Feline Leukemia Virus (FeLV in Cats)

Feline Leukemia Virus (FeLV in Cats)

Animals Affected



Feline leukemia virus, or FeLV, is a virus that infects cats.   It is related to FIV and HIV, the virus that causes human AIDS.  There is no evidence that FeLV can infect humans.

FeLV does not sicken every cat that is infected with it.   However, some cats that are infected with FeLV suffer suppression of the immune system or develop cancers.   These cats often die from the virus.

FeLV is much more contagious than FIV or HIV.   It can be spread by casual contact.   Cats may contract the virus by sharing food or water with an infected cat.  Mothers can pass the virus to kittens.  The virus can be spread among cats that groom each other or fight with each other.

Cats that are kept exclusively indoors in stable households have almost no chance of contracting FeLV.  Maintaining cats in an indoor environment is the most effective way to prevent transmission of the virus.  A vaccine against FeLV is available, but it may not be 100% effective.  The FeLV vaccine has been linked to the development of tumors in some cats.  The vaccine is recommended only for cats that have access to the outdoors.

Although infection with FeLV is deadly in many cases, it is estimated that 2/3 of cats infected with the virus will not show signs of serious illness.  Most of these cats survive. Most cats that become sick due to FeLV infection succumb to the virus.


Some cats that are infected with FeLV show no symptoms whatsoever.  Others may show mild, transient symptoms.   Members of a final group become ill and may exhibit the following symptoms.

Risk Factors and Prevention

  • Access to outdoor areas is the main risk factor.  Cats that do not go outside have almost no chance of contracting FeLV.
  • Young male cats, aged 1 – 6 years, have the highest rate of FeLV infection.
  • FeLV can spread among cats living in the same household.
  • Kittens born to an FeLV-infected mother may contract the disease from her.

A vaccine against FeLV is widely used.   Experts warn that the vaccine may not be 100% effective.  However, the vaccine does appear to help prevent infection with the Virus.

Vaccines against FeLV have been linked to the development of tumors at the injection site in some cats.  Because of this, most veterinarians recommend vaccinating only at-risk individuals.


Approximately 2/3 of FeLV-infected cats suffer no long-term complications from the disease.  The remaining 1/3 generally suffer chronic health problems.  Most members of this group die within two years of diagnosis.  The following complications also are common in this group.

  • Because FeLV suppresses the immune system, infected cats often develop recurrent or persistent infections with bacteria, fungi, or parasites.  This is similar to the complications of infection with FIV.
  • FeLV may lead to development of cancers such as lymphoma or leukemia.
  • As was mentioned above, stomatitis is a common complication of FeLV infection.


Basic blood or saliva tests can detect the presence of the virus.  In rare instances, advanced tests on bone marrow or other tissues are performed.

Cats that test positive for FeLV should be re-tested periodically.  Some members of this group eliminate or suppress the virus and may test negative in the future.


There are few effective treatments for FeLV-infected cats.   In most cases, treatment focuses on eliminating bacterial infections or cancers that develop as a result of immune system suppression, rather than combating the virus itself.

Some FeLV-infected cats may benefit from the use of immune system stimulators, such as interferon.  Results of studies involving antiviral drugs (such as AZT, which is used to combat HIV) have not been promising.


Cats that are infected with FeLV may be at increased risk from the diseases that are spread by fleas.  Appropriate flea control should be used in all FeLV-infected cats.

Medications that weaken the immune system, such as prednisone, should be avoided if possible in any cat that has tested positive for FeLV in its lifetime.  These cats also should avoid extreme stress because it can suppress the immune system.

To prevent spreading the disease to other cats in the household, FeLV-infected cats must be strictly isolated from uninfected cohorts.  There must be no contact between the cats. Food bowls, water bowls, bedding, and brushes must not be shared.

FeLV-infected cats should not be fed raw meat or allowed to hunt, because they may be at increased risk from the diseases and parasites that are often present in raw meats.

FeLV-infected cats should be kept indoors. This prevents spread of the disease to other cats in the environment.  It also reduces the likelihood of the infected cat developing infections with bacteria, fungi, and parasites.


FeLV does not appear to be capable of infecting humans.  However, FeLV-infected cats are at increased risk of contracting diseases such as ringworm, Salmonella, and E. coli which do pose a threat to people.  Owners of FeLV-infected cats should be aware of this.

Copyright © Eric Barchas, DVM.  All rights reserved.
The contents of this page are provided for general informational purposes only. Under no circumstances should this page be substituted for professional consultation with a veterinarian.