Feline Leukemia Virus (FeLV)

TL;DR. Feline Leukemia Virus (FeLV) is a common retrovirus in cats that compromises the immune system and bone marrow, often leading to severe anemia, cancers like lymphoma, and life-threatening secondary infections.

FeLV replicates within the lymphoid tissues and bone marrow of infected cats.

What is it?

Feline Leukemia Virus (FeLV) is a widespread and serious viral infection that affects cats worldwide. As a retrovirus, FeLV has a unique way of reproducing inside a cat's body. Once a cat is exposed, the virus targets and replicates within the lymphoid tissues and bone marrow. This replication disrupts the normal production of blood cells and weakens the cat's immune system, leaving them highly vulnerable to secondary infections, anemia, and certain types of cancers.

To understand how FeLV takes hold, it helps to look at how the virus operates at a cellular level. A leading veterinary internal medicine reference explains:

"The virus produces reverse transcriptase, which catalyzes the reaction, resulting in the formation of a DNA copy (provirus) of FeLV viral RNA in the cytoplasm of infected cells; the provirus is inserted into the host cell genome. On subsequent host cell divisions the provirus serves as a template for new virus particles formed in the cytoplasm and is released across the cell membrane by budding."

By embedding its own genetic material directly into the cat's DNA, the virus becomes a permanent part of the host's cells. This allows it to persist and replicate silently before clinical signs appear. The consequences of this cellular hijacking are diverse and severe. FeLV can cause profound immunodeficiency, destroy red blood cells, and trigger the uncontrolled cell division that leads to leukemia or lymphoma.

Causes & risk factors

FeLV is primarily spread through close, friendly contact between cats. The virus is shed in high quantities in saliva, as well as in nasal secretions, urine, feces, and milk. Common routes of transmission include mutual grooming, sharing food and water bowls, and bite wounds sustained during fights. Mother cats can also pass the virus to their kittens during pregnancy or through nursing.

Outdoor cats, intact male cats, and cats living in multi-cat households where the infection status is unknown are at the highest risk of contracting the virus. Kittens are highly susceptible to infection, whereas older cats develop a degree of natural resistance as their immune systems mature. There are no known breed predispositions to FeLV; any cat, regardless of breed, can contract the virus if exposed.

Signs to watch for

Because FeLV affects multiple body systems—particularly the blood-producing organs and the immune system—the symptoms can be incredibly varied. Some cats may remain asymptomatic for months or even years, while others show rapid decline.

Common Signs

• Anorexia and Weight Loss: A gradual or sudden loss of appetite, leading to noticeable wasting.
• Depression and Lethargy: A marked decrease in energy, playfulness, and normal activity.
• Nonregenerative Anemia: The bone marrow fails to produce new red blood cells, resulting in pale gums, weakness, and low energy.
• Azotemia: An accumulation of nitrogenous waste products in the blood, indicating kidney dysfunction or failure.
• Hyperbilirubinemia and Bilirubinuria: High levels of bilirubin in the blood and urine, which can cause the urine to look dark or tea-colored.
• Thrombocytopenia: A low platelet count, which impairs the blood's ability to clot and can lead to bruising or abnormal bleeding.
• Neutropenia: A dangerously low level of neutrophils (a type of white blood cell), which severely compromises the cat's ability to fight off infections.

Occasional Signs

• Icterus (Jaundice): A yellowing of the skin, whites of the eyes, and gums due to liver dysfunction or severe red blood cell destruction.
• Ocular Changes: Cloudy eyes, squinting (blepharospasm), constricted pupils (miosis), or unequal pupil sizes (anisocoria).
• Gastrointestinal Issues: Vomiting, diarrhea, and difficulty swallowing (dysphagia).
• Stomatitis: Severe, painful inflammation of the mouth and gums, making eating difficult.
• Urinary Changes: Increased drinking (polydipsia), increased urination (polyuria), protein in the urine (proteinuria), or urinary incontinence.
• Reproductive Failure: Infertility, stillbirths, or spontaneous abortions in pregnant queens.
• Neurological Issues: Weakness, uncoordinated movement (ataxia), partial paralysis of the hind limbs (paraparesis), or weakness in all four limbs (tetraparesis).
• Lameness and Behavioral Changes: Unexplained limping or sudden shifts in temperament.
• Dyspnea: Labored or difficult breathing, which is a major red flag.

Lethargy and pale gums are common signs of severe anemia caused by FeLV.

How vets diagnose it

Diagnosing FeLV requires a combination of clinical evaluation and specific laboratory tests. Because the virus moves through different stages in the body, your vet may need to perform multiple types of tests to understand the exact status of the infection.

To identify the virus, vets rely on several key diagnostic tools:

• ELISA (Enzyme-Linked Immunosorbent Assay): This is the standard screening test, typically performed in the clinic using a small blood sample. It detects the presence of the FeLV p27 core protein in the blood. A positive ELISA test indicates that the virus is circulating in the bloodstream (viremia).
• IFA (Immunofluorescence Assay): If a cat tests positive on an ELISA, your vet may recommend an IFA test sent to an outside laboratory. The IFA detects the virus within white blood cells and platelets, indicating that the infection has progressed to the bone marrow. Cats that are IFA-positive are highly likely to remain infected for life.
• PCR (Polymerase Chain Reaction): This highly sensitive molecular test detects viral DNA or RNA. It is particularly useful for identifying cats with low-level or "regressive" infections where the virus is hidden in the cells but not actively circulating in large amounts.
• Complete Blood Count (CBC) and Chemistry Profile: These tests evaluate the secondary impacts of the virus, checking for nonregenerative anemia, low white blood cells, low platelets, and signs of kidney or liver damage.

Your vet will interpret these results carefully, as some cats can clear the initial stage of infection (transient viremia) and test negative later. It is also important to note that other testing methods have limitations. As noted in veterinary literature:

"Antibody titers to FeLV envelope antigens (neutralizing antibody) and against virus-transformed tumor cells have been detected in research studies, but the diagnostic and prognostic significance of results from these tests is unknown. Real-time PCR assays are more sensitive than conventional PCR for FeLV infections, but validated and standardized assays are not currently available in the United States."

ELISA testing is the primary screening tool used to detect the FeLV p27 core protein in a cat's blood.

Treatment options

There is currently no definitive cure for Feline Leukemia Virus. Once the virus has permanently integrated into the cat's bone marrow cells, it cannot be completely eradicated. However, veterinary medicine offers several supportive and targeted therapies to manage the symptoms, suppress viral replication, and improve the cat's quality of life.

Antiretroviral Therapy

Your vet may consider specific antiviral medications to help slow down the replication of the virus.

• Zidovudine: This is a nucleoside reverse transcriptase inhibitor (NRTI). It works by blocking the reverse transcriptase enzyme that the virus uses to copy its genetic material. While it does not cure the infection, it can help reduce viral load and improve clinical signs in some symptomatic cats.

Immunomodulators and Cytokines

These therapies aim to regulate or boost the cat's natural immune response to help them fight off the virus and secondary opportunistic infections.

• Interferon Omega: A veterinary-licensed antiviral cytokine that has been shown to improve survival rates and clinical signs in FeLV-infected cats.
• Human Recombinant Interferon Alfa: Another cytokine option used to stimulate the immune system, though its long-term efficacy can be limited as cats may eventually develop antibodies against the human-derived protein.

Immunostimulants

• Propionibacterium acnes injection: A biologic response modifier that non-specifically stimulates the immune system to help the body combat chronic viral infections.
• Acemannan: A non-specific immunostimulant and antiviral substance derived from aloe vera, sometimes used as an adjunctive therapy to support immune function.

Supportive and Symptomatic Care

Because FeLV causes a wide range of secondary complications, targeted supportive care is vital:

• Blood Transfusions: For cats suffering from severe, life-threatening nonregenerative anemia.
• Antibiotics: To treat secondary bacterial infections that take advantage of the cat's compromised immune system.
• Fluid Therapy: Crucial for managing dehydration, especially in cats experiencing renal failure or severe vomiting and diarrhea.
• Nutritional Support: High-calorie diets, appetite stimulants, or feeding tubes may be necessary for cats experiencing severe anorexia and weight loss.

Prognosis

The prognosis for cats with progressive, persistent FeLV infection is guarded. The majority of persistently viremic cats die within 2 to 3 years of diagnosis due to complications such as severe immunodeficiency, opportunistic infections, nonregenerative anemia, or aggressive cancers like lymphoma.

However, the outcome can vary depending on the cat's individual immune response. Some cats experience a "regressive" infection, where their immune system successfully contains the virus, keeping it dormant. These cats can live normal, healthy lives for many years, though they may still be at risk of reactivation if they become severely stressed or immunocompromised later in life.

Prevention

Preventing exposure is the most effective way to protect cats from FeLV.

• Vaccination: Highly effective vaccines are available. Vaccination is strongly recommended for all kittens, as well as adult cats that go outdoors, live with FeLV-positive housemates, or are otherwise at risk of exposure.
• Testing and Isolation: All new cats or kittens should be tested for FeLV before being introduced to a household with other cats. FeLV-positive cats must be kept strictly indoors and housed separately from FeLV-negative cats to prevent transmission.
• Hygiene: The virus does not survive long in the environment (usually only a few hours on dry surfaces) and is easily destroyed by common household disinfectants, soap, and water. Regular cleaning of food bowls, litter boxes, and bedding is essential in multi-cat environments.

When to call your vet

If your cat has been diagnosed with FeLV, close monitoring is essential. You should contact your veterinarian if you notice any changes in their behavior, appetite, or energy levels.

Seek immediate emergency veterinary care if your cat displays any of the following red-flag signs:

• Difficulty breathing (dyspnea) or rapid, shallow breathing
• Extreme lethargy, collapse, or inability to stand
• Very pale or white gums (indicating severe anemia)
• Uncontrolled bleeding or sudden bruising
• Persistent vomiting or diarrhea leading to severe weakness
• Sudden neurological changes, such as seizures, severe disorientation, or loss of use of their limbs

Sources

• Internal Medicine, 5th Edition, pages 1384, 1385, 1386, 1387.