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Diagnosis, treatment of tick-borne diseases
Some cases difficult to detect or fail to respond to therapies



Cytauxzoonosis is an emerging infectious disease of cats in North America, caused by the protozoal parasite Cytauxzoon felis. It is transmitted via the tick vector Dermacentor variabilis and possibly other tick species such as Amblyomma americanum.

Cats typically present acutely, and historically the mortality rate is high (over 90 percent). More than 90 percent of the cases are diagnosed between April and September. Outdoor cats are at higher risk for infection, and there appear to be hyper-endemic areas of C. felis transmission. Bobcats appear to be the reservoir host and only rarely develop severe disease. Most cats die within five to seven days of the onset of clinical signs. The majority of clinical signs are due to obstruction of small vessels with schizont-laden macrophages, resulting in ischemia and thrombosis.

The most common signs are lethargy, depression and fever. Pancytopenia is the classic hematologic finding for cytauxzoonosis, but there may only be reductions of one or two cell lines in affected cats. Thrombocytopenia and leukopenia appear to be the most common hematologic abnormalities.

Hemolytic anemia is most prominent in seven to 14 days after presentation. Hyperbilirubinemia and increased serum ALT and ALP concentrations (often not proportional to the degree of hyperbilirubinemia) are common serum chemistry profile findings. Physical examination typically reveals fever and hepatosplenomegaly. Cats often are dyspneic, moribund, hypothermic and neurologic in the end stages of disease.

Cytologic diagnosis is the most common and rapid means of diagnosing cytauxzoonosis. The earliest stage of infection is the multiplication of schizonts in macrophages. These infected macrophages can be identified in tissue aspirates (particularly the liver, lung and spleen) or on the feathered edge of peripheral blood smears. These infected macrophages frequently are mistaken for platelet clumps and can measure nearly 100 microns in diameter.

In endemic areas hepatic aspirates may be warranted in highly suspicious cases.

The parasite also may be identified in red blood cells on Wright Giemsa-stained blood smears as the classic signet ring. There are no commercially available serologic tests. PCR is now available, sensitive and specific and can be performed rapidly to aid in the diagnosis or confirmation of cytauxzoonosis.

Supportive care with intravenous fluids and anticoagulants are the standard of care for the treatment of cytauxzoonosis.

Heparin is the anticoagulant of choice (100-300 U/kg SQ TID). Anti-protozoal therapies have been administered to cats with cytauxzoonosis but the effect on outcome is not clear because no controlled studies have been performed.

Imidocarb dipropionate (2 mg/kg IM once every two weeks) or diminazine aceturate have been recommended. Pre-treatment with atropine (0.05 mg/kg SQ once) appears to reduce the cholinergic side effects associated with imidocarb dipropionate. Atovaquone (10 mg/kg PO Q24) and azithro-mycin (15 mg/kg PO TID) combination therapy in combination for 10 days with aggressive supportive care appears to be a promising treatment with survival rates approaching 60 percent in an uncontrolled study.

Other antibiotics frequently are administered to cats with cytauxzoonosis, presumably to prevent secondary infections because many cats are neutropenic, although some antibiotics (doxycycline and clindamycin) do have anti-protozoal activity.

If cats survive more than seven days, the prognosis for long-term survival is excellent. Prospective testing of cats in the same household with infected cats has identified carriers.

Dr. Hoskins is owner of Docu-Tech Services. He is a diplomate of the American College of Veterinary Internal Medicine with specialities in small animal pediatrics. He can be reached at (225) 955-3252, fax: (214) 242-2200 or e-mail:


Source: DVM360 MAGAZINE,
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