Also known as bobcat fever, cytauxzoonosis is a serious and often fatal parasitic infection caused by the protozoal organism Cytauxzoon felis. Its natural host is the bobcat (Lynx rufus), who harbor the parasite with mild or subclinical effects. The parasite has also been found in the Florida panther. Ticks are responsible for transmitting the infection to domesticated cats.
Cytauxzoonosis can be found in the United States, particularly south-central, southeastern and mid-Atlantic regions of the United States. Outdoor cats are affected more than indoor cats, presumably due to greater chances of exposure to ticks. The incidence of disease is higher in the spring and summer months when ticks are more active.
Outdoor/free-ranging cats are at greater risk of catching cytauxzoonosis, particularly those who roam wooded areas. This disease affects cats only, it can not be transmitted from cat to cat, cat to human or tick to human.
How do cats become infected with C. Felis?
Ticks are the intermediate host, meaning that they transmit the parasite from the bobcat to the domestic cat. This occurs during feeding when saliva containing the parasite is injected into the cat. Below is a brief outline of how the parasite is transmitted and affects your cat.
Pathogenesis of cytauxzoonosis:
This can get a little complicated in places, in order to understand what happens, it is important to know about the life cycle of this parasite.
The parasitic infection has two phases. The leukocytic or tissue phase and the erythrocytic (red blood cell) phase.
- The bobcat is the natural reservoir and intermediate host of this organism is the Lone Star Tick. When red blood cells infected with merozoites (erythrocytic stage) are ingested when the tick feeds, once released inside the tick stomach, they split into two forms and sexually reproduce to form a zygote. This is able to differentiate into an ookinete and replicate by asexual reproduction.
- The ookinete then penetrates the stomach wall of the tick and migrates to the salivary glands where it asexually reproduces forming sporozoites which are released into the tick saliva.
- When the tick feeds on its next host (your cat), saliva containing the sporozoites are injected into the bloodstream. Once inside the cat, sporozoites enter macrophage cells throughout the body where they reproduce. Merozoites bud from sporozoites, rapidly increasing the size of the macrophages. As this happens, vascular obstruction from the swollen macrophages occurs in the small blood vessels of many vital organs such as the spleen, liver, and lungs. This leads to tissue necrosis due to inadequate blood supply as well as a severe immune response. It is this phase (the leukocytic/tissue phase) that is so fatal to cats.
- In the next phase, merozoites break out of the macrophages and enter the red blood cells (erythrocytic phase). This phase is less pathogenic than the leukocytic phase.
What are the symptoms of cytauxzoonosis?
Symptoms of cytauxzoonosis occur between 5-15 days of infection, and may initially be nonspecific. They can vary depending on the organ(s) affected, early symptoms may include:
- Anorexia (loss of appetite).
- Pale mucous membranes due to anemia.
As the infection progresses, additional symptoms may include:
- Difficulty breathing.
- Jaundice (yellowing of the mucous membranes).
- Enlarged lymph nodes.
- Enlarged spleen and/or liver.
- Pain due to enlarged spleen and/or liver.
- Dark brown urine.
- A high temperature is one of the first symptoms of cytauxzoonosis, however as the cat approaches death, his temperature can drop below normal.
How is cytauxzoonosis diagnosed?
Your veterinarian will perform a physical examination of your cat and obtain a medical history from you including onset of symptoms. He will need to perform some tests to determine the cause. These may include:
- Biochemical profile which may reveal hyperbilirubinemia due to the breakdown of red blood cells, hypoalbuminemia (low blood albumin) which may be indicative of liver failure, elevated liver enzymes.
- Complete blood count may reveal low red blood cell count. As the red blood cells become parasitised, they are marked for destruction by the immune system. Low white blood cells (leukopenia) and thrombocytopenia (low platelets) may also be revealed.
- Polymerase chain reaction test of whole blood may detect the parasite in acutely sick cats as well as carriers.
- Blood smear may reveal the parasite inside the red blood cells. The tissue phase occurs, which is by far the most devastating to the cat, occurs before the red blood cell phase, so the parasite may not yet be seen in the red blood cells.
- Fine needle biopsy of the liver and/or spleen to check for the parasite.
- Abdominal X-rays may reveal an enlarged liver and/or spleen.
How is cytauxzoonosis treated?
This disease used to almost always be fatal, however, new treatments do offer some hope, increasing the survival rate from around 5% to 50-60%. Even so, the prognosis is guarded, early treatment is vital.
Recently treatments have improved the mortality rate of this disease. The antimalarial drug atovaquone used in conjunction with the antibiotic azithromycin have shown promise. Both of these medications are given for ten days.
Aggressive supportive care while your cat recovers is also necessary. This may include intravenous fluids, feeding tube, heparin (a blood thinner) may be given to prevent disseminated intravascular coagulation which can occur with this disease. Blood transfusion may be necessary for the severely anemic cat.
Cats who have recovered can still carry the parasite in their blood so should be kept indoors so they don't become a reservoir of infection to other cats (via tick bites).
There is no vaccine available for this disease, the best way to reduce your cat's chances of developing this parasite is to keep him indoors to reduce exposure.
If he does go outside, proper tick prevention is a must. This may be in the form of a topical application and/or a tick collar. If you are using more than one tick prevention product on your cat, check with your veterinarian as it is possible to double up on parasitic control causing toxicity. Even this isn't 100% foolproof, your cat should be checked daily for ticks. Start at the head and work your way down his body, not forgetting to check inside the ears and between the toes.
Bobcat photo courtesy of Linda Tanner.
Lone Star tick photo courtesy of Benjamin Smith.