CONDITION
Feline Leukaemia Virus (FeLV)
Feline leukaemia virus is a retrovirus that can infect cats through close contact—typically via saliva during grooming, sharing food bowls, or through bite wounds. Once inside the body, the virus may be cleared by the immune system, may persist at low levels without causing illness, or may establish a progressive infection that weakens immunity over time. The outcome often depends on factors such as the cat's age at exposure, the strength of their immune response, and the amount of virus encountered. Owners often arrive on this page after a routine blood test has detected the virus, or because their cat has developed signs that may include recurrent infections, poor coat condition, weight loss, pale gums, or general lethargy. In some cases, cats can carry the virus for months or years without obvious illness, while others may develop problems relatively soon after infection. The virus itself does not cause one single disease; rather, it can lead to a range of complications by compromising the immune system or affecting bone marrow function. This page explores what FeLV infection can look like in practice, the mechanisms by which the virus affects the body, how infection is confirmed and monitored, and the range of approaches used to support affected cats. The focus is on understanding what may be happening and what options exist, rather than prescribing a single path forward.
Why this matters now
Feline leukaemia virus is one of the most significant infectious diseases in cats, though vaccination has substantially reduced its prevalence. The virus spreads through close contact—mutual grooming, shared food bowls, and bite wounds. Kittens are most susceptible, whilst adult cats often resist persistent infection. FeLV affects the bone marrow and immune system, with consequences ranging from mild to severe.
Following exposure, outcomes vary considerably. Some cats clear the virus entirely. Others become persistently infected, with the virus remaining in the bloodstream. Persistently infected cats face increased risk of developing anaemia, immune suppression, or cancers—particularly lymphoma and leukaemia. However, the timeline is unpredictable; some cats remain healthy for years whilst others develop problems sooner.
Signals & patterns
Early signals
Positive test result
Many cats are diagnosed through routine screening before any clinical signs appear.
Vague illness
Initial infection may cause non-specific signs such as mild fever or reduced energy.
Enlarged lymph nodes
Lymph node swelling may occur early and can persist.
Kitten from unknown background
Kittens from multi-cat environments or with unknown maternal history carry higher risk.
Later signals
Recurrent or persistent infections
Immune suppression leads to difficulty fighting off other pathogens.
Pale gums
Anaemia causes visible pallor of the mucous membranes.
Weight loss despite eating
Metabolic changes and disease progression can cause weight loss.
Masses or swellings
Lymphoma may present as lumps in various locations including the intestines, chest, or lymph nodes.
Click to read about the biological mechanisms
How this is usually investigated
Diagnosis involves detecting viral antigen in the blood. Interpretation requires understanding that test results can change over time, particularly in recently exposed cats.
ELISA antigen test
IFA test
Retesting of positive cats
Complete blood count
Testing for concurrent FIV
Options & trade-offs
There is no cure for FeLV, but infected cats can be supported to maximise quality of life. Management focuses on preventing secondary problems and monitoring for complications.
Isolation from uninfected cats
Keeping FeLV-positive cats separate to prevent transmission.
Trade-offs: Prevents spread within a household. Some owners maintain separate living spaces for positive and negative cats.
Supportive and preventive care
Maintaining good nutrition, parasite control, and prompt attention to any illness.
Trade-offs: Supports overall health. May help extend quality life, though the underlying infection remains.
Regular monitoring
Periodic blood tests and health assessments to detect problems early.
Trade-offs: Allows early intervention. Frequency depends on individual cat's status and any emerging concerns.
Treatment of FeLV-associated diseases
Managing anaemia, infections, or cancers as they develop.
Trade-offs: Some conditions respond to treatment. Chemotherapy for lymphoma can achieve remission in some cases.
Quality of life assessment
Ongoing evaluation of comfort and wellbeing as circumstances change.
Trade-offs: Helps guide decisions throughout the cat's life. Individual circumstances vary greatly.
Common misconceptions
"A positive FeLV test means the cat will die soon"
Whilst FeLV does shorten average lifespan, many cats live for years with good quality of life. The course of infection is highly variable.
"FeLV is the same as feline AIDS"
FeLV and FIV are different viruses with distinct characteristics. FeLV tends to be more directly pathogenic, whilst FIV causes gradual immune decline. Some cats are infected with both.
"Vaccinated cats are completely protected"
FeLV vaccines are highly effective but not 100% protective. Testing before vaccination and avoiding exposure to infected cats remain valuable precautions.
Related conditions
Feline Immunodeficiency Virus (FIV)
Feline immunodeficiency virus is another retrovirus that weakens immunity in cats, and while the viruses have different mechanisms and transmission routes, both can lead to recurrent infections and similar patterns of immune compromise. Cats can carry both viruses simultaneously, and testing for one often prompts testing for the other.
Lymphoma in Cats
Feline leukaemia virus infection is associated with a substantially increased risk of lymphoma in cats, particularly certain forms that affect the chest, kidneys, or nervous system. The virus can alter the behaviour of lymphocytes over time, sometimes contributing to the uncontrolled growth that defines lymphoma.
Toxoplasmosis
Cats with progressive FeLV infection may be more vulnerable to toxoplasmosis becoming clinically apparent, because the weakened immune response can allow the parasite to spread more widely or reactivate from dormant tissue cysts. In healthy cats, Toxoplasma infection often remains subclinical, but immune compromise can shift that balance.
Chronic Upper Respiratory Disease in Cats
Cats with FeLV infection can be more prone to chronic upper respiratory disease, as the impaired immune function may reduce the ability to clear or control respiratory viruses and bacteria that would otherwise resolve or remain dormant. Recurrent or persistent nasal discharge, sneezing, or conjunctivitis in an FeLV-positive cat may reflect this altered immune landscape.
Feline Stomatitis
Feline stomatitis can occur in FeLV-positive cats, and the virus may contribute to the severity or persistence of oral inflammation by disrupting normal immune regulation in the mouth. The relationship is not straightforward—many cats with stomatitis test negative for FeLV—but the virus can be one factor that worsens or prolongs the condition in affected individuals.
Understanding the difference between initial positive tests and confirmed persistent infection helps interpret results. Noting any changes in energy, appetite, or appearance allows early recognition of problems. Creating a stress-reduced environment may support immune function. Knowing that some cats live well for extended periods helps maintain realistic optimism.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS