Cytauxzoonosis in Cats
Also known as “bobcat fever,” this blood parasite is hard to diagnose and also hard to treat.
In 1987, during my senior year at the University of Florida College of Veterinary Medicine, the teaching hospital began seeing a few puzzling cases of acutely ill cats. All of these cats had fevers, were lethargic and refused all food. All of the cats were anemic, and most were jaundiced. Sadly, despite aggressive treatment, all of the cats died within a week of hospitalization. After much investigative work, a diagnosis was finally elucidated. These cats had succumbed to cytauxzoonosis, a parasitic blood disorder.
Geography is expanding
Cytauxzoonosis is a life-threatening blood disorder caused by the parasite Cytauxzoon (“site-awk-zo-an”) felis. It used to be considered a rare disease, confined to a small geographic area (the first reported case was in Missouri in 1976). Since then, the geographic range has progressively expanded, and has now been confirmed in domestic cats in 17 states (see related sidebar on page 6).
The organism has been documented in bobcats from Pennsylvania and North Dakota, although no domestic cats have yet been reported with the disease there. For many years, cytauxzoonosis was reported in North America and South America, but in recent years the infection has also been documented in Europe (in the south of Spain, in France and in Italy).
Ticks are the cause
Cats acquire the illness through the bite of a tick, usually the Lone Star tick. The tick picks up the parasite from bobcats, which are the reservoir host. Bobcats, when infected with Cytauxzoon felis, develop only mild clinical signs of illness. The bobcats recover — and then become persistent carriers. When a tick feeds on an infected bobcat, the tick acquires the pathogen. These ticks can bite and infect other bobcats, or they can bite a domestic cat instead, which leads to severe illness and often death. All felids are susceptible to infection; however, cytauxzoonosis has never been described in a non-felid.
Outdoor cats at higher risk
Because ticks transmit the disease, it should be no surprise that infection is far more likely in cats that are allowed outdoors, especially in rural areas where tick contact is more common. Cats of any age and either sex may be infected, although young adults are more commonly affected. Most infections occur between April and September.
The most common signs are poor appetite and lethargy, as well as high fever and jaundice. Dehydration, rapid and/or labored breathing, rapid heart rate, pale gums, abdominal or generalized pain, increased vocalization, enlarged lymph nodes and enlarged spleen and/or liver have also been reported.
How it is diagnosed
In order to diagnose it, it must first be on the veterinarian’s list of possibilities. In endemic areas, veterinarians must suspect cytauxzoonosis for any acutely ill cat with a high fever and outdoor exposure during the spring, summer or early fall. Of the routine tests we perform on sick cats — a complete blood count, serum chemistry panel, urinalysis, X-rays — the complete blood count is the most useful diagnostic test. Microscopic identification of the parasites inside the red blood cell confirms the infection.
The parasite has been described as “signet ring” shaped. Unfortunately, a major limitation of this test is that the parasite may not be visible inside the red blood cells early in the disease. Evaluating a blood smear every day increases the chances of finding the parasite, but it may be several days before the parasite is identified. Initially, only a few red blood cells may be parasitized. As the disease progresses, however, up to 50 percent of the red cells may become infected.
There is a molecular test that is very sensitive and specific for identifying infected cats, but this test must be mailed out to a diagnostic laboratory. Given the rapid progression of the disease, treatment should not be delayed while waiting for test results because 90 percent of cats infected with Cytauxzoon felis die from the disease (most within a week of onset of clinical signs). That’s why it is imperative to begin treatment in any cat suspected of having the disease — even if a definite diagnosis has not yet been made. Many antiprotozoal drugs have been tried. Recently, the administration of the antimalarial drug atovaquone — in combination with the antibiotic azithromycin —has shown the best results, with 60 percent of naturally infected cats surviving.
Treatment is administered for ten days. Supportive care is essential during the treatment period because these cats are critically ill. Intravenous fluids, pain medication, anti-nausea drugs and aggressive nutritional support are important aspects of treatment, as these supportive measures keep the cat alive while the antiprotozoal drugs and the cat’s own immune system do their work. Severely anemic cats may require a blood transfusion. Cats fortunate enough to recover from the illness are apparently immune from ever getting the disease again, but they may continue to harbor the parasite in their red blood cells for months or years.
Tick control is essential
Currently, there is no vaccine for cytauxzoonosis, so prevention is based on tick control. A recent study showed that a collar containing 10 percent imidacloprid/4.5 percent flumethrin (Seresto™, Bayer) was effective for prevention of Cytauxzoon felis transmission. In that study, two groups of cats (with and without a collar) were exposed to ticks that were infected with the organism. None of the cats wearing the collar contracted the disease, while 90 percent of cats without the collar got infected.
When cats become carriers
Although infected cats cannot transmit the disorder to other cats through physical contact, cats that have recovered from the disease may become carriers — able to transmit the organism to ticks —and therefore indirectly to other cats. It goes without saying that the most sensible way to prevent infection is to keep cats completely indoors, thereby eliminating tick exposure.
An effective alternative to the atovaquone/azithromycin combination would be highly welcome because atovaquone is expensive, hard to obtain and also difficult to administer to cats. (Atovaquone is a thick, viscous, citrus-flavored liquid that must be given every eight hours, and the majority of cats greatly resent being medicated with it.)
However, until a better treatment is discovered, the combination of these two drugs offers the best chance for survival and remains the treatment of choice. — Arnold Plotnick, DVM, DACVIM