Feline Leukaemia Virus (FeLV): Transmission, Testing, and Living With It
Feline leukaemia virus, commonly referred to as FeLV, is one of the most significant infectious diseases affecting domestic cats worldwide. Despite its name, the virus does not exclusively cause leukaemia — it is better understood as a retrovirus that compromises the immune system, leaving infected cats vulnerable to a wide range of secondary conditions. For many owners, a positive FeLV diagnosis raises immediate and frightening questions. Understanding how this virus actually behaves can help you make informed, compassionate decisions for your cat.
What Is Feline Leukaemia Virus?
FeLV is a retrovirus, meaning it integrates its genetic material into the host cell's DNA. Once inside the body, it primarily targets rapidly dividing cells, particularly those in the bone marrow and immune system. This is why infected cats often suffer from anaemia, immune suppression, and, in some cases, lymphoma and leukaemia — conditions that arise from the virus's interference with normal cell replication and immune function.
The virus exists in several forms. Some cats exposed to FeLV mount an effective immune response and eliminate the virus entirely — these cats are said to be transiently infected and go on to develop immunity. Others become progressively infected, meaning the virus integrates permanently into bone marrow cells and the cat remains infectious for life. A third group, termed regressively infected, carry the virus in a latent state without testing positive on standard antigen tests, though they may still be subject to future health complications.
How Is FeLV Transmitted?
FeLV spreads most efficiently through prolonged, close contact between cats. The virus is shed in high concentrations in saliva, making mutual grooming and shared food and water bowls the most common transmission routes. Nasal secretions, urine, faeces, and milk can also carry the virus, though at lower concentrations. Biting is a particularly effective transmission route, which is why outdoor, intact male cats who fight are at significantly elevated risk.
The virus does not survive long outside a host — it is fragile in the environment and easily destroyed by standard household disinfectants. This means FeLV is not easily spread through brief, casual contact or contaminated surfaces in the way that some other feline pathogens are. It requires sustained, intimate contact, which is why the greatest risk is between cats that live together or socialise closely outdoors.
Kittens are more susceptible than adults, and in-utero transmission from an infected queen to her kittens is well documented. Kittens born to FeLV-positive mothers carry a high risk of progressive infection, and many will not survive their first year of life.
Testing for FeLV
Testing for FeLV is strongly recommended for all cats, particularly those being introduced to a new home, cats with access to the outdoors, and any cat with unexplained illness. The standard test used in veterinary practice is an ELISA (enzyme-linked immunosorbent assay) test, which detects FeLV antigen — specifically the p27 protein — in the blood. These tests can be run as in-house snap tests with results in minutes, or sent to external laboratories for confirmation.
A positive ELISA result should always be confirmed with an additional test — either immunofluorescence assay (IFA) or PCR — to distinguish between transient and progressive infection. Cats testing positive on ELISA but negative on IFA may still clear the virus with time and should be retested after twelve weeks. A confirmed positive on both tests indicates progressive infection, and those cats should be considered persistently infectious.
It is worth noting that false positives can occur, particularly in low-prevalence populations, which is another reason confirmatory testing matters before making irreversible decisions about a cat's care.
Health Implications of a Positive Diagnosis
A progressively FeLV-infected cat faces a range of potential health challenges stemming from the virus's suppression of normal immune function. These include:
- Anaemia, which may be regenerative or non-regenerative depending on the mechanism
- Increased susceptibility to bacterial, viral, and fungal infections
- Lymphoma, the most common FeLV-associated cancer
- Leukaemia and other bone marrow disorders
- Reproductive failure in breeding queens
- Neurological signs in some cases
Prognosis varies considerably. Some cats with progressive infection live for several years with good quality of life, while others decline more rapidly. Regular veterinary monitoring — typically every six months — allows early detection of complications and timely intervention.
Living With an FeLV-Positive Cat
An FeLV-positive diagnosis does not mean the end of a good life for your cat. With attentive care, many cats with FeLV can remain comfortable and happy for years. The key priorities are keeping the cat indoors to prevent exposure to additional pathogens, feeding a nutritionally complete diet, and avoiding raw meat, which carries a risk of bacterial and parasitic infection that an immunocompromised cat is less equipped to handle.
FeLV-positive cats should be kept separate from FeLV-negative cats. This is not merely about protecting other cats — it is also about protecting the positive cat from infections that healthy cats could shrug off but that may be serious for an immunosuppressed individual.
Vaccination and Prevention
A vaccine against FeLV is available and is classified as a non-core vaccine in the UK — meaning it is recommended based on individual risk rather than universally. Cats with outdoor access or those in multi-cat environments with unknown FeLV status should be vaccinated. The vaccine does not offer complete protection, but it significantly reduces the likelihood of progressive infection following exposure.
Retesting all cats in a household when a new cat is introduced, and quarantining new arrivals before integration, are straightforward measures that reduce transmission risk substantially. FeLV is a serious diagnosis, but with knowledge, care, and regular veterinary support, affected cats can still live a life worth living.