Feline Leukemia Virus (FeLV) infection

Overview

FeLV is a retrovirus that suppresses the immune system and can cause cancers (especially lymphoma) and severe anemia. It's one of the most significant infectious causes of illness and death in cats worldwide, though prevalence has dropped substantially in regions with good testing and vaccination access.

Symptoms

• Weight loss (moderate, common)
• Pale gums (severe, common)
• Recurrent infections (moderate, common)
• Enlarged lymph nodes (severe, uncommon)
• Lethargy (moderate, common)

Causes

Infection with feline leukemia virus, a retrovirus specific to cats.

Risk Factors

Unvaccinated cats, outdoor/indoor-outdoor cats, cats in multi-cat households with an infected cat, kittens born to an infected queen, bite-wound exposure from fighting.

Transmission

Primarily through prolonged close contact — mutual grooming, shared food/water bowls, bite wounds — and from an infected queen to her kittens in utero or through nursing. It does not survive long outside the body and is not transmitted through casual brief contact. FeLV spreads through prolonged close contact — mutual grooming, shared food/water bowls, and bite wounds all transmit the virus, which is present in saliva, nasal secretions, and other body fluids, making it genuinely more contagious through casual contact than FIV specifically, and a real consideration for multi-cat households with an infected cat rather than only relevant to fighting/biting.

Incubation Period

Variable — some cats clear the infection within weeks, others become progressively or persistently infected over months to years before signs appear.

Disease Stages & Progression

After exposure, a cat may (1) effectively clear the virus, (2) become regressively infected (virus present but suppressed, usually not shedding, occasional reactivation risk), or (3) become progressively infected (persistent active infection, the group at highest risk of FeLV-related disease over time). After exposure, a cat's immune system either successfully clears the virus (a genuine possibility, particularly in adult cats with a robust immune response), develops a regressive infection (virus present but suppressed and not actively replicating, generally not contagious to other cats), or develops progressive infection, where the virus actively replicates and the cat becomes both persistently infected and a transmission risk to other cats, with this last category carrying the most significant long-term health implications.

Early Warning Signs

Often none for a long time in progressively infected cats; early signs when present are nonspecific: reduced appetite, weight loss, poor coat condition, recurrent minor infections.

Advanced Symptoms

Persistent fever, pale gums (anemia), enlarged lymph nodes, chronic respiratory or GI infections, oral disease, neurological signs in some cases, and cancers (especially lymphoma) in a meaningful proportion of progressively infected cats over time.

Diagnostic Methods

In-clinic ELISA antigen test is the standard first screen; positive results, especially in low-risk cats, are typically confirmed with a second method (PCR or IFA) since a single test can occasionally be a false positive. An ELISA blood test is the standard initial screen, similar to FIV testing; a positive result, particularly in a cat without strong known exposure risk, is typically confirmed with a follow-up test (often IFA or PCR) partly because the distinction between regressive and progressive infection genuinely matters for prognosis and management, and a single ELISA doesn't reliably distinguish between them.

Veterinary Examination Process

Full physical exam focusing on lymph nodes, gum color, body condition, and any signs of secondary infection; history of exposure risk (outdoor access, multi-cat household, unknown history in a newly adopted cat) is important context.

Recommended Lab Tests

FeLV ELISA antigen test, confirmatory PCR or IFA, complete blood count, and additional testing directed at any specific complication found (e.g., lymph node aspirate if lymphoma is suspected).

Diagnostic Imaging

Not primary for diagnosis; chest/abdominal imaging may be used if lymphoma or another FeLV-associated cancer is suspected, to look for masses or enlarged lymph nodes/organs.

Blood Test Indicators

Complete blood count may show anemia or abnormal white cell counts; specific FeLV antigen tests (ELISA, often run in-clinic) and confirmatory PCR testing establish diagnosis and infection status.

Biopsy / Histopathology

Performed if a mass or enlarged lymph node is found, to characterize a suspected FeLV-associated cancer such as lymphoma.

Differential Diagnoses

Feline immunodeficiency virus (FIV), other causes of chronic anemia, other causes of lymphoma/lymphadenopathy — testing distinguishes these rather than symptoms alone.

Home Monitoring Guidance

Regular weight checks, watching appetite and energy level, and prompt vet visits for any new or worsening sign rather than waiting, since FeLV-positive cats have a reduced margin for tolerating untreated secondary infections.

Veterinary Treatment Options

There is no cure for FeLV infection itself. Management focuses on supportive care, prompt and thorough treatment of secondary infections, and monitoring for FeLV-associated cancers so they can be caught and treated as early as possible. As with FIV, there is no cure, and management focuses on supportive care, infection prevention, and prompt treatment of secondary illness; progressively-infected cats specifically should be kept strictly indoors both to protect them from additional infection exposure and to prevent transmission to other cats.

Surgery Requirements

Not required for the viral infection itself; may be needed to remove an accessible tumor if a solid FeLV-associated cancer develops.

Recovery Timeline

Not applicable to the underlying infection, which is generally lifelong once progressively infected; secondary infections and complications are treated on their own timelines.

Possible Complications

Increased susceptibility to other infections, chronic anemia, lymphoma and other cancers, and a shortened lifespan on average for progressively infected cats compared to uninfected cats.

Long-Term Management

Indoor-only living to reduce exposure to other pathogens and to protect other cats, twice-yearly veterinary checkups (more frequent than a typical healthy cat), prompt attention to any new symptom, and keeping FeLV-positive cats separated from FeLV-negative cats in a multi-cat household unless all cats are confirmed positive.

Prevention

Vaccination (a non-core vaccine recommended based on lifestyle risk — see the Vaccination Center), testing all new cats before introducing them to a household with existing cats, and avoiding unsupervised outdoor access in areas with unknown-status cats.

Vaccination Availability

An FeLV vaccine is available and is generally recommended for kittens and any cat with outdoor access or exposure to cats of unknown FeLV status; it is not 100% protective, so testing and exposure reduction remain important alongside vaccination.

Prognosis

Variable — cats that clear the infection or become regressively infected often do well; progressively infected cats have a shortened average lifespan (commonly cited as a median of a few years from diagnosis in older literature, though outcomes vary widely and modern supportive care has improved this), with cause of death usually related to secondary infection or cancer rather than the virus directly. Prognosis varies considerably based on infection status — cats who successfully clear the virus or develop regressive infection generally have a good long-term outlook, while progressively infected cats face a meaningfully higher risk of FeLV-associated illness (including certain cancers and immune suppression) and, on average, a shorter lifespan than uninfected cats, though individual cats vary and good supportive care genuinely helps.

References

American Association of Feline Practitioners (AAFP) Retrovirus Testing and Management Guidelines; WSAVA; International Cat Care feline retrovirus resources.

Medications Used

• Antibiotics for secondary bacterial infections (specific choice depends on the infection)
• Antiemetics and appetite stimulants as needed for supportive care
• Chemotherapy protocols if lymphoma develops, directed by an oncologist or internal medicine specialist

Estimated Treatment Cost by Country

Frequently Asked Questions