Feline leukemia virus (FeLv) is a common infection of cats which is caused by a retrovirus. A retrovirus is a type of virus that hijacks the host cell and makes copies of itself. This virus is highly infective to cats, however, it is unstable in the environment and susceptible to most disinfectants. FeLv is transmitted mainly through the saliva of infected cats to noninfected cats by licking, biting, or other social contact. A cat may also transmit the virus via infected ocular secretions and urine, or from an infected mother to her kittens during pregnancy

How is it transmitted?

A cat exposed to this virus may produce antibodies, but these antibodies do offer immune protection as conventional antibodies do. Instead, the antibodies can actually cause organ damage and immunosuppression. Some infected cats are able to eliminate the virus and become resistant to its effects. However, in cats that do not become resistant, the virus spreads from the lymph nodes to the bone marrow- where it replicates and enters the blood stream. The virus then spreads to other tissues, including the salivary glands. Once FeLv reaches the salivary glands, it can then spread to other cats through saliva.

The Feline Leukemia virus suppresses the immune system and makes the cat vulnerable to infections and some types of cancer. In fact, researchers estimate that one third of all cancer related deaths in cats result from this virus.

Some cats may remain completely normal, but can carry the virus and spread it to other cats.  These cats may eventually become ill, especially if stressed or if they have a weakened immune system. 

Signs of Feline Leukemia 

A cat may have the Feline Leukemia virus and be completely healthy.  Since FeLv causes the animal to be susceptible to infections, the clinical signs seen with this disease are related to other conditions and may include:

• Inappetence (lack of hunger)

• Respiratory disease

• Weight loss

• Dehydration 

• Fever

• Diarrhea

• Lethargy

• Vomiting

• Infertility

• Enlarged lymph nodes

• Weakness

Diagnosis

There are two common types of diagnostic tests for FeLv.  One is an enzyme linked immunosorbent assay, also known as an ELISA test.  This test is ran in the clinic and only takes a few minutes for the results. The other type of test for FeLV is the immunofluorescent (IFA) assay, which is sent out to an outside diagnostic laboratory for analysis.

In both, the tests detect antigens, or substances that produce an immune response, to FeLV in the blood.  False negative results can occur if the animal is not producing antigens to the virus. In those cases, a special test that involves collecting a sample of the bone marrow could be performed to confirm a diagnosis, however, this test is rarely performed. Since the virus can be latent in the body, diagnostic testing can be confusing and may need to be repeated to confirm a diagnosis. 

Treatment:

Unfortunately, a cat with Feline Leukemia will have decreased immunity which makes them susceptible to various infections and cancers. Infected cats should be kept away from other cats, to keep the virus from spreading. 

Immunotherapy with a medication called interferon has been used to help treat clinically ill cats. Oral interferon has been used successfully to treat clinically ill cats. Interferon is an immune booster and helps the cat produce an immune response.

Antiviral Medication  There have been several studies where FeLV positive cats have been treated with human antiviral medications. Unfortunately, there is no treatment that has proved effective in clearing the virus from the cat’s body. 

 Prevention

The prevalence of cats is much higher in cats that have access to the outdoors because the virus spreads through both fighting and social contact.  The best way to prevent exposure of your cat is to keep your cat indoors and away from potential infected cats. With a multiple cat household, it is recommended to check all new cats for the virus before allowing them contact with other cats in your household.

There are many brands of Feline Leukemia vaccines available.  Some are made from a virus that has been activated and there is one type that is a genetically altered virus.  Both have passed safety and efficacy studies and require an initial vaccination and then a booster within 3-4 weeks with annual revaccinations.    In general, it is recommended to test cats before vaccination.