Feline Cytauxzoonosis

Article #5 CE Feline Cytauxzoonosis Peter J. Bondy, Jr, DVM, MS, DACVIM Leah A. Cohn, DVM, PhD, DACVIM Marie E. Kerl, DVM, DACVIM, DACVECC University of Missouri ABSTRACT: Cytauxzoon felis is a protozoal organism transmitted to cats through a tick bite. Region- ally restricted to the south central and southeastern United States, C. felis infection of domestic cats is usually fatal.The parasite life cycle includes both a tissue and an erythrocytic phase.The clinical disease course is rapid, with onset of fever, lethargy, and anorexia 5 to 20 days after infection and death within a week of initial signs. Leukocytosis, hemolytic anemia, icterus, and elevated liver enzymes are usually present. Definitive diagnosis is based on microscopic identification of parasites. Effective medical therapy remains elusive, although several cats have reportedly survived. ytauxzoon felis is a protozoal organism schizont phase of the genus Theileria occurs in that causes fatal illness in domestic lymphocytic cells.3 The infected macrophage cats. It is related to other Cytauxzoon cells occlude venules in the liver, spleen, lung, C 1 spp of African ungulates and was first recog- and lymph nodes (Figure 2). The schizont nized in Missouri in 1976.1 Geographically phase is most closely associated with clinical limited primarily to the south central and disease, and the degree of schizogony is southeastern United States (Figure 1), C. felis reflected in the severity of illness.4 In domestic seems to infect only felidae and therefore cats, the schizont burden is extensive, whereas it poses no zoonotic or agricultural risk.2 Diag- is usually small and brief in mildly affected nosing cytauxzoonosis in cats is based on com- species such as the bobcat.5 patible clinical signs and identifying the Fission of the schizonts results in formation organisms in tissue or blood. No form of ther- of merozoites.6 Merozoites are released when apy has been proven effective, and most ill cats the infected macrophage ruptures; these die despite supportive and/or antimicrobial merozoites undergo endocytosis by erythro- treatment. cytes. In erythrocytes, these forms are referred C. felis is a protozoan organism belonging to to as piroplasms. Although visualization of the order Piroplasmida and the family Theileri- piroplasms on a peripheral blood smear is the idae. Like its relatives in the genus Theileria, most direct and simple way to diagnose infec- C. felis exists in distinct erythrocytic and non- tion, it is the schizont phase that leads to erythrocytic life phases. After infection, the or- venous congestion, thrombotic disease, organ ganism undergoes an asexual failure, and ultimately death.4,7,8 Piroplasms Send comments/questions via email reproductive phase referred to may persist for life after recovery from schi- [email protected], as schizogony. Schizonts of C. zogony in both domestic and nondomestic fax 800-556-3288, or web felis occur in mononuclear cats without apparent clinical consequences CompendiumVet.com phagocytic cells, whereas the for infected animals.9,10 January 2005 69 COMPENDIUM 70 CE Feline Cytauxzoonosis develop into their next life stage in an intermediate host, such as ticks, to produce the virulent schizont form. Therefore, transfusion from cats that have recovered to ones that are naive would not result in illness even if erythrocytes contain piroplasms.10,19 However, transfusion from recently infected cats during clinical illness or just before onset of illness can transmit infection because circulating monocytes can contain schizonts.7,19 Blood-donor cats should be in good health, kept free of ectoparasites, and ideally housed indoors to minimize this very small risk. Close contact between cats in the absence of tick vectors does not Figure 1. US states with confirmed cases (in yellow) of pose a risk of disease transmission.18 Although multi- C. felis infection. ple cats within a household are commonly infected, this circumstance is more likely related to common exposure to infected tick populations than to direct PROPOSED LIFE CYCLE AND cat-to-cat contact. TRANSMISSION Although its natural life cycle is not completely CLINICAL PRESENTATION understood, C. felis is a tickborne disease.11 Bobcats are Feline cytauxzoonosis is most commonly reported in persistent carriers of this organism after infection and middle-aged cats, although it can occur at any age.8,10 are the presumed reservoir for C. felis.2,12 Ticks presum- Sex and breed predilections have not been identified.8 ably transmit the organism from one cat to another by Outdoor cats are more likely to acquire this disease, pre- feeding.11 The only tick that has been experimentally sumably because of increased exposure to tick vectors.8,10 demonstrated to be a competent vector of C. felis is Der- The greatest risk of infection seems to occur in the macentor variabilis, although many other species of ticks spring and early summer, presumably when the tick vec- may feed on wild or domestic cats.11,13 Although domes- tors are most active. Sixty-one of 81 (75%) cases evalu- Only felidae are susceptible to infection with C. felis. tic cats generally succumb to infection and are consid- ated at the University of Missouri Veterinary Medical ered terminal hosts, the eastern bobcat (Lynx rufus Diagnostic Laboratory and Veterinary Medical Teach- rufus) usually develops mild or subclinical infection.2,5,12 ing Hospital over the past 5 years were seen from May Extensive investigation has shown that nonfelidae through July. species, including immunodeficient mice, cannot be The onset of clinical disease occurs 1 to 3 weeks infected.2 However, both clinical and nonclinical infec- after infection.4,18 Clinical signs are nonspecific and tion has been demonstrated or at least strongly sus- include anorexia, lethargy, dyspnea, icterus, and pected in several felidae, including a captive white tiger pallor.1,3,4,9,10,17 Physical examination usually demon- (Panthera tigris), Florida panthers (Felis concolor coryi), a strates pyrexia (often marked), but hypothermia is Texas cougar (Felis concolor), and cheetahs (Acinonyx common in moribund cats.1,8–10 Tachypnea and tachy- jubatus).14–17 cardia are typical, with or without overt respiratory Infection can be accomplished experimentally by distress.4,9 Abdominal palpation often reveals inoculating schizont-containing tissue.18 However, splenomegaly and/or hepatomegaly.1,4,8 Altered menta- inoculating erythrocytes that contain piroplasms only tion, vocalization, seizures, and coma may occur in the results in persistent erythroparasitemia without the later stages of disease.18,20 The disease course is rapid, clinical illness that results from the schizont phase of and most cats succumb within 1 week of initial clinical infection.11 It appears that the piroplasms must illness.7,8,21 COMPENDIUM January 2005 Feline Cytauxzoonosis CE 71 Figure 2. Hematoxylin-and-eosin–stained photomicrographs of a histologic section of an infected spleen. (Original magnification ×10) A splenic vein lined by schizont-laden macrophages in different stages. (Original magnification ×40; courtesy of Dr. Susan Turnquist) DIAGNOSIS Although historical and physical findings are nonspecific, a patient with acute onset of fever, pallor, icterus, and splenomegaly or hepatomegaly in an endemic region should immediately prompt clinicians to suspect cytauxzoonosis. The differential diagnosis might include Mycoplasma haemofelis infection (formerly Haemobar- tonella felis), cholangitis or cholangiohepatitis, immune- mediated hemolytic anemia, retroviral disease sequelae, toxoplasmosis, and feline infectious peritonitis. Diag- nostic testing should be directed toward both confirm- ing the infection and eliminating diagnostic differentials from consideration. Of the routine imaging and initial laboratory tests, only examination of peripheral blood Figure 3. Wright’s-stained peripheral blood smear smears can confirm the diagnosis. Visualizing erythro- exhibiting C. felis (arrows) within erythrocytes. (Original cytic piroplasms is sufficient evidence to confirm infec- magnification ×100) tion. Piroplasms are most often shaped as 1 to 1.5 µm signet rings (Figure 3), but “safety pin” and tetrad forms are also observed, as are chains of organisms resembling cocci.9,17 Although piroplasms are a specific finding, they used to confirm the presence of C. felis8 (Figure 4). are not present in all infected cats, particularly early in Pancytopenia can often be found via a complete blood the disease course.1,3,8–10 In fact, piroplasms may be cell count, although thrombocytopenia and neutropenia absent in up to 50% of cases at the initial illness7 but can be inconsistent.8,9,22 Anemia is typically normocytic, may persist indefinitely in cats that have recovered from normochromic, and nonregenerative because of the infection.10 Infected macrophages on the feathered edge acute nature of the illness.22 Moderate to marked of a peripheral blood smear may also be visualized and thrombocytopenia is believed to be related to consump- January 2005 COMPENDIUM 72 CE Feline Cytauxzoonosis diagnosing C. felis infection, but splenomegaly and hepatomegaly would be expected.4,7,8 For cats in which piroplasms have not been identified, samples from fine- needle aspiration of the lymph nodes, spleen, or liver may provide evidence of infection. These organs are typically heavily infiltrated with schizont-loaded macrophages, and infected cells may be readily identifiable by cytologic examination8,22 (Figure 4).

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