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Feline Leukemia by Frank Utchen, DVM

Question: Dr. Utchen, I recently adopted a cat from an animal shelter and when I took her to my vet she tested positive for feline leukemia. She seems perfectly healthy right now, but what can I expect down the line? Is my other cat at risk of getting leukemia?


The Feline Leukemia Virus (FeLV) is in the same family of viruses as the human AIDS virus (HIV). Although not contagious to people FeLV is essentially as serious to cats as HIV is to us humans.

The Feline Leukemia Virus is found in an infected cat's saliva and is spread from cat to cat through biting, although the virus itself dies in a matter of just a few minutes when outside of a cat's body on inanimate surfaces.

However, when the virus is introduced into a cat's system by a bite wound the virus can induce several forms of disease, or the cat can succeed in fighting off the virus.

When a cat first contracts the virus, there is a 30-40% chance he will actually fight off the virus and become immune to it. This occurs during the first few months a cat has the virus in his system. After that, if the virus is still present, it means the virus has won the first battle and has actually spliced it's DNA into the cat's DNA and can no longer be fought off.

For those cats that do not fight off the virus in the first few months, they will be carriers of the virus for the rest of their lives. In this case, there is no outward indication that a cat has the virus in its system. Statistically, about half of these cats will become ill and die within the first few months, and most of the remainder will die within 3 years. There are a few cats, about 5%, that are believed to carry the virus indefinitely without becoming sick.

When a cat becomes sick with the virus, it can develop the cancer of leukemia (hence the name of the virus) or it can develop other separate conditions, usually related to suppression of the cat's immune system, similar to the way AIDS affects humans. Virtually any symptoms can be a result of FeLV: respiratory infections, gum infections, vomiting, diarrhea, even eye infections.

Experimental work has been done using an immune stimulant called Interferon, which cats can take orally in liquid form every day to help boost their failing immune system.

Experts are divided on how effective this is. If a cat becomes ill with bacterial infections as a result of a suppressed immune system, antibiotics are needed.

The key when owning a cat who carries FeLV is to minimize the cat's exposure to infection, and eliminate contact of the cat with other cats.  Keeping a carrier cat indoors is almost always recommended and fortunately most cats are content to stay indoors.

There is an effective immunization against FeLV that cats that go outdoors should be given. In most cases this immunization is needed every 3 years. For cat's living indoors, with no direct contact with other cats, the vaccine is not absolutely necessary, as the only way for a cat to contract the virus is to be bitten by an already infected cat.

For a cat who already is discovered to be a carrier of the virus, immunization will not help, although it will not hurt either if a carrier cat receives the vaccine inadvertently. For this reason, we recommended doing a simple blood test on all newly adopted cats to be sure they aren't already carriers, prior to immunizing them. The blood test is done on just a few drops of blood and takes a veterinarian 10 minutes to perform.

If your other cat is not already immunized against FeLV, that cat should be tested for FeLV, and if they test negative (don't have the virus) they should be immunized against FeLV with a series of 2 injections given 3 weeks apart. After that, the immunizations are given every 3 years. I have never seen a cat who was immunized ever contract the virus from another cat.

If your newly adopted cat appears at all ill, consult your veterinarian immediately and be aggressive with treatment for any emerging problems. Remember your cat's immune system is very likely suppressed, but with proper care you can help keep him healthy as long as possible.

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