Feline Leukemia Virus (FeLV)

Feline Leukemia Virus (FeLV) is a retrovirus in the oncovirus family. FeLV is considered to be the most prevalent cause of severe illness and death in domestic cats. This virus can create malignancy (cancer called lymphosarcoma). FeLV causes thirty three percent of all cancer deaths in cats. Some people refer to FeLV as the "catastrophic" virus because of its ability to wreak havoc with the cat's immune system and produce such a wide variety of illnesses.

There are four possible outcomes in cats that encounter FeLV:

Cats that can make effective antibodies that neutralize the Feline Leukemia Virus will have a transient viremia (virus circulating in the blood stream) and clear the virus from their system. This represents 40% of FeLV challenged cats.
Cats that fail to produce virus-neutralizing antibodies will have FeLV persisting in their blood stream. The virus reaches and replicates in the bone marrow. Within three years of becoming infected, 80% of these cats will die from FeLV related disease. This represents 30% of FeLV challenged cats.
Cats that have viral latency do not have whole virus present in their cells. Instead there are cells in the bone marrow and lymph nodes that are 'transformed'. These transformed cells have the capability of becoming reactivated and infective under situations of stress and illness in the cat. Fifty percent of these cats will maintain this latent state. Forty percent of cats will clear the virus from their system. Ten percent of cats will reactivate the transformed cells and become viremic again. This latent state represents about 30% of FeLV challenged cats.
Cats that become immune carriers have created effective antibodies that neutralize the virus, but the virus still lives inside cells of the cat's body. Under normal circumstances the virus is trapped in the cell and killed if it emerges from the cell. If the cat becomes stressed or its immune system is compromised, the virus can emerge and the cat will shed FeLV, potentially infecting other cats. This represents 1-2% of FeLV infected cats. The course that the virus takes depends on the ability of the cat's immune system to neutralize the virus.

FeLV is spread by direct contact with tears, saliva and possibly urine and feces of an infected cat. Prolonged close contact or a bite is required for successful transmission of the virus because it is quickly inactivated by warmth and drying. It does not survive long outside the cat.

The clinical signs of FeLV vary widely from a general malaise, to anaemia, leukemia, weight loss, enlarged lymph nodes, reproductive problems, diarrhea, and tumor development. This virus attacks and disables the immune system so the cat may also present with a myriad of signs related to secondary infections and diseases.

Tests can be conducted on saliva, blood and bone marrow. At different stages of the virus' infection it will appear in these various locations. It can be elusive to detect and may require retesting to verify the test results at a later date. . The carrier state can be difficult to detect if, for example, the virus is present in the bone marrow but absent from the peripheral blood or if only 'transformed' cells are present. Testing for this virus can be challenging and frustrating. Speak to your veterinarian about testing for your individual cat's situation.

There is a vaccine available that, when used in an FeLV negative cat, can help the cat create effective antibodies that kill the virus and reduce the likelihood of a cat remaining in the carrier state. FeLV can exist in a cat in a dormant state for many years. Stress, illness and immunosuppressive drugs can cause it to come out of this dormant state and become actively destructive in the cat. Thus a vaccinated untested young kittens may still get FeLV if the kitten was positive prior to the vaccination. Frustratingly a kitten or cat that tested negative and is vaccinated can also get FeLV. This can happen if the test was a false negative: for example the infection had not yet reached the blood or saliva at the time of testing or if the virus is latent within 'transformed' cells within the cat. Testing is useful, just keep in mind that the test may need to be repeated or verified by a subsequent test.

There is no specific treatment for cats with disease associated with the FeLV. Treatment is aimed at addressing the signs that appear in each individual cat. Cats can survive long periods of time in an FeLV positive status. If possible avoid situations and drugs that stress or reduce the cat's immunity.

Prevention includes testing and vaccinating cats that are at risk: this is cats that go outdoors, are exposed to other cats and cats that live with an FeLV positive cat. Reduce exposure of your cat by keeping it indoors and away from cats with an unknown FeLV status. FeLV is a debilitating and potentially fatal disease, so if your cat tests positive, keep him or her indoors for the cat's own protection and to eliminate the chance your cat will infect others.

For more information on Feline Leukemia Virus, talk to your veterinarian.