upon rupture, schizonts release merozoites that infect red blood cells (RBCs), appearing as piroplasms. While RBC parasitemia is typically low (1–4%), higher levels have been documented. Cats surviving acute infection may become chronically parasitemic without clinical signs.

Clinical Presentation:

Incubation Period: Approximately 5–14 days post-infection.
Early Signs: Depression, lethargy, anorexia, and high fever (up to 106°F or 41°C).
Progressive Signs: Icterus (jaundice), lymphadenopathy, hepatosplenomegaly, dyspnea, and in severe cases, hypothermia and vocalization indicative of pain.
Fatality: Without treatment, death often occurs within 2–3 days after peak fever.

Diagnosis:

Laboratory Findings: Leukopenia with toxic neutrophils, thrombocytopenia, normocytic normochromic anemia, hyperbilirubinemia, and hypoalbuminemia.
Microscopic Examination: Identification of piroplasms in RBCs on peripheral blood smears or schizonts in cytologic samples from affected tissues (e.g., lymph node, spleen).
Molecular Testing: Polymerase chain reaction (PCR) assays can confirm the presence of C. felis DNA.

Treatment:

Pharmacotherapy: The combination of atovaquone (15 mg/kg) and azithromycin (10 mg/kg) administered orally every 8 hours for 10 days has shown improved survival rates. Alternatively, imidocarb dipropionate (5 mg/kg intramuscularly, two doses 14 days apart) has been used, though with lower efficacy.
Supportive Care: Intravenous fluid therapy to address dehydration, blood transfusions for severe anemia, nutritional support, and management of complications such as disseminated intravascular coagulation (DIC).

Prognosis:

Historically, cytauxzoonosis was considered nearly universally fatal. However, with early and aggressive treatment, survival rates have improved, with some studies reporting up to 60% survival with atovaquone and azithromycin therapy.

Prevention:

Tick Control: Regular use of veterinarian-approved acaricides and tick preventatives.
Environmental Management: Limiting outdoor access during peak tick activity (April through September) and managing the environment to reduce tick habitats.
Monitoring: Regularly inspecting cats for ticks and promptly removing any found.