Cytauxzoonosis (Bobcat Fever) in Cats

What is cytauxzoonosis?

Also known as bobcat fever, cytauxzoonosis is a severe and often fatal blood-borne parasitic infection caused by the protozoal organism Cytauxzoon felis. Its natural host is the bobcat (Lynx rufus), who harbours the parasite with mild or subclinical effects.

Distribution

The distribution of Cytauxzoon felis is south-central, southeastern and mid-Atlantic regions of the United States. Outdoor cats are affected more than indoor cats, presumably due to higher chances of exposure to ticks. The incidence of the disease is higher in the spring and summer months when ticks are more active.

Outdoor/free-ranging cats are at higher risk of catching cytauxzoonosis, particularly those who roam wooded areas. This disease affects cats only, and it can not be transmitted from cat to cat, cat to human or tick to human.

Transmission

Ticks are the intermediate host, which means that they transmit the parasite from the bobcat to the domestic cat, which occurs when the tick injects infected saliva into the cat during feeding.

Pathogenesis

This can get a little complicated in places, to understand what happens, it is essential 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 the intermediate host of this organism is the Lone Star Tick. 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 can 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.

Clinical signs

Symptoms develop 5-15 days after infection, and may initially be nonspecific. They can vary depending on the organ(s) affected.

Early symptoms:

• Fever
• Lethargy
• Anorexia (loss of appetite)
• Dehydration
• Pale mucous membranes due to anemia

Advanced symptoms:

• Difficulty breathing
• Jaundice (yellowing of the mucous membranes)
• Enlarged lymph nodes
• Enlarged spleen and/or liver
• Pain due to an 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

Diagnosis

Your veterinarian will perform a physical examination of your cat and obtain a medical history from you including the onset of symptoms.

Diagnostic workup:

• 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 to look for the parasite inside the red blood cells. The tissue phase – 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.
• Radiographs may reveal hepatomegaly or splenomegaly.

Treatment

This disease used to almost always be fatal; however, new therapies do offer some hope, increasing the survival rate from around 5% to 50-60%. 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 has shown promise. The duration for both medications is ten days.

Aggressive supportive care while your cat recovers will 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. A blood transfusion may be necessary for the severely anemic cat.

Cats who have recovered can still carry the parasite in their blood, so they should be kept indoors, so they don’t become a reservoir of infection to other cats (via tick bites).

Prevention

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 minimise 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. Check your cat 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.