What is Feline Leukemia Virus?

Feline Leukemia Virus (FeLV) is a retrovirus that suppresses the immune system and can cause various cancers, particularly lymphoma. It is one of the most significant infectious diseases in cats, responsible for more cat deaths than any other infectious agent. FeLV inserts its genetic material into the cat's own cells, which is why there is no cure — once a cat is persistently infected, the virus remains for life.

However, not all cats exposed to FeLV become permanently infected. A cat's immune response determines the outcome: some cats fight off the virus completely, some become latent carriers, and others become persistently viremic (actively infected and shedding virus).

Symptoms

FeLV itself may not cause immediate symptoms. Cats can carry the virus for months or years before becoming ill. When disease develops, it can manifest as:

  • Chronic weight loss and poor appetite
  • Recurrent infections (respiratory, skin, urinary)
  • Persistent fever
  • Pale gums (anemia)
  • Enlarged lymph nodes
  • Reproductive problems (infertility, abortions)
  • Lymphoma or other cancers
  • Gradual deterioration over months to years

The median survival time for persistently infected cats is approximately 2.5 years after diagnosis, though some cats live much longer with good care.

Transmission

FeLV is spread through prolonged close contact between cats:

  • Mutual grooming (saliva is the primary route)
  • Shared food and water bowls
  • Shared litter boxes
  • Bite wounds
  • From mother to kittens (in utero or through nursing)

The virus is fragile outside the body and survives only minutes to hours in the environment. Casual or brief contact is generally low risk.

Testing

Testing before vaccination is essential because the vaccine is ineffective in already-infected cats. Two tests are commonly used:

  • ELISA (SNAP test): Detects FeLV antigen in the blood; available as an in-clinic rapid test
  • IFA: Confirms persistent infection by detecting virus in white blood cells

All cats should be tested when first adopted, before exposure to other cats, and after any potential exposure to an infected cat.

Vaccination

The FeLV vaccine is classified as a non-core vaccine, recommended primarily for:

  • Cats that go outdoors
  • Cats in multi-cat households where FeLV status of all cats is unknown
  • Kittens (who are more susceptible to persistent infection)

Vaccination Schedule

  • Kittens: Two doses starting at 8 weeks, 3–4 weeks apart
  • Adult cats: Two doses 3–4 weeks apart
  • Boosters: Annually for at-risk cats; may be discontinued for strictly indoor cats

Living with an FeLV-Positive Cat

FeLV-positive cats can live comfortable lives with proper management:

  • Keep them strictly indoors to prevent exposing other cats
  • Feed a high-quality, balanced diet
  • Regular veterinary checkups every 6 months
  • Promptly treat any infections or illnesses
  • Keep in a low-stress environment
  • Do not introduce them to FeLV-negative cats
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