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Feline Leukemia Virus Infection

General Information

Feline leukemia virus (FeLV) is one of the most common and destructive of all cat viruses. It is highly contagious and is spread primarily by saliva during cat fights, grooming or mating. The virus is also spread by blood, urine and feces. Kittens may become infected while still in the womb, when the mother bites off the umbilical cord or during nursing.

Not all cats exposed to FeLV become infected. About 50% of exposed cats have immune systems that destroy the invading virus. In general, if a cat remains infected with the virus for longer than 90 days it will become persistantly infected. If a cat that is not sick is tests positive for virus it may be appropriate to retest in 90 days to see if the body has eliminated the virus from circulation. The cat must be isolated from other cats during this time since it will be contagious during the time the virus is in circulation. If the cat still has virus in circulation after 90 days it will, in general, remain infected and eventualy die of the disease. Some persistantly infected cats have inactive virus in their bone marrow, and these virus particles may later become active when the cat becomes ill from another disease, stress or certain drugs. These cats are difficult to identify by standard blood testing. The majority of infected cats can be detected by a simple blood test.

Of the cats persistently infected, about 25% will die within 1 year and 75% will die within 3 years. Some may live a normal life but tend to have various chronic illnesses. If a cat is overtly sick at the time of diagnosis it's prognosis is not as good as the cat that appears outwardly healthy. It is impossible to predict how long a good quality of life may last for these cats. It is important to remember that eventually all persistantly infected cats die from the disease and that they must be kept isolated from other cats to avoid spreading this deadly disease. These cats will also be immunosuppressed and therefor should not be around people that are immunosuppressed.


There are no signs specific for FeLV infection. The main effect of the virus is to disrupt the cat's immune system. While anemia is the most common disorder caused by the virus, cancer and various other diseases are common. Disorders commonly associated with FeLV infection include: chronic respiratory disease; chronic infection of the mouth, gums and tongue; chronic eye disease; frequent or chronic skin disease; reproductive disease (abortion, stillbirths and kitten deaths); frequent or chronic urinary tract infections; chronic digestive tract disease; and other systemic diseases (infectious peritonitis, hemobartonellosis, toxoplasmosis, polyarthritis).


Vaccination before exposure to the virus is the second best means of preventing FeLV infection. The vaccine is probably about 70 % effective in preventing FeLV infection. Without vaccination, isolation from other cats is the only means of prevention. Strict isolation from physical contact with other cats is essentially 100 % effective in preventing feline leukemia.

Important Facts

* Infected cats are at high risk for developing cancer or other
life-threatening disease.

* Indoor cats are at low risk for developing FeLV infection.

* Outdoor cats are at high risk for developing FeLV infection.

* Currently, there is no uniformly effective treatment for cats
infected with FeLV.