FeLV (Feline Leukaemia Virus): Testing, Prevention and Living with FeLV

Feline leukaemia virus (FeLV) is a retrovirus that infects cats worldwide and remains one of the most significant infectious causes of illness and death in domestic cats. Unlike many viral infections that cause acute disease, FeLV is insidious — it integrates its genetic material into the cat's own DNA, potentially leading to a range of chronic, life-threatening conditions including lymphoma, leukaemia, anaemia, and severe immune suppression.

"FeLV is not an immediate death sentence. With proper management, many FeLV-positive cats live comfortable lives for years. But early detection through testing is absolutely essential — both for the individual cat and for preventing spread to others." — Dr. Sarah Chen, DVM

FeLV is classified into several subgroups (FeLV-A, B, C, and T), each associated with different disease manifestations. FeLV-A is the most common and is present in all naturally infected cats. It causes immune suppression and is the transmissible form of the virus. FeLV-B arises from recombination with endogenous retroviral sequences and is associated with tumour development (lymphoma). FeLV-C, though rare, causes severe red blood cell aplasia (pure red cell aplasia), while FeLV-T targets T lymphocytes and causes profound immune deficiency.

The virus is shed primarily in saliva, but also in nasal secretions, urine, faeces, and milk. Transmission occurs most commonly through prolonged close contact — mutual grooming, shared food and water bowls, shared litter trays, and bite wounds. A mother cat can also transmit the virus to her kittens in utero or through nursing. Casual or brief contact is generally considered low risk, though not zero risk.

Understanding FeLV is the first step in protecting your cat. Whether you are bringing home a new kitten (see our new kitten checklist) or managing a multi-cat household, knowledge about testing, vaccination, and transmission prevention is essential.

Testing for FeLV is a cornerstone of responsible cat ownership and veterinary medicine. Early detection allows for proper management, prevents transmission, and informs vaccination decisions.

When Should Cats Be Tested?

• All new cats and kittens — Before they are introduced to a household, regardless of their apparent health status. This applies to adopted, rescued, stray, and purchased cats.
• After potential exposure — If your cat has had contact with a known or suspected FeLV-positive cat, had a bite wound from an unknown cat, or escaped outdoors.
• Before initial FeLV vaccination — Vaccination does not benefit a cat that is already infected.
• When illness develops — Cats presenting with unexplained weight loss, chronic infections, anaemia, lymphoma, or immune-mediated disease should be tested.
• Annually for at-risk cats — Outdoor cats, cats in multi-cat households, and cats with ongoing exposure risk benefit from regular screening.

Testing Methods

• SNAP ELISA test (in-clinic) — Detects FeLV p27 antigen in blood. Results are available in approximately 10 minutes. This is the most common screening test and is highly sensitive. A positive result indicates viraemia (virus present in the bloodstream) but should be confirmed with a secondary test.
• IFA (Immunofluorescent Antibody) test — Detects virus within white blood cells and platelets, indicating bone marrow infection. A positive IFA generally indicates a more advanced, persistent infection and is associated with a poorer prognosis. IFA is often used as a confirmatory test following a positive ELISA.
• PCR (Polymerase Chain Reaction) — Detects proviral DNA integrated into the cat's genome. PCR can identify cats with latent (regressive) infections that may test negative on ELISA and IFA. It is the most sensitive test available and is particularly useful in discordant or ambiguous cases.

Understanding Results

A positive ELISA should always be confirmed, ideally with an IFA or PCR test, because false positives can occur (though they are uncommon). If the initial ELISA is positive but the IFA is negative, the cat may be in the early stages of infection and could potentially clear the virus. Retesting in 30 to 60 days is recommended. Cats that remain ELISA-positive and IFA-positive at retest are considered progressively infected and are persistently viraemic.

Not every cat exposed to FeLV develops the same outcome. The cat's age, immune status, viral load, and duration of exposure all influence what happens after infection. Modern veterinary understanding recognises four main outcomes.

1. Abortive Infection

Cats with robust immune systems may mount an effective immune response that eliminates the virus before it establishes a significant infection. These cats will test negative on all tests and develop immunity. This outcome is most common in healthy adult cats with brief, low-level exposure.

2. Regressive (Latent) Infection

In this outcome, the cat's immune system controls the virus but does not completely eliminate it. The virus becomes integrated into the cat's bone marrow DNA in a dormant state. These cats typically test ELISA-negative but PCR-positive. They are not actively shedding the virus and are generally not considered contagious under normal circumstances. However, severe stress, immunosuppression, or certain medications (such as corticosteroids) could potentially reactivate the virus. Most regressively infected cats live normal lifespans.

3. Progressive Infection

This is the most concerning outcome. The cat's immune system fails to control the virus, which replicates continuously in the bone marrow and is shed in large quantities. These cats test positive on ELISA, IFA, and PCR. They are persistently viraemic and contagious to other cats. Progressive infection is associated with the development of FeLV-related diseases and a shortened lifespan — approximately 83 percent of progressively infected cats die within three years of diagnosis, though some live considerably longer with excellent care.

4. Focal (Atypical) Infection

Rarely, the virus may establish a localised infection in specific tissues (such as the mammary glands, bladder, or eyes) without widespread systemic viraemia. These cats may show intermittent or discordant test results and can be challenging to classify. Close monitoring and repeat testing are necessary.

Understanding which category your cat falls into is crucial for making informed decisions about management, housing, and prognosis. Your veterinarian can guide you through interpreting test results and developing an appropriate care plan.

FeLV is often referred to as a "helper virus" because its primary impact is not through a single disease but through its ability to cause or facilitate a wide spectrum of conditions. Progressively infected cats may develop one or more of the following complications.

Immune Suppression

The most common consequence of FeLV infection is progressive immune suppression. The virus destroys or impairs the function of key immune cells, leaving the cat vulnerable to opportunistic infections that a healthy cat would easily fight off. Chronic or recurrent upper respiratory infections, oral infections (stomatitis and gingivitis), skin infections, urinary tract infections, and slow-healing wounds are all common in FeLV-positive cats.

Cancer (Lymphoma and Leukaemia)

FeLV is the most common cause of cancer in cats. The virus can induce malignant transformation of lymphocytes, leading to lymphoma (solid tumours of lymphoid tissue) or lymphoid leukaemia (cancer of blood-forming cells). Lymphoma can develop in the intestines, kidneys, liver, spine, nasal cavity, or lymph nodes. Mediastinal lymphoma (in the chest) is particularly associated with FeLV. Treatment with chemotherapy is possible, but the prognosis is generally more guarded in FeLV-positive cats than in FeLV-negative cats with the same cancer type.

Anaemia

FeLV can cause anaemia through several mechanisms: direct suppression of red blood cell production in the bone marrow, immune-mediated destruction of red blood cells (haemolytic anaemia), and the rare FeLV-C subgroup causing pure red cell aplasia. Severe anaemia may require blood transfusions and carries a guarded prognosis.

Other Conditions

• Reproductive failure — FeLV-positive queens may experience infertility, foetal resorption, abortion, or give birth to weak, infected kittens.
• Neurological disease — Including peripheral neuropathies, urinary incontinence, and anisocoria (unequal pupil sizes).
• Immune-mediated conditions — Including glomerulonephritis (kidney inflammation) and polyarthritis (joint inflammation).

Regular veterinary monitoring is essential for FeLV-positive cats. Early detection of complications through routine examinations and blood work — as outlined in our guide to vet visit frequency — can significantly improve outcomes and quality of life.

Preventing FeLV is far more effective than treating it. A combination of testing, vaccination, and management strategies offers the best protection for your cat.

FeLV Vaccination

An effective FeLV vaccine is available and is recommended as a core vaccine for all kittens by many veterinary guidelines (including the AAFP). The initial series consists of two doses given two to four weeks apart, starting as early as eight weeks of age. A booster is given one year later. Subsequent boosters are recommended annually or every two years for at-risk cats (outdoor access, multi-cat households). Low-risk indoor-only adult cats may not need ongoing FeLV boosters after the initial series, but this should be discussed with your veterinarian.

Important: the vaccine does not provide 100 percent protection, and it cannot treat or eliminate existing infection. This is why testing before vaccination, and combining vaccination with exposure reduction, is essential.

Exposure Prevention Strategies

• Test before introducing new cats — Every new cat entering your home should be tested for FeLV and ideally quarantined until results are confirmed. See our cat vaccination guide for the full recommended testing and vaccination timeline.
• Keep cats indoors — Outdoor access dramatically increases the risk of FeLV exposure through contact with stray or feral cats. Indoor-only living is the single most effective prevention strategy.
• Separate FeLV-positive cats — If you have an FeLV-positive cat in your household, they should be kept strictly separated from FeLV-negative cats. Do not share food bowls, water dishes, or litter trays. The virus does not survive long in the environment (minutes to hours on dry surfaces), but direct or close indirect contact poses a real risk.
• Spay and neuter — Reduces roaming behaviour and therefore reduces exposure risk. Also prevents transmission to offspring.

A diagnosis of FeLV-positive status can be frightening, but it is important to know that many FeLV-positive cats live comfortable, happy lives for years with appropriate care. Here is how to give your FeLV-positive cat the best possible quality of life.

Veterinary Care

• Twice-yearly veterinary examinations — More frequent check-ups allow early detection of complications. Each visit should include a thorough physical examination, weight assessment, and discussion of any changes in behaviour, appetite, or activity level.
• Annual blood work — Complete blood count, chemistry panel, and urinalysis help monitor for anaemia, immune changes, organ dysfunction, and early signs of cancer.
• Prompt treatment of illness — Because the immune system is compromised, infections that might be minor in a healthy cat can become serious quickly. Do not adopt a "wait and see" approach — seek veterinary attention at the first sign of illness.
• Avoid immunosuppressive medications — Corticosteroids and other immunosuppressive drugs should be avoided or used with extreme caution, as they can accelerate disease progression.

Home Environment

• Indoor-only living — Protects your cat from exposure to secondary infections and prevents transmission to other cats in the neighbourhood.
• High-quality nutrition — A well-balanced, premium diet supports immune function. Avoid raw meat diets, which carry a risk of bacterial contamination that an immunocompromised cat may be unable to handle.
• Stress reduction — Stress suppresses the immune system. Provide a calm, stable environment with consistent routines, hiding places, vertical spaces, and plenty of enrichment.
• Parasite prevention — Keep up with flea, tick, and worm prevention as recommended by your vet. Parasitic infections place additional strain on an already compromised immune system.

Emotional Support

FeLV-positive cats deserve love, companionship, and engagement just like any other cat. They can play, purr, and bond deeply with their owners. Many FeLV-positive cats enjoy years of excellent quality of life. Focus on making every day a good day, and lean on your veterinary team for support and guidance as you navigate the journey together.

With regular veterinary care, a safe indoor environment, good nutrition, and lots of love, your FeLV-positive cat can live their best life. For guidance on building the right care schedule, explore our vet visit frequency guide by age.