What Is Feline Leukemia (FeLV)?

OBACCP Feline Leukemia in Cats

What Is Feline Leukemia (FeLV)?

Feline Leukemia Virus (FeLV) is a highly contagious, serious viral disease that affects only cats. Fortunately, there are many ways we can protect our cats from exposure to this disease. It is estimated that 2.3% of cats in the western United States are infected with feline leukemia.

How does Feline Leukemia cause disease?

The Feline Leukemia Virus can cause bone marrow suppression. One of the functions of bone marrow is to produce the cells of the immune system that fight off infection. Therefore, this disease can result in a suppressed immune system.  The immune system normally protects the cat from common infectious agents (bacteria, viruses, etc) that do not generally cause disease in healthy cats. However, if a cat’s immune system is weakened, as with feline leukemia, these same organisms can cause disease — known as secondary or opportunistic infection. Although the disease only affects cats, it is recommended that immunocompromised people (ie chemo patients and human AIDS patients) not reside with FeLV positive cats because these cats are more likely to harbor opportunistic infections that could be transmitted to immunocompromised humans. This viral disease is also associated with the development of certain types of cancer (i.e. lymphoma, leukemia) in affected cats.

How does a cat get FeLV?

The most common way cats get the disease is through saliva and casual contact with an infected cat (i.e. mutual grooming, sharing bowls, sharing litter areas, touching noses, etc.). Since this disease is highly contagious between cats, it is important to completely isolate new cats of unknown viral status and get them tested before intermingling them with other household cats, and to also wash hands between handling cats. Kittens are more susceptibleto FeLV.Cats who are at greater risk of exposure are those who are allowed outdoorsand encounter infected cats. The virus can also be transmitted from mother to kittens.

What is the outcome of disease?

Cats infected with FeLV have four possible outcomes. They can:

  1. Succumb to the infection and develop FeLV associated diseases, such as cancer. The development of FeLV associated diseases such as bone marrow suppression or cancer poorly affects the cat’s prognosis.
  2. Overcome the virus but the viral “DNA” is incorporated into the cat’s own DNA; these cats may or may not develop illness at some point in their lifetime. Those cats that are infected and have no clinical signs may remain asymptomatic for months to years or for life. These cats should still be considered contagious to other cats.
  3. Completely eliminate the virus from their system.
  4. Limit the virus so that it is confined or localized to a small region of the body, such as a mammary gland (this is rare).

FeLV positive cats must be kept indoors away from unaffected cats so they don’t spread the disease to other cats.  They should be spayed/neutered, provided with good nutrition, avoid raw diets, and need regular visits to their veterinarian.  With proper care many FeLV cats can live months to years in apparent good health. At some point, however, their health will decline and veterinary care will be required.

How do we test for FeLV?

A simple in-house blood test can be performed to test for FeLV (ELISA or “snap” test). If the test is positive it could mean that the cat has FeLV or the test could simply be wrong. A positive snap test should be confirmed by an IFA test, which is another blood test but with more accurate and conclusive results. If the snap test is negative, it could mean that the cat is not infected with FeLV or that the cat was recently infected (up to 30 days ago) but won’t test positive yet.

How can we protect our cats from FeLV?

  • Keep tame cats exclusively indoors.
  • Vaccination is highly recommended for cats determined to be at risk (i.e., cats that go outdoors or cats who encounter new cats that have not been viral tested). Vaccination will NOT interfere with testing for the disease.
  • Always isolate and test new cats for FeLV before allowing them to intermingle with your existing pets.


Article written by Dr. Amanda Page, DVM. Published with permission. For more information, see Cornell University’s online brochure on FeLV.