CONDITION

Feline Leukaemia Virus (FeLV)

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

Purpose: To detect FeLV antigen in the bloodstream
Considerations: Point-of-care tests provide rapid results. A positive result indicates current infection but not necessarily permanent infection in recently exposed cats.

IFA test

Purpose: To confirm positive ELISA results and assess likelihood of persistent infection
Considerations: Detects virus in white blood cells. A positive IFA suggests bone marrow infection and likely persistent viraemia.

Retesting of positive cats

Purpose: To determine whether infection is transient or persistent
Considerations: Cats may clear the virus within weeks to months. Retesting after 30-60 days helps clarify infection status.

Complete blood count

Purpose: To check for anaemia and abnormalities in blood cell lines
Considerations: Bone marrow effects can cause anaemia, low white cells, or low platelets.

Testing for concurrent FIV

Purpose: To assess for dual infection which worsens prognosis
Considerations: Routine to test for both retroviruses simultaneously.

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

Misconception:

"A positive FeLV test means the cat will die soon"

Reality:

Whilst FeLV does shorten average lifespan, many cats live for years with good quality of life. The course of infection is highly variable.

Misconception:

"FeLV is the same as feline AIDS"

Reality:

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.

Misconception:

"Vaccinated cats are completely protected"

Reality:

FeLV vaccines are highly effective but not 100% protective. Testing before vaccination and avoiding exposure to infected cats remain valuable precautions.

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