Feline Leukemia Virus (FeLV)

Feline Leukemia Virus (FeLV) is a virus that impacts cats; can cause cancer and blood disorders; and can compromise the immune system, making them more susceptible to infections. FeLV only affects cats and does not pose any risk to other species. This virus is found worldwide and affects approximately 2-3% of cats in the United States.

Prevention

FeLV is not considered a population risk in shelters, but screening cats who will be co-housed with unfamiliar cats is important. However, since FeLV is spread via prolonged, close contact and bite wounds, FeLV-infected cats should not be housed with uninfected cats in the shelter environment. All cats should be screened for infection prior to group housing. 

FeLV-positive cats can be safely housed with the general shelter population when housed in individual housing units or in small group housing with other FeLV-positive cats. Because FeLV can cause immune-suppression, extra care should be taken to prevent handling unhealthy cats before handling FeLV-positive cats. 

There are several FeLV vaccines available that appear to be highly protective against progressive infection and subsequent associated conditions. This vaccine is considered non-core and is not included in most shelter vaccine protocols. However, it can be considered for owned cats at a higher risk for infection, such as cats allowed outdoors or those who may co-mingle with FeLV-infected cats. 

The virus is short-lived in the environment and is not spread through casual handling of infected cats. Routine sanitation will prevent the transmission of retroviruses.

Clinical Signs

FeLV does not cause any signs of disease in early infection. Over time, a progressively infected cat’s health is likely to deteriorate, and the cat may show signs of illness such as:

• Inappetence
• Weight loss
• Enlarged lymph nodes
• Pale gums
• Neurologic disease
• Eye conditions
• Chronic inflammation
• Signs of secondary infections

Not all cats exposed to FeLV will develop an infection. The course of the disease is complex, and three different scenarios are possible:

• Abortive infection: the cat’s immune response eliminates the virus before it can spread to the rest of the body. 
• Regressive infection: some of the virus spreads to organs and bone marrow, but the immune response eliminates the circulating virus soon after infection. These cats are not as contagious as progressively infected cats and are less likely to develop FeLV-associated diseases. 
• Progressive infection: the cat’s immune response fails to contain the virus and it spreads to the rest of the body. These cats are always infectious to other cats and are at high risk for FeLV-associated disease.

It is important to note that a cat’s infection status might change over time. For example, if a cat with a regressive infection experiences immune suppression, the virus may be reactivated, causing the infection to become progressive. 

Testing Recommendations

FeLV-infected cats develop cancer, immune-mediated disease, and blood disorders more commonly than uninfected cats. If a cat has recurrent or chronic infection or an illness not responding to treatment, FeLV status should be investigated. In addition, any kitten showing signs of disease that would require significant medical intervention should be tested for FeLV. The median survival time after diagnosis for progressively FeLV-infected cats is 2.5 years.

Transmission

The FeLV virus is shed primarily in saliva, but can also be excreted in nasal secretions, feces, milk, and urine. The typical routes of transmission are via nursing, mutual grooming, and sharing water or food dishes, but transmission via bite wounds is also possible. It can be transmitted to kittens in utero or during nursing or grooming after birth. 

Diagnosis

Testing for FeLV infection is complex due to the different levels of infection. The most common test used to screen for FeLV infection is a combination FeLV/FIV point of care (POC) blood test which detects the FeLV antigen. If the POC test is positive, this could mean that the infection will become abortive (clear the virus), regressive, or an actual progressive disease. For further confirmation of a positive result, a quantitative PCR can be performed to determine if the infection is regressive or progressive. Regardless of infection type, cats testing positive on a POC test are likely to be contagious at that point in time.

Note About Testing in Shelters

The decision to test cats for FeLV in a shelter must be evaluated in consideration of the best allocation of resources. Since the prevalence of infection in healthy cats is relatively low, shelters may elect to limit testing to situations where the diagnostic value of the test will be most beneficial. These situations include when a cat is more likely to be infected (symptomatic or history of exposure) or to avoid in-shelter risk (e.g., group housing, blood donors). When cats are not tested in the shelter, adopters should be aware so they can alert their personal veterinarian.

Treatment & Management

While there is no specific treatment for FeLV infection, the level of management depends on the type of infection. Abortive or regressive infections do not typically need any treatment or supportive care. Progressively infected cats will likely need supportive care for any FeLV-associated disease, which can range from antibiotics for infections to more significant interventions, including humane euthanasia for blood disorders or cancer.

Once a cat is diagnosed with regressive or progressive FeLV infection, a veterinarian should regularly examine them and provide routine preventive care. Stress can lead to an increased susceptibility to illness, so it’s important to maintain a low-stress environment for these cats.

Outcome decisions for cats testing positive for FeLV should be based on prognosis, quality of life, and the organization’s capacity for care. In general, healthy cats testing positive for FeLV on a POC test are still good candidates for adoption or transfer. However, humane euthanasia should be considered for sick, progressively infected cats due to concerns regarding their prognosis and quality of life.   

Foster and Adoption Considerations

FeLV-infected cats may be considered for foster and adoption programs depending on a shelter’s resources. Marketing and education can help prevent prolonged lengths of stay. FeLV infection status should be a consideration when adopting into a home with other cats. The risk to resident cats is reduced if the cats share the same FeLV status, if resident cats are fully vaccinated against FeLV, or if the infected cat has a regressive infection. However, the adopter’s veterinarian is best suited to gauge the level of risk.

Resources and Downloads

• ASV Position Statement on FeLV and FIV Testing and Management in Animal Shelters
• AAFP Feline Retrovirus Testing and Management Guidelines
• Infectious Disease Management in Animal Shelters Textbook
• Webinar: Who Should I Be Testing? Untangling the Mystery of FeLV and FIV in Shelter Cats
• FeLV Testing and Management Sample Protocol