Feline health can be complex, but few diagnoses evoke as much concern as Feline Leukemia Virus, commonly abbreviated as FeLV. This contagious virus is one of the leading causes of illness and death in domestic cats worldwide. Affecting the immune system and predisposing cats to a range of secondary infections, cancers, and chronic conditions, understanding this virus is essential for prevention, diagnosis, and management.
Whether you’re a seasoned veterinary professional or a devoted cat owner, this guide offers a deep dive into the key facts, clinical manifestations, transmission, diagnosis, and prevention strategies associated with FeLV.
What Is Feline Leukemia Virus?
Feline Leukemia Virus is a retrovirus, specifically a gammaretrovirus, meaning it integrates into the host cat’s DNA and can remain latent for extended periods. Discovered in the 1960s, FeLV remains a major threat, though its prevalence has declined due to increased testing, vaccination, and awareness.
Unlike leukemia in humans, FeLV is not a cancer itself, but it can cause cancer, most notably lymphoma and leukemia in cats. It also severely compromises the immune system, making infected cats vulnerable to common infections and diseases.
Transmission: How Cats Get Infected
FeLV is primarily spread through prolonged close contact, especially among cats that groom one another, share food bowls, or live in the same household or colony. The virus is shed in saliva, but it can also be found in nasal secretions, urine, feces, and milk.
Key Transmission Routes:
- Mutual grooming and licking
- Bite wounds, especially from fighting
- Shared food/water dishes and litter boxes
- From mother to kittens, either in utero or during nursing
Kittens and young cats are more susceptible due to their immature immune systems, while healthy adult cats are typically more resistant unless exposed repeatedly.
Who Is at Risk?
FeLV affects cats of all breeds and genders, though several factors increase vulnerability:
- Outdoor cats
- Stray and feral cats
- Kittens under one year of age
- Cats in multi-cat households or shelters
- Unvaccinated individuals
In general, indoor-only cats that live alone or with uninfected companions are at significantly lower risk.
Symptoms and Clinical Signs of FeLV
FeLV can produce a wide variety of clinical signs, depending on the stage of infection and affected organs. In some cases, a cat may appear healthy for months or even years before symptoms emerge.
Common Symptoms Include:
- Weight loss
- Lethargy and weakness
- Fever
- Pale gums (anemia)
- Loss of appetite
- Enlarged lymph nodes
- Diarrhea
- Persistent infections (skin, urinary tract, respiratory)
Some cats develop neurological issues, reproductive problems, or malignancies such as lymphosarcoma—a cancer of the lymphatic system strongly associated with FeLV.
The Four Stages of FeLV Infection
Understanding the progression of the virus helps inform treatment and prognosis:
- Abortive Infection
The cat’s immune system eliminates the virus before it establishes infection. These cats are not contagious and will test negative over time. - Regressive Infection
The virus becomes dormant within the bone marrow. These cats may test positive on some tests but not be actively contagious. Reactivation is possible. - Progressive Infection
The virus actively replicates, and the cat sheds it in large amounts. These cats are infectious and prone to illness. - Focal (Atypical) Infection
The virus localizes in a specific tissue, such as the bladder or mammary glands. This is the least common and most difficult to detect.
Diagnostic Testing for FeLV
Accurate diagnosis is crucial for managing infected cats and preventing transmission. The primary tests used are:
1. ELISA (Enzyme-Linked Immunosorbent Assay)
This in-house test detects viral antigen in the blood and is ideal for screening. A positive result should be followed up with a confirmatory test.
2. IFA (Immunofluorescent Antibody Test)
This laboratory test detects antigen within white blood cells and bone marrow, indicating a more advanced infection.
3. PCR (Polymerase Chain Reaction)
PCR detects viral DNA and helps identify latent or regressive infections, especially in ambiguous cases.
Routine testing is recommended for:
- Newly adopted cats
- Cats with unknown health backgrounds
- Sick cats with compatible symptoms
- Before vaccination or entering group housing
Treatment Options and Prognosis
Currently, there is no cure for FeLV. However, supportive care and preventive health strategies can extend a cat’s quality and length of life.
Treatment Strategies:
- Regular veterinary exams (every 6 months)
- Prompt treatment of secondary infections
- Nutritious diet and stress minimization
- Antiviral medications (e.g., zidovudine, interferon omega) in select cases
- Chemotherapy for associated cancers
- Indoor living to reduce exposure to pathogens
The prognosis varies. Some cats with regressive infections can live normal lifespans, while those with progressive disease often survive 2–3 years post-diagnosis.
Prevention Is the Best Medicine
Preventing infection is more effective than managing it after the fact. Responsible pet ownership and veterinary diligence play a crucial role.
Core Prevention Steps:
- Vaccinate at-risk cats (especially outdoor and shelter cats)
- Test all new cats before introduction to the household
- Keep infected and uninfected cats separate
- Prevent roaming to reduce exposure
- Spay and neuter to limit fighting and transmission
While the FeLV vaccine isn’t 100% effective, it significantly reduces the risk of infection. Annual boosters are recommended for high-risk cats, while low-risk cats may not require ongoing vaccination after their initial series.
Living With a FeLV-Positive Cat
An infected cat can still lead a fulfilling life with proper care and attention. Here are a few practical tips:
- Prioritize routine veterinary care
- Maintain a clean, calm environment
- Feed high-quality, well-balanced diets
- Avoid raw food to prevent bacterial exposure
- Monitor for signs of illness and act quickly
FeLV-positive cats should be kept indoors to prevent spreading the virus and protect them from additional infections or injuries.
Final Thoughts
Feline Leukemia Virus remains one of the most serious infectious diseases in domestic cats, but advancements in diagnostics, vaccines, and awareness have dramatically improved outcomes. Early detection, prevention, and dedicated care can make a profound difference in an affected cat’s life.
If you suspect your cat may be at risk or showing signs of illness, consult your veterinarian promptly for testing and guidance. Remember, with proactive care and compassion, even cats living with this virus can enjoy comfort and love for years to come.
Want to read about another devastating virus in cats? Check out “Feline Infectious Peritonitis (FIP) in Cats“.
