Feline Leukemia Virus (FeLV)
One of the most important infectious diseases of cats worldwide. A retrovirus that suppresses the immune system and causes cancer (lymphoma, leukemia). Infected cats may live years with good management but are highly contagious to other cats.
Last updated: 2026-05-10
Severity
severe
When to Act
See Vet Soon
Symptoms & Signs
Weight loss and poor coat
Progressive wasting despite adequate nutrition.
Recurrent infections
Immunosuppression leads to frequent bacterial, fungal, or viral infections that don't resolve normally.
Lethargy and fever
Persistent or intermittent fever of unknown origin.
Stomatitis/gingivitis
Severe oral inflammation, often resistant to treatment.
Lymphoma/leukemia
Enlarged lymph nodes, mediastinal mass causing breathing difficulty, or bone marrow infiltration.
Anemia
Pale gums, weakness from non-regenerative anemia (bone marrow suppression).
Behavioral Changes to Watch For
Pets can't tell us what's wrong. These behavioral changes are often the first clues that something is wrong.
🐾 Progressive lethargy and withdrawal
Cat becomes increasingly inactive and disengaged.
What You May Notice:
Your cat that used to be active sleeps 20+ hours a day and shows no interest in play or interaction.
Causes & Risk Factors
Causes
- •Infection with feline leukemia virus (FeLV) — a gammaretrovirus
- •Transmitted through saliva, nasal secretions, urine, feces, and milk
- •Prolonged close contact needed: mutual grooming, shared food/water bowls, bite wounds
- •Kittens can be infected in utero or through nursing
Risk Factors
- ⚠Kittens (highest susceptibility — most adult cats with strong immunity can clear the virus)
- ⚠Outdoor cats (exposure to infected cats)
- ⚠Multi-cat households with unknown FeLV status
- ⚠Cats in shelters or feral colonies
How It's Diagnosed
- 1In-clinic ELISA (SNAP FeLV test) — screens for the p27 antigen
- 2IFA (immunofluorescent antibody) — confirms progressive infection
- 3PCR — detects proviral DNA; confirms integration into the host genome
- 4All cats should be tested before vaccination and before introducing to a household
Treatment Options
Supportive Care and Management
No cure — treatment focuses on managing secondary complications and maintaining quality of life.
Steps
- 1.Indoor-only lifestyle to prevent transmission and protect from pathogens
- 2.High-quality nutrition — avoid raw food (risk of foodborne infection in immunosuppressed cats)
- 3.Prompt treatment of any infections with longer antibiotic courses
- 4.Regular veterinary monitoring every 6 months
- 5.Antiviral drugs (AZT/zidovudine, raltegravir) — limited efficacy but may help some cats
- 6.Interferon omega (Virbagen Omega) — available in some countries
Expected Outcome
Extended quality life — some cats live years with good management.
Precautions
- !Must be strictly isolated from FeLV-negative cats
- !Immunosuppressed cats should not receive live vaccines
Common Medications Used
| Medication | Usage | Important Notes |
|---|---|---|
| AZT (Zidovudine) | Antiretroviral — may reduce viral load and improve clinical signs | Limited efficacy. Can cause bone marrow suppression with long-term use. |
Prevention
- ✓FeLV vaccination — recommended for all kittens and at-risk adult cats
- ✓Test all cats before introducing to a household
- ✓Keep FeLV-positive cats strictly indoors and isolated from FeLV-negative cats
- ✓Spay/neuter infected cats to reduce fighting and transmission behaviors
When to See a Veterinarian
- ⚠️Unexplained weight loss or fever
- ⚠️Recurrent infections that don't resolve
- ⚠️Enlarged lymph nodes
- ⚠️Any new cat before introduction to household (testing)
Frequently Asked Questions
Can FeLV-positive cats live with negative cats?
Prognosis
Variable. Approximately 30% of exposed cats develop progressive infection (persistently viremic). About 30% develop regressive infection (virus integrates but is controlled). About 40% abort the infection. Persistently infected cats have a median survival of 2-4 years. Lymphoma carries a poor prognosis.
References
- [1] AAFP — FeLV Management Guidelines
- [2] European Advisory Board on Cat Diseases
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