What you should know about... Cytauxzoonosis

Q: What do practitioners need to know about how cytauxzoonosis, or bobcat fever, is transmitted? Is it zoonotic? What population of cats is most at risk?

Dr. Leah A. Cohn

A: Cytauxzoonosis is transmitted by the bite of a tick, especially the lone star tick (Amblyomma americanum). It cannot be transmitted directly from cat to cat, even through close contact.

Cytauxzoonosis is not a zoonotic infection. In fact, the protozoal pathogen (Cytauxzoon felis) infects cats of every kind, but only cats. Bobcats, cougars, and even tigers and lions can be infected, but the pathogen cannot be transmitted to nonfelid animals.

The cats at greatest risk of contracting cytauxzoonosis are outdoor cats living in the South Central and Southeastern United States during the spring, summer, and early autumn.

Q: What is the prevalence of cytauxzoonosis in the United States, and in which regions are we seeing this infection?

A: Infections were first recognized in the South Central United States—Missouri, Arkansas, and Oklahoma. Today, the disease has a greatly expanded geographic region, extending all the way to the Mid-Atlantic states and throughout the Southeast as well as the South Central states. The pathogen has even been identified in bobcats, the reservoir host, as far north as North Dakota.

The prevalence of infection is not known. In Oklahoma, the diagnosis accounted for almost 1.5% of feline hospital admissions at the Veterinary Medical Teaching Hospital; most of these cats suffered from the acute illness. In some areas of the country, the infection is completely unheard of. In others (like parts of southern Missouri) I am aware of clinics that see three cases a week throughout the spring and early summer.

Bobcats, and domestic cats that survive initial infection, can remain healthy chronic carriers of the pathogen. In some regions, prevalence of infection in bobcats is as high as 50%. In a study of domestic cats from a trap-neuter-release program, only two of 494 cats in Florida and one of 75 cats in Tennessee were found to be presumably healthy carriers.1

Q: Are there typical presenting signs of cytauxzoonosis in domestic cats?

A: An owner first notices that his or her previously healthy cat goes off food and acts lethargic. The cat may cry out as if in pain and may develop respiratory effort or distress. The veterinarian may recognize a high fever, icterus, pallor, splenomegaly or lymphadenomegaly, and sometimes central nervous system signs including obtundation. Laboratory findings usually include evidence of hemolytic anemia, disseminated intravascular coagulation, and hepatopathy. Often, affected cats die within five to seven days of the appearance of the very first signs.

Cats that do survive initial infections remain chronic carriers for years. These cats are clinically healthy, but small numbers of infected red blood cells may be observed on a peripheral blood smear.

Q: How do we diagnose this infection in domestic cats?

A: Microscopic examination of a peripheral blood smear will reveal signet-ring-shaped piroplasms in the red blood cells (Figure 1) of most acutely infected cats, but since illness can occur before the red blood cell stage of infection, negative smears should be re-evaluated the next day. Alternatively, schizont-distended mononuclear cells accompany clinical illness (Figures 2A & 2B). These large cells are commonly found on aspirates of lymph nodes or spleen and are occasionally observed on blood smears. The infection can also be confirmed by polymerase chain reaction testing.

1. Signet-ring shaped Cytauxzoon felis. (Courtesy of Dr. Marlyn Whitney, University of Missouri Veterinary Medical Diagnostic Laboratory.)

Q: What is the latest on how we should treat cytauxzoonosis in domestic cats?

A: In a recent study, the combination of an antimalarial drug, atovaquone, and the antibiotic azithromycin was demonstrated to result in improved survival of acutely ill cats compared with cats treated with the antiprotozoal drug imidocarb dipropionate.2 All cats also received supportive care including fluid therapy and blood transfusions when required. While only 26% of the imidocarb-treated cats survived, 60% of those treated with the combination drug therapy lived. However, the antimalarial drug is expensive and not readily available from nearby pharmacies.

2A & 2B. Schizont-laden macrophage (2A 100X; 2B 1000X). (Courtesy of Dr. Marlyn Whitney, University of Missouri Veterinary Medical Diagnostic Laboratory.)

Q: How do we prevent cytauxzoonosis in domestic cats?

A: Since the infection is transmitted only by ticks, tick prevention is key. Because there are no acaricides that can prevent tick bites perfectly, in endemic regions the most effective prevention is to apply products such as fipronyl and to keep cats indoors to minimize exposure.

Leah A. Cohn, DVM, PhD, DACVIM

Department of Veterinary Medicine and Surgery

College of Veterinary Medicine

University of Missouri

Columbia, MO 65211

REFERENCES

1. Haber MD, Tucker MD, Marr HS, et al. The detection of Cytauxzoon felis in apparently healthy free-roaming cats in the USA. Vet Parasitol 2007;146(3-4):316-320.

2. Cohn LA, Birkenheuer AJ, Brunker JD et al. Improved survival of cats with acute cytauxzoonosis treated with atovaquone and azithromycin as compared to imidocarb dipropionate. J Vet Intern Med 2011;25(1):55-60.