Feline Leukemia, Basic Understanding
Drs. Foster & Smith Educational Staff

Part 1 of 3: Feline Leukemia - Is Your Cat at Risk? Part 1 of 3: Feline Leukemia - Is Your Cat at Risk?
Although not all cats that are exposed to the virus develop disease symptoms, it does not pay to take a chance. Protect your cat from being exposed to the disease and have her immunized.

There are three different stages of infection.

In this stage, which occurs 3-4 weeks after exposure to the virus, and involves 35-40% of the population, large numbers of virus particles are found in the bloodstream. Cats in the acute phase usually do not show signs of disease. If they do, the signs are usually mild.

In this stage, cats are infected with FeLV but they kill off most of the virus. The remaining virus is held in check by the cat’s immune system. They show no sign of disease and they usually don’t shed any virus. This involves 5-10% of the population.

In this stage, involving 30% of the population, persistent infection occurs. These cats will not mount an adequate immune response and will remain permanently infected. These are the cats that shed large amounts of virus and become ill and die of FeLV-related diseases.

Feline leukemia is a cancerous disease caused by feline leukemia virus (FeLV). Infection with FeLV is a major cause of illness and death in domestic cats. It is estimated that 2-3% of healthy cats are infected with FeLV. The number is significantly higher in the stray cat population, reaching as high as 11%.

FeLV is a type of virus called a retrovirus. That puts it in the same family as feline immunodeficiency virus (FIV) and human immunodeficiency virus (HIV, the virus that causes AIDS). Retroviruses are species-specific. This means a feline retrovirus will only infect cats; a human retrovirus will only infect humans. Additionally, this virus is very unstable and dies quickly outside of the animal.


This virus is spread in the cat population by contact with secretions from infected cats. It takes a large amount of virus and a prolonged exposure to infect a cat.

Saliva of infected cats usually contains large amounts of FeLV. Therefore, the most common mode of transmission is through nose-to-nose contact. Bites are also a very efficient way of transmitting the virus. Other ways that a cat can transmit FeLV are through grooming, licking, and sharing dishes and litter pans.


The prognosis for cats infected with FeLV depends on the immune status of the cat, the cat’s level of exposure to the virus, and its vaccination history. Disease caused by FeLV is very serious, and it is estimated that fewer than 20 percent of persistently infected cats will survive more than three years.


No known treatment exists for FeLV. Some treatments that have been investigated include antiviral drugs, such as AZT, which has many side effects. Other treatment protocols include drugs that stimulate the immune system, such as ImmunoRegulin and interferon.

Specific cancers associated with FeLV have their own treatment protocols. Other problems created by the virus, such as bacterial infections, are treated symptomatically.

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