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Veterinary Forum July 2007 (Vol 24, No 7)

Clinical Report: "Rapid diagnosis needed for cytauxzoonosis"

by Sophia Yin, DVM, MS (Animal Science)

    A cat presents with sudden lethargy, anorexia and fever. The diagnostic differentials for these clinical signs are numerous and include infection, immune-mediated disease and toxicosis. In many cases, a veterinarian might try a short course of antibiotics, which could be of some value if the cause is bacterial infection, and wait to see whether there is improvement.

    But for one particular tick-borne disease, cytauxzoonosis, a few days' delay waiting for the wrong treatment to improve the condition can nullify the cat's already slim chances of survival. And veterinarians in newly endemic areas are likely to miss this diagnosis, unless they are looking for it.

    Disease epidemiology changing

    Many veterinarians might think that this protozoal disease caused by Cytauxzoon felis is not a worry for them because it has remained very geographically limited — primarily in the Midwest and Southeast. However, says Adam Birkenheuer, DVM, PhD, DACVIM, assistant professor of internal medicine at North Carolina State University College of Veterinary Medicine and a researcher in vector-borne diseases, the epidemiology of the disease is changing. "Over the past 6 to 8 years, we've been receiving reports of Cytauxzoon infection in areas where it hasn't surfaced before. For example, North and South Carolina, Virginia and Kentucky have all reported cases in the past year. So it seems to be on the move," he continues.

    In that same time, endemic states have reported an increased incidence of the disease.

    "Some hospitals in endemic areas see five or six cases a week during the spring and summer when ticks carrying the organisms are active," Birkenheuer says.

    Cytauxzoonosis tends to occur in hot spots, even in endemic areas. For instance, says Birkenheuer, in certain households or neighborhoods, five or six cats may become infected or die. "When it happens once in an area, it seems to repeat a lot. So if you see just one case, it's unlikely to remain a single anomaly."

    Short life cycle, deadly outcome

    This tick-borne disease differs from others because a veterinarian must make a rapid diagnosis to have the best chance of saving the animal. Domestic cats show signs of infection 1 to 3 weeks after being bitten by an infected tick (usually Dermacentor variabilis) and usually die within 5 to 7 days of clinical presentation.

    During this short life cycle, the parasite may progress through two stages in the cat: The first stage is the schizogenous stage in which schizonts undergo asexual reproduction within macrophages. As the schizonts multiply within the macrophages, they cause cells to balloon to such a degree that they clog blood vessels. Any tissue that requires blood supply is affected and undergoes ischemic necrosis, leading to disseminated intravascular coagulation (DIC) and, later, consumptive anemia. The classic finding on blood work is pancytopenia caused by leukocytes mobilizing in necrotic tissue or enlarged cells becoming stuck in tissue blood vessels. Biochemical profiles frequently reveal hyperbilirubinemia with mild to moderate increases in liver enzymes.

    Luckily, a diagnosis of cytauxzoonosis is not too difficult if the veterinarian remembers to include it as a diagnostic differential. "Clinicians in endemic areas are often able to recognize the disease on presentation even before conducting definitive diagnostics," Birkenheuer explains. "The finding of rapid onset of lethargy, depression and fever in a healthy cat that goes outdoors during the spring or summer and seeing mild central nervous system signs from ischemia can point to the disease."

    At this stage, a finding of organisms within erythrocytes on a blood smear or histologic identification of schizonts in tissues is diagnostic. When the macrophages can no longer expand, the schi-zonts break out and are endocytized by red blood cells. Then, the classic signet ring structures can be observed in red blood cells. By the time this second stage is evident, cats are farther along in the infectious stage and the prognosis is poorer.

    In fact, until recently, nearly 100% of cats died within a week of presentation, regardless of the stage. Fortunately, Birkenheuer and his colleague Leah Cohn, DVM, have been studying drug combinations that are proving hopeful. "So far, clinical trials are showing that with a combination of atovaquone and azithromycin coupled with aggressive supportive therapy, 60% of cats are surviving and returning to normal," Birkenheuer said. Birkenheuer and Cohn also are actively recruiting hospitals in endemic regions to participate in their prospective clinical trial, which is comparing the atovaquone-azithromycin treatment with imidocarb dipropionate.

    In the meantime, for cats not yet infected, Birkenheuer does recommend treatment with acaricidals. But because some cats have succumbed to the disease despite acaricidal treatment, keeping cats indoors is the most effective preventive precaution.

    NEXT: Diagnostic Dilemma: "Disappearing Joints"


    Did you know... Some ticks induce an ascending flaccid paralysis in dogs and children; this is caused by a protein toxin elaborated by various species after a few days of feeding.Read More

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