Feline leukemia virus (FeLV), is a retrovirus, so named because of the way it behaves within infected cells. All retroviruses, including feline immunodeficiency virus (FIV) and human immunodeficiency virus (HIV), produce an enzyme, reverse transcriptase, which permits them to insert copies of their own genetic material into that of the cells they have infected.

Although related, FeLV and FIV differ in many ways, including their shape: FeLV is more circular while FIV is elongated. The two viruses are also quite different genetically, and their protein consituents are dissimilar in size and composition. Although many of the diseases caused by FeLV and FIV are similar, the specific ways in which they are caused differs.

FeLV-infected cats are found worldwide, in the United States, approximately 2 to 3% of all cats are infected with FeLV. Rates rise significantly, 13% or more in cats that are ill, very young, or otherwise at high risk of infection.

Cats infected with FeLV serve as sources of infection. Virus is found in saliva and nasal secretions, but also in urine, feces, and milk from infected cats. Cat-to-cat transfer of virus may occur from a bite wound, but rarely through the shared use of litter boxes and feeding dishes. Infected mother cast will transmit the disease to her kittens, either before they are born or while they are nursing. FeLV doesn’t survive long outside a cat’s body, most likely less than a few hours.

Cats at greatest risk of infection when living with infected cats, being left outside where they might be bitten by infected cats and kittens born to infected mothers. Kittens are much most susceptible, and therefore are at the greatest risk of infection.

Feline leukemia virus adversely affects the cat’s body in many ways. It is the most common cause of cancer in cats, it may cause various blood disorders, and it may lead to a state of immune deficiency that hinders the cat’s ability to protect itself against other infections. Bacteria, viruses, protozoa, and fungi that may be found in the everyday environment where they usually do not affect healthy animals can cause severe illness in those with weakened immune systems.

During the early stages of infection, it is common for cats to exhibit no signs of disease at all. However, over time the cat’s health may deteriorate or be characterized by recurrent illness interspersed with periods of relative health. Signs can include loss of appetite, weight loss (followed by severe wasting in the late disease progress), poor coat condition, enlarged lymph nodes, persistent fever, pale gums and inflammation, diarrhea, seizures, a variety of eye conditions. Unspayed females might experience abortion of kittens.

At an early stage of virus infection some cats are able to mount an effective immune response, eliminate the virus from the bloodstream, and halt progression to the secondary viremia stage.

At the secondary viremia a persistent infection of the bone marrow and other tissue is developed. This is a point of no return. The majority of cats at the second stage will be infected for the rest of their lives. Blood testing is necessary to detect that virus.

The only sure way to protect cats is to vaccinate and prevent their exposure to FeLV-infected cats. Make sure that newly adopted cats don’t carry the virus. Keep cats indoors, or supervise them outdoors.

Unfortunately, many FeLV-infected cats are not diagnosed until after they have lived with other cats. In such cases, all other cats in the household should be tested for FeLV. Ideally, infected and non-infected cats should then be separated to eliminate the potential for FeLV transmission.

The life expectancy of a cat infected with FeLV can vary. With appropriate care and under ideal conditions, infected cats can remain in apparent good health for many months, maybe up to 2 - 3 years. If your cat has already experienced one or more severe illnesses as a result of FeLV infection, or if persistent fever, weight loss, or cancer is present, a much shorter survival time can be expected.

It is recommended that pregnant women, people with suppressed immune systems, people with AIDS, the very young, and the very old avoid contact with FeLV-infected cats.