Everything You Need to Know About Feline Leukemia Virus

What is Feline Leukemia Virus?

Feline Leukemia Virus (FeLV) is a serious life-threatening viral infectious disease that affects kittens and cats. The main viral structures include an envelope, RNA, and several proteins. Once the virus enters the cytoplasm of the host cell, it uses an enzyme called reverse transcriptase to create DNA from RNA. Retroviruses tend to cause serious illnesses such as HIV, AIDS, and cancer in humans. 

How is FeLV Transmitted?

FeLV can be transmitted from one cat to another via saliva, urine and feces. It is most commonly spread via saliva from grooming. It is also possible to be transmitted to kittens in utero as it is able to cross the placental barrier (protects the unborn), as well as being transmitted through milk during nursing. Luckily, not every cat or kitten exposed to this virus becomes infected. In some cats, the immune system is able to clear the virus. If not cleared, the virus causes infection of the bone marrow that can never be completely eradicated. Once in the bone marrow, the virus can either cause a progressive infection or remain latent (present, but not currently causing clinical symptoms). A latent infection has the potential of reactivating at any point in time and causing immunosuppression. Cats with a latent form of FeLV are carriers meaning their are still able to transmit the infection to other cats. Indoor cats are much less likely to be exposed and become infected compared to outdoor cats. 
When adopting a cat or kitten, it is important to keep them separated from other cats in your household until you are able to have him/her tested for FeLV, FIV (Feline Immunodeficiency Virus) and other contagious infectious diseases that can be easily transmitted in a shelter setting. A cat that tests positive for FeLV should remain isolated from other cats indefinitely. 

What are Symptoms of FeLV?

The bone marrow is a major site responsible for producing red and white blood cells (RBC, WBC). Since FeLV infects the bone marrow several other secondary diseases can result, such as anemia (decreased RBCs) and other cytopenias (decreased blood cells), as well as cancers like lymphoma and leukemia. White blood cells are necessary for fighting off infections and illness. The bone marrow is able to produce and release more WBCs in response to infection. If the infected bone marrow is unable to do this, cats infected with FeLV will experience more infections of greater pathology as well as immunosuppressive syndromes.

 Clinical symptoms include, but are not limited to:

  • pale, dry gums
  • enlarged lymph nodes
  • weight loss
  • inappetance
  • weakness or lethargy
  • fever
  • diarrhea
  • chronic bladder, respiratory and/or skin infections
  • icterus mucous membranes (sign of liver failure)
  • stomatitis (infection/inflammation of the mouth)

How is FeLV Diagnosed?

FeLV is typically diagnosed through enzyme-linked immunosorbent assay (ELISA) and immunofluorescent assay (IFA) tests. An ELISA snap test is able to detect FeLV antigens in whole blood or serum and can be performed in-house. It is the best test at detecting early and late stage viremia and is commonly used as a screening test for FeLV. An immunofluorescent assay detects antigens in white blood cells (leukocytes) and is typically performed to confirm an ELISA positive test. Polymerase chain reaction (PCR) is another method of testing that detects viral DNA, but is less frequently utilized. Re-testing within a month is recommended, as an infection may be recent and show up as a false negative. 

How Can I Prevent FeLV?

There is currently no treatment or cure for FeLV. Supportive care can help make an infected patient more comfortable if they are suffering from symptoms of a secondary condition, such as anemia or an infection. There is a vaccination for FeLV, however, it is not 100% effective. For cats less than 1 year old (highest risk group), the FeLV vaccine is considered a core vaccine, which means it is highly recommended for all cats by veterinary professionals.


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