Feline Leukemia Virus: The Contagious Cancer That Vaccination Almost Eliminated

The Virus That Changed How We Think About Cat-to-Cat Contagion

When FeLV was identified in the 1960s and 70s, it was everywhere -- a common finding in multi-cat households, catteries, and shelters. It was the paradigm case that established cats as reservoirs for horizontally transmitted oncogenic viruses. The discovery that a cat could give another cat cancer through grooming and shared dishes fundamentally changed how veterinarians approached multi-cat household disease prevention.

The mechanism: FeLV is a retrovirus that integrates into the cat's DNA. In some cats, the immune system clears the virus within weeks (regressive infection -- these cats are not contagious and are protected from reinfection). In others, the virus establishes a progressive infection: persistent viremia, immunosuppression, and eventual development of lymphoma, anemia, or secondary infections that the compromised immune system can't handle. The outcome of FeLV exposure depends heavily on the cat's age and immune status at the time of exposure -- kittens and immunocompromised cats are at far higher risk of progressive infection than healthy adult cats.

Testing: Who Needs It and When

All new cats entering a household should be tested for FeLV (and FIV) before contact with resident cats. The standard test is an ELISA antigen test detecting p27 protein in the blood -- available in-clinic within minutes. A positive ELISA result should be confirmed with an IFA (immunofluorescence assay) or PCR, as false positives occur. Kittens can have maternal antibody interference in early testing -- retest at 16 weeks if a young kitten tests positive.

Test cats before annual boosters (FeLV vaccination recommendations depend on lifestyle risk). Cats with unknown history adopted as adults should be tested; cats with known outdoor exposure should be tested annually if they have contact with indoor cats. All sick cats with unexplained weight loss, chronic infections, or lymphoma should be tested regardless of vaccination history -- stress can reactivate latent infections.

FeLV-Positive Cats: What Life Looks Like

A newly diagnosed FeLV-positive cat with no current symptoms may live 2-3 years or more with appropriate care. Management: keep strictly indoors (to prevent transmission to other cats and to protect the immunosuppressed cat from infections from other animals), feed a high-quality diet, avoid raw meat (pathogen risk to the compromised immune system), have biannual vet checkups with bloodwork, treat secondary infections promptly and aggressively, and consider prophylactic dewormers and flea prevention.

FeLV-positive cats should live only with other FeLV-positive cats or alone -- not with FeLV-negative cats. Some studies suggest low-level transmission risk in household cohabitation, but the ethical position of most veterinarians is to keep positive and negative cats separated. Humane euthanasia is appropriate when quality of life is poor from lymphoma, severe anemia, or chronic uncontrolled infection -- not at diagnosis.

Vaccination: Who Should Get It

FeLV vaccination is available and effective. AAFP categorizes it as lifestyle-based (not core for all cats): recommended for kittens as part of the initial series, then continued for cats with outdoor access or contact with cats of unknown FeLV status. Indoor-only cats with no exposure risk can stop boosters after the kitten series (though some vets continue annual boosters given the severity of infection if exposure occurs). The vaccine doesn't cure infection -- it prevents progressive infection in exposed cats, and it significantly reduces transmission risk.