Feline Leukemia

Feline leukemia virus, also known as FeLV, is one of the most common infectious diseases of cats worldwide.  In the United States, it is estimated that 3% of cats are infected with feline leukemia virus.  Outdoor cats, unneutered males, and cats with concurrent diseases are at the highest risk for being infected with feline leukemia.

What is Feline Leukemia Virus (FeLV)?

Feline leukemia is a virus that affects domestic cats and some species of wild cats.  Kittens are most susceptible to infection with feline leukemia virus, but any cat can be infected.  The virus causes suppression of the immune system, making the cat susceptible to secondary infections.  Cats with FeLV are also more likely to develop cancer and certain blood disorders.  Unfortunately, FeLV is usually fatal.  Studies show that 80-90% of cats die within 3-4 years of diagnosis.

Is Feline Leukemia Contagious?

Feline leukemia is highly contagious between cats.  It is often called “the friendly cat disease” because it can easily be spread between cats, even those that do not fight.  Infected cats shed the virus primarily in their saliva, and also in blood, tears, urine, feces, nasal secretions, and milk.  Cats can become infected through direct contact with an infected cat, through grooming, bite wounds, or by sharing bowls or litter boxes with an infected cat.  Feline leukemia is specific to cats and is not contagious to humans, dogs, or other species.

Can Cats With Feline Leukemia Live With Other Cats?

It’s recommended that FeLV positive cats be the only cat in the household or only live with other FeLV positive cats.  This is because feline leukemia is contagious to other cats through close contact such as grooming, sharing food and water bowls, and using the same litter box.  Although we can vaccinate healthy cats against feline leukemia virus, no vaccine is 100% effective.  Because of this, it is recommended that cats positive for feline leukemia be separated from cats that are negative for the virus.  Any new cat being adopted into a household with existing cats should be tested for feline leukemia prior to addition to the household.

Types of Feline Leukemia Virus Infection

After exposure to feline leukemia virus, there are three possible infection outcomes for the exposed cat:

• Abortive Infection. After exposure to the virus, the cat’s immune system mounts a response and clears the virus.  The cat will test negative for the virus on antigen and PCR tests, but will test positive on antibody tests.
• Regressive Infection. After exposure, the cat’s immune system mounts a response but does not clear the virus.  Cats with regressive infection do not shed the virus, are not infectious to other cats, and are at low risk of developing FeLV-associated diseases.  However, regressive infections can reactivate following periods of stress or immunosuppression, leading to viremia and a risk for the development of FeLV-associated diseases.
• Progressive Infection. After exposure, the cat’s immune system does not mount an immune response and the virus replicates unchecked.  These cats actively shed the virus and are infectious to other cats.  These cats are at risk for developing FeLV-associated diseases and will have a shortened lifespan.  Progressively infected cats test positive on antigen and PCR tests.

Feline Leukemia Symptoms

Cats infected with feline leukemia can have a wide variety of symptoms.  Many cats have no symptoms during the initial stages of the infection.  Signs of feline leukemia may vary depending on the concurrent disease or secondary infection affecting the infected cat.  Infected cats may experience recurrent illnesses or progressively deteriorating health.

Signs of feline leukemia include:

• Weight loss
• Loss of appetite
• Fever
• Lethargy
• Enlarged lymph nodes
• Pale gums
• Vomiting
• Diarrhea
• Neurologic abnormalities
• Abortions and reproductive problems
• Inflammation of the mouth and gums
• Eye conditions

Diagnosing Feline Leukemia

To establish a diagnosis of feline leukemia virus, your veterinarian may recommend some or all of the following tests:

• Physical Examination. Your veterinarian will perform a comprehensive head-to-tail physical examination on your cat.  Your veterinarian will also ask you questions about your cat’s history, including your cat’s vaccination status and exposure to other cats.
• Blood Work. A complete blood count and biochemistry panel may be performed.  Cats with feline leukemia often have anemia, decreased white blood cell counts, and decreased platelet counts.  Biochemistry findings can be variable depending on the body system affected by feline leukemia and secondary infections.
• Antigen Tests. In-clinic antigen test kits are commonly used to test for feline leukemia virus and may be used if your cat displays symptoms of feline leukemia or as a screening test for healthy cats.  The test uses a sample of your cat’s blood to detect a viral protein.  Most cats test positive within 30 days of exposure to the virus.  Some cats that test positive may be only transiently infected and may mount an effective immune response against the virus.  Retesting in 30 days may be recommended for some cats.
• Antibody Tests. The indirect immunofluorescent antibody test (IFA) is performed on blood or bone marrow samples and does not become positive until the bone marrow is infected.  IFA is sometimes used to confirm a positive antigen test result.  A positive result usually indicates a progressive infection.  Cats with regressive infection usually do not test positive on IFA.
• PCR Tests. PCR testing can detect both progressive and regressive infections.  This test can be performed on whole blood, bone marrow, or other tissues.  PCR testing is often used to confirm a positive antigen test result and to distinguish between progressive and regressive infections.

Treatment for Feline Leukemia

Unfortunately, there is no cure for feline leukemia.  Treatments with antiviral medications, AZT, and interferons have been studied in cats but results have been variable.  For most FeLV positive cats, treatment focuses on management of concurrent diseases or infections.  Good nutrition, regular preventive care, and routine wellness exams are essential to maintain good health in an FeLV positive cat.  Infected cats should be kept strictly indoors to reduce the risk of secondary infections and to prevent the spread of feline leukemia to other cats.  It is recommended that infected cats maintain regular preventive health care, including vaccinations and spaying or neutering.

Feline Leukemia Prognosis

The prognosis for feline leukemia varies depending on the type of feline leukemia infection.  In general, cats with FeLV can have a good quality of live for several years.  Cats with regressive infection can live healthy lives with a normal lifespan.  Cats with progressive feline leukemia often develop FeLV-associated diseases and have a shortened lifespan.  The median survival time for cats with FeLV is 2.5 years following diagnosis.

Feline Leukemia Vaccine

The most effective way to prevent feline leukemia is vaccination.  Feline leukemia is a non-core vaccine, which means it is recommended for certain cats based on their risk of exposure to this virus.  According to the 2020 guidelines from the American Association of Feline Practitioners and the American Animal Hospital Association, the feline leukemia vaccine is recommended for all kittens less than 1 year of age and all at-risk adult cats.  Your veterinarian can help you determine whether your adult cat should receive the feline leukemia vaccine.

In addition to vaccination, keeping cats indoors also helps prevent feline leukemia as this reduces your cat’s risk of exposure to infected cats.  Any new cat being introduced to the home should also be tested for feline leukemia prior to introduction.

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