Overview of Feline Leukemia Virus (FeLV)

Dog Breeds

The feline leukemia virus is a highly contagious retrovirus that can cause immunosuppression, secondary opportunistic infections and a number of neoplastic (cancerous) and hematologic (blood) abnormalities in cats.

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Definition of FeLV

The feline leukemia virus is a highly contagious retrovirus that can cause immunosuppression, secondary opportunistic infections and a number of neoplastic (cancerous) and hematologic (blood) abnormalities in cats. In fact, FeLV infection is one of the leading causes of death among companion felines and is responsible for more feline disease than any other identified infectious agent. It also contributes to cancer and a number of other diseases, due to its immunosuppressive effects.

Causes of FeLV

Most FeLV-positive cats become infected by direct contact with saliva or blood from the oral or nasal secretions of infected cats. This commonly occurs either through mutual cat-to-cat grooming, playing, shared water or food dishes, weeping wounds or cat bites. The virus is shed to a lesser extent in urine, feces and tears. FeLV can be transmitted through blood transfusions, and also in utero from an infected queen to her unborn kittens. Young newborns tend to be more susceptible than older cats to clinical disease and often contract the infection from the saliva of their infected mothers. Because this virus is not very hardy and is highly susceptible to environmental conditions such as heat and disinfectants, environmental contamination is an uncommon cause of FeLV infection.

However, not all cats exposed to FeLV become clinically ill. Some mount an effective immune response and eliminate the virus entirely. It appears that prolonged or repeated exposure to the virus is necessary for infection, especially in otherwise healthy adult cats. Stress, sickness, poor hygiene, overcrowding and poor nutrition can all weaken a cat’s immune system and make it more susceptible to infection.

Some cats mount an ineffective or incomplete immune response and become viremic, meaning that the virus replicates in their lymphoid tissue and bone marrow and enters circulation. If an effective immune response eventually kicks in, some of these cats will become latent carriers, while others will become persistently infected and will shed the virus continually in their saliva and other body secretions. It is this latter group of cats that is most likely to develop blood abnormalities, anemia, cancer, chronic opportunistic infections or other diseases due to the progressive weakening of their immune systems caused by FeLV.

There are three subgroups of FeLV. Subgroup A is the most common and is present in all cats with observable disease and viremia (presence of the virus in circulation). It is responsible for the immunosuppression that makes FeLV-positive cats susceptive to so many other infections and illnesses. Subgroup B occurs in combination with subgroup A in about one-half of cats and appears to be responsible for FeLV-associated cancers. Subgroup C is uncommon and occurs together with subgroup A in cats that develop nonregenerative anemias and bone marrow complications.

Prevention of FeLV Infection

There is no fool-proof way to prevent FeLV infection, although keeping cats indoors and away from free-roaming strays is perhaps the best prevention. Uninfected (naïve) cats should be kept away from FeLV-positive cats and should not share their food and water bowls or litter boxes. They should not live in the same household due to the high risk of contagion. There is an FeLV vaccine that may be useful in high-risk cats, such as those kept primarily or exclusively outdoors. However, it has been associated with vaccine-induced injection site sarcomas and currently is not recommended for use in indoor-only animals. New cats should not be introduced into multi-cat households or catteries without first being tested twice (3 months apart) with an IFA test (see, PetWave article on FeLV – Diagnosis and Test). They should be isolated from other cats during this process. In addition, cats should be tested for FeLV before being bred, and should not be used in a breeding program if they are FeLV-positive.

Special Notes

Feline leukemia virus has not been shown to be transmittable from infected cats to people. However, it reportedly can replicate in human cells in a laboratory setting. People with weakened immune systems (such as those who are HIV-positive), and women who are pregnant or considering becoming pregnant, probably should avoid contact with FeLV-positive cats as a precautionary measure. Unfortunately, there is no current cure for FeLV infection.

Symptoms and Types