What is Cytauxzoon felis?

Cytauxzoon (pronounced “sight-oz-o-un”) organisms are blood parasites that were first recognized in Africa as parasites of antelopes and other ruminants. Cytauxzoon organisms were not discovered in the U.S. until 1976 when Cytauxzoon felis was first described as a feline blood parasite affecting and killing cats from forested areas. Outside of Africa and the US, this feline disease has also been reported in wild and domestic cats in areas of Europe, South America, and Asia.

Initially, there was concern that livestock were next as this was a ruminant infection as far as anyone knew, but research showed only cats could be infected. It appears that in the U.S., the natural host is the bobcat, where most of the time the infection is minor, but it tends to be much more severe (and even deadly)  in domesticated cats.

Cytauxzoon felis is a single-celled organism that infects both the blood and various body tissues of the cat. The blood cell stage of the parasite called a piroplasm, is not particularly harmful. The schizont stage, where the parasites move into various organs and body tissues - including the lymph nodes, brain, liver, lungs, and spleen- is the problem.

The schizont infects immune cells that line blood vessels, which causes them to swell and block off the blood vessels in which they reside. Any tissue with blood vessels (almost every tissue in the body) can be affected, but most symptoms seem to relate to the resulting red blood cell destruction and inflammation.

Signs

Signs start vaguely with fatigue and fever but quickly progress. Other signs may include: anorexia, diarrhea and vomiting, jaundice (yellowing of the skin and white part of the eyes, labored breathing, lymph node enlargement, neurological issues, pain, spleen enlargement, and weight loss. Intensive care is required for cats to have the best chance of survival.

Without treatment, many domestic cats die within one week of showing signs, with a two to three-week incubation period from the initial infection.

Cats with clinical signs of Cytauxzoon felis are more commonly immunocompromised or have secondary infections and immune-mediated diseases.

How The Infection Spreads

Cytauxzoon felis is spread by tick bites. The ticks usually implicated are the American Dog Tick (Dermacentor variabilis) and the Lone Star Tick (Amblyomma americanum). Bobcats and infected domestic cats carry the Cytauxzoon piroplasms in their blood; ticks feed on the bobcats and then drop off and molt to their next life stage. They are still carrying the Cytauxzoon piroplasm when they attach to their next host, and if that next host is a domestic cat, a potentially lethal infection results. The organisms are home to the immune cells lining the blood vessels as described above, and if the host lives long enough, the schizonts will eventually produce offspring (the piroplasms). Bobcats do not get sick during this process. It is the local bobcat population that infects the local tick population.

A lone star tick. Image courtesy of CDC.

Feline red blood cells infected with Cytauxzoon felis. The piroplasm (Cytauxzoon felis' blood cell form) is the dot seen inside some of the red blood cells. Image courtesy Dr. Alice Wolf.

Diagnosis

The cat typically has a fever, with or without jaundice, and is brought to the veterinarian for evaluation. In most cases, the piroplasms are fairly obvious when the blood sample is evaluated. The tissue phase of the infection with the schizonts comes first,  so when blood tests are first done, it's possible that piroplasms aren’t present yet. Because of the rapid progression of the infection, piroplasms will likely be seen in a few days if they are not at first, so sometimes a second blood evaluation is needed.

Your veterinarian might suggest other tests, like a PCR test to find the parasite's DNA and confirm the diagnosis, a blood clotting test, a urinalysis to check kidney function, and X-rays to evaluate the lungs.

Because piroplasms sometimes have variable sizes, they are sometimes mistaken for Mycoplasma hemofelis, a much more treatable infection.

Cytauxzoon organisms are larger and have a thick “dot” on their ring shape.

If the diagnosis is to be made post-mortem (after death), it is usually easy to find the schizonts in many body tissues.

Treatment

Prompt treatment with medications and intensive supportive therapy are needed for recovery.

A combination of atovaquone and azithromycin is the most effective treatment. If atovaquone, an anti-malaria drug, is not readily available, it can be obtained through compounding pharmacies.  Another drug, imidocarb, can be used in the meantime.

Supportive Therapy and Monitoring

Cats must be hospitalized and supported with intravenous fluids, have their vitals monitored closely, and be given supplemental feeding as needed. An anti-coagulant medication is also given to prevent inappropriate blood cell clotting and vessel clogging. Pain medications may be beneficial, although NSAIDs (nonsteroidal anti-inflammatory drugs) should be avoided.

There are milder strains of this disease, where some cats have survived without treatment (as do most bobcats) but there is not enough research yet to know about these differences. These cats continue to have piroplasms in their blood but seem to have no effect from this. It is yet unknown how this can happen. (Different strains of Cytauxzoon, effective medication, and genetics are all theories.)

Your cat will need to be monitored closely with continued supportive care and it may take several days (average 4-7 days) for improvement. Your veterinarian will recommend recheck exams and lab work as necessary.

Prognosis (Outcome) and Prevention

Since mortality is quite high with this infection and treatment is still being researched, prevention is crucial. The most effective prevention is to keep your cat indoors where there is no tick exposure, especially if you live in an area with large populations of wild cats.

The next best prevention is to use a tick control product on your cat; note that there are not nearly as many tick products for cats as there are for dogs. Do not use dog products on cats. Talk to your veterinarian about the best tick prevention for your cat, and how to protect them from this and other tick-borne diseases.