Feline leukemia virus (FeLV)

FeLv is a devastating disease in cats. It is responsible for more deaths than any other feline infectious disease. Three percent of domestic cats are infected with FeLV but it is believed that in the feral population the infection rate may be as high as 11%. Although FeLV does not infect humans, it is very infectious between cats. FeLV belongs to the same family of viruses as FIV which has been discussed in another article.

The science of FeLV

When a cat is exposed to FeLV, there are four possible outcomes. Thirty percent of cats have bodies which are are resistant to infection and produce a strong immune response to the virus. About 30-40% of cats become infected and are unable to clear the infection, and as a result become persistently infected. These cats excrete the virus in their saliva and are a permanent threat to other cats. About 25-30% of cats have bodies which cannot resist the virus. However, these cats do not become persistently infected immediately. Instead, the virus hides in the bone marrow for up to 30 months. Eventually, these cats either clear the virus or become persistently infected. Finally, a in small percentage of cats the infection becomes sequestered. In these animals the virus hides from the immune system, predominantly in the the bone marrow. They do not ever clear the virus but they are also not contagious and are unlikely to develop the full blown disease.

FeLV is capable of producing a wide variety of associated diseases and symptoms including degenerative diseases, such as anemia, liver disease, intestinal disease and reproductive problems. In some cats, the virus produces cancerous diseases, such as lymphosarcoma and leukemia. Many cats suffer from suppressed immune systems and a variety of illnesses, depending on which organ is involved. Cats whose immune systems are depressed by FeLV are susceptible to a wide variety of infectious diseases and other problems, such as chronic respiratory infections, chronic gingivitis and stomatitis, feline infectious peritonitis, poor healing of wounds and abscesses and chronic generalized infections.

There are three types of FeLV:

  1. FeLV-A is found in all FeLV cats and it is mainly responsible for severe immunodeficiency.

  2. FeLV-B is found in about 50% of infected cats and causes cancers

  3. FelV-C is rare, only seen in about 1% of infected cats and is responsible for severe anemia.

About the virus

FelV is a retrovirus, an enveloped RNA virus which uses specific enzymes to translate its own RNA into DNA and incorporate that DNA into the body's DNA. Retroviruses were only discovered in late 1960s and did not gain their name till 1974.

In retroviral infection, a virus infects a new host through receptor proteins on cells at the infection site, much like a key fits into a lock. Once a cat is infected, the virus gains a foothold by undergoing a series of genetic mutations designed to invade new sets of receptors, allowing it to continually evade detection, attack, and ultimately shut down the body's defenses. This shutdown occurs when mutated versions of the virus infect and destroy the body's T cells, which are critical to immune function. Recently, studies on FeLV identified another factor in the infection process: a secondary retroviral receptor (or cofactor) that is crucial for the mutated, or T-cell adapted, virus to do its work. Without this receptor, appropriately dubbed FELIX, the virus would be unable to set up shop. Specific blocking of FELIX may bring a new way to treat FeLV in future.


There are two main tests for FeLV:

ELISA - enzyme-linked immunosorbent assay - The most common screening test for FeLV. It is a rapid test which is usually carried out at veterinary clinics. Vaccination for FeLV does not affect test results since the tests look for viral antigens, not antibodies.

IFA - the immunofluorescent antibody test - is used most commonly as a confirmatory test when false positive or false negative results are suspected from an ELISA test.

How is FeLV spread?

The virus does not survive outside the cat's body for long. Therefore infection normally happens through contact. The virus is found in saliva, urine and other secretions of infected cats. Cats sharing food bowls and litter trays can pass the infection to uninfected cats. Infected mothers pass the infection to their unborn kittens, all of whom are usually infected at birth. Bites following cat fights are a common means of spreading the disease. This is why FeLV is more common in cats which are allowed to roam outside. In general kittens are more susceptible, especially those under 4 months of age. These have the least ability to mount an effective immune response and so are most susceptible to FeLV.

Cats older than six months are relatively - but not absolutely - resistant, but high-level exposure to infected cats can overcome this resistance.

How can I protect my cat from FeLV infection?

There is a vaccine against FeLV, and all cats should be vaccinated. If your cat is an indoor cat the vaccine is less important but still it is recommended as a precaution. If you have a cat which is infected make sure that the contact between the FeLV+ cat and healthy cats is kept to a minimum. Ordinary household detergents and bleach effectively kill this virus; so make sure that food bowls and litter trays are cleaned regularly.


There is no cure for FeLV. The main ways of treating FeLV+ cats are protecting them from exposure to other diseases, ensuring good nutrition, giving regular vaccinations with killed virus vaccines (but not with a feline leukemia vaccine), reducing stress, controlling parasites, and early and aggressive treatment of any symptoms that appear. Because of the suppression of the immune system any bacteria infections have to be treated with antibiotics. In severe bacterial infection the vet can prescribe Propionibacterium acnes (Immunoregulin) Immunregulin is an immune stimulant that may increase the response of antibiotics to fight resistant secondary infections. It needs to be injected IV (intravenously) to insure that a sufficient concentration reaches the proper cells. Low-dose oral Interferon Alpha is also an immunostimulat and can be prescribed to help combating secondary infections.

Specific cancers associated with FeLV have their own chemotherapy treatment protocols. However, cats with cancer associated with FeLV have an average survival time of 6 months even with aggressive chemotherapy.

FeLV+ cats are susceptible to FIA (feline infectious anemia) which is caused by the parasite Mycoplasma (formerly known as Haemobartenella felis). This parasite destroys RBC's and can cause or worsen anemia. FIA is treated with antibiotics like tetracycline, Doxycycline, and Baytril, and possibly steroids (e.g. prednisone). If the anemia is severe a blood transfusion may be needed. Cats are good at hiding anemia so you need to watch closely for the signs which include pale gums, weakness, deep breathing, or possibly eating litter or licking concrete or walls.

Loss of appetite is common in FeLV+ cats and requires veterinary attention because just a few days of anorexia in cats can have serious consequences like liver damage.

FeLV+ cats are also susceptible to periodontal disease which can in turn cause other health problems; as infected gums bleed, bacteria can spread infection throughout the body. Your vet should exam your cat's mouth and treat any problems.

Virbagen Omega (rFeIFN) is a recombinant feline interferon which is authorized for use in cats as a treatment for FeLV and/or FIV in non-terminal clinical stages.

The vet can also prescribe so-called "Combination therapy". This is a "cocktail" of drugs, like those used for AIDS in humans. They contain ati-viral drugs such as AZT. This type of treatment is still very much experimental.


There is a good vaccination against FeLV. It is recommended that after the initial vaccination cats are given annual boosters. On average, FeLV vaccines prevent infection in about 80 to 90% of cats.

Living with an FeLV-infected cat

If you have a cat infected with FeLV you can help to prolong her life by limiting exposure to infection. As we have said earlier, FeLV positive cats have a very suppressed immune system. It is very important to keep your cat indoors. She should not be fed with raw meat or eggs since there is a risk of bacterial contamination. Always keep her dishes and litter tray very clean.

Regular checks with the vet are important to spot any developing problems in early stages. Your cat should be given any routine vaccinations, but it may be better to use vaccines based on killed pathogens. The vet will be able to advise you on this.

Unfortunately cats with FeLV are also prone to cancer. Treatment with chemotherapy can be attempted but think before making that decision. The chemotherapy will probably not prolong your cat's life for more that few months and there may be serious side effects. So you will have to weigh the benefits of the chemotherapy with the quality of your cat's life.

Note: This information is for guidance only. It is not intended to replace consultation with a licensed practitioner.

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