Remo Lobetti, BVSc (Hons), MMedVet (Med), Dip ECVIM (Internal Medicine), MRCVS
Cytauxzoon felis is a tick-borne protozoal disease of domestic and wild cats. In domestic cats the disease is usually fatal. The disease was first described in 1973 and since then has predominantly been reported in the south central and southeastern areas of North America.
Cytauxzoon has both an erythrocytic and tissue phase, whereas the babesias only have an erythrocytic phase, which is morphologically indistinguishable from the erythrocytic phase of Cytauxzoon. There is no serological cross-reactivity between the small babesias (B. felis, B. equi, and B. rodhaini) and Cytauxzoon, however on PCR there is enough similarity between these organisms to re-classify them as a separate family.
Cats acquire the infection in rural and wooded areas as probably accidental and dead-end hosts. The bobcat appears to be the reservoir host for the disease. The highest incidence of the disease is in the summer months and in cats that roam and thus have exposure to ticks.
The disease is thought to be transmitted by Dermacentor variabilis.
Schizonts develop primarily within monocytes where they undergo multiple divisions by both schizogony and binary fission. The infected monocytes tend to marginate on the blood vessel endothelium of most organs and often end up occluding the blood vessel. As the disease progresses the schizonts form merozoites that separate and eventually fill the host cell. The host cell then ruptures and releases the merozoites into the blood and tissue fluid. The merozoites then invade other monocytes/macrophages as well as erythrocytes. In the erythrocyte they divided, then leave the cell to enter other erythrocytes.
As the organism invades and parasitizes red blood cells haemolysis and anaemia results. As the organisms multiply and the disease progresses there is enlargement of the spleen and lymph nodes. Occlusion of major vessels of the spleen, lymph nodes, lungs and other organs occur.
The course of the disease is generally less than a week. Consistent clinical signs are severe lethargy, anorexia, icterus, and depression. Most cats are dehydrated and can show pale mucus membranes, dyspnoea, and fever with hypothermia developing prior to death. The fatality rate is close to 100 %.
Although generally a fatal disease there have been reports of cats surviving the disease. Some of these cats showed a persistent erythro-parasitaemia. Possible reasons for these cats surviving the disease is infection with a less virulent strain or heightened immune-competency in these cats.
The primary haematological abnormalities are mild to moderate normomocytic normochromic non-regenerative anaemia, mild to marked leukopaenia with neutropaenia and band neutrophilia, and marked thrombocytopaenia.
Urine analysis may show hypersthenuria, bilirubinuria, haemoglobinuria, and proteinuria.
Changes on serum biochemistry are elevated ALT activity, hyperbilirubinuria, hyperglycaemia, hypoalbuminaemia, and hypokalaemia.
The diagnosis of C. felis is made by demonstrating organisms within infected erythrocytes on a blood smear or histological identification of schizonts in tissues. Differentiation needs to be made between C. felis and the smaller feline parasites, Babesia felis and Haemobartonella felis, as well as Howell-Jolly bodies. PCR can be used to differentiate between these feline parasites.
The treatment of domestic cats with cytauxzoonosis has largely been unrewarding as the disease is highly fatal in cats that develop clinical signs.
Drugs that have been used are diminazene aceturate (administered intramuscularly at a dosage of 2 mg/kg subcutaneously and repeated in 2-4 weeks) and imidocarb dipropionate (administered subcutaneously at a dose of 5 mg/kg, and repeated in 2 weeks) may have some efficacy.
Paravaquone, buparaquone, sodium thiacetarsamide, and tetracyclines have not been effective.
Supportive therapy is essential and should be based on thorough patient assessment and ongoing monitoring, and appropriate laboratory testing.
The primary means of disease prevention is the control of the vector tick by routinely dipping or spraying cats, using tick collars, or spot-on preparations, and spraying of the premises. Ectoparasitic agents (such as fipronil) may also be effective. Another control measure is to avoid areas where the likely vectors and reservoir animals are present.