Feline leukemia virus (FeLV) is an infectious virus in cats, which decreases the immune system’s ability to fight infection, affects the bone marrow’s ability to make blood cells, and predisposes cats to developing cancer at an early age. 

FeLV is usually diagnosed by a blood test, called an ELISA, run by a veterinarian.

What is Feline Leukemia Virus Infection?

• Feline leukemia virus (FeLV) is an infectious virus in cats, which decreases the immune system’s ability to fight infection, affects the bone marrow’s ability to make blood cells, and predisposes cats to developing cancer at an early age.  It is usually spread from cat to cat through bites, grooming, shared food and water dishes, and shared litter boxes but may be transmitted from a mother to her kittens.  The virus is most commonly acquired as a kitten (<16 weeks old) and male cats allowed outdoors are more commonly affected (likely due to fighting).  Not all cats exposed to the disease will become permanently infected, however those that do will usually die at a young age.

Clinical Signs

• There are no signs specific only to FeLV, however permanently infected cats usually begin acting sick months to years after infection.  They do not want to eat, they are losing weight, they are depressed and not acting normally, or they may have a fever.  FeLV can affect almost any body system, but it most commonly depresses the immune system (the body’s ability to fight infection) and predisposes cats to developing cancer. 
• Depression of the immune system makes cats very susceptible to infections which may affect almost any organ in the body (eyes, mouth, nose, lungs, stomach and intestines, kidneys, lymph nodes, red blood cells, nervous system, reproductive system.).  Signs will vary depending on the body system affected.
• Cats with FeLV develop cancer at a high rate at a much earlier age than cats without FeLV.  The most common type of cancer in these cats is lymphoma (cancer of the lymphocytes—a type of white blood cell), but other cancers of the blood cells (leukemia) are also possible.

Diagnosis

• FeLV is usually diagnosed by a blood test, called an ELISA, run by a veterinarian. It is recommended that any new pet cat or kitten (no matter what age) be tested for FeLV before being integrated into the home. 
• The blood test results of the ELISA are overall very reliable, but occasionally tests will need to be repeated or other forms of testing, such as IFA blood test, or tissue or bone marrow biopsy, will need to be done.  Any cat that tests positive on the ELISA should have an IFA blood test run to confirm the positive test result.  If the second test also shows positive the cats should be retested in 3 months, as some cats are later able to clear the infection.

Treatment

• There is no cure for FeLV infection, but secondary effects of the virus may be temporarily managed.  Treatment with drugs, such as interferon, which modulate the immune system and antiviral drugs may make sick cats feel better and increase their lifespan, but they will not cure the disease.  Sick cats need supportive care, typically antibiotic therapy, but in severe cases may need hospitalization, IV fluids, feeding tubes, or blood transfusions, to help them overcome infections.   FeLV infected cats with cancer can be treated with chemotherapy which may improve quality and duration of life.  Cats with FeLV should be kept indoors separate from cats without FeLV (even if they are vaccinated) to prevent spread of FeLV to other cats as well as prevent the FeLV infected cat from being exposed to other infections.  In addition to being separated from the FeLV infected cats, the other normal healthy cats in the home should be vaccinated against FeLV.  Cats with FeLV should be vaccinated yearly against respiratory diseases, should be kept on flea preventative, and should not be allowed to hunt or be fed undercooked meat.

Prognosis

• About 1/3 of cats exposed to the virus will become permanently infected.  These cats typically die within 2-3 years. About ¼ of these permanently infected cats will develop cancer (usually lymphoma).  The remaining ¾ of these cats will usually die or be euthanized due to an infection.

Prevention

• Have all new cats tested for FeLV by a veterinarian before integrating them into the household. 
  Keep cats indoors, but if they go outdoors have the cats vaccinated against FeLV (It is better to keep the cats indoors as the vaccine is not 100 % protective.)
  Spay and neuter outdoor cats to decrease fighting. 
  Do not allow unaffected cats to mix with known infected cats or share food and water dishes. 
  Avoid keeping large populations of cats in a small area.

References

• Barr, M.C.  Feline Leukemia Virus Infection (FeLV).  In Tilley, L.P. and Smith, W.K. (Eds.)  The 5-minute Veterinary Consult: Canine and Feline, 3rd Ed.  Lippincott Williams &Wilkins: Philadelphia, 2004.
• Greene, C.E.  Feline Leukemia Virus Infection Update on Diagnosis and Prognosis. In World Small Animal Veterinary Association World Congress Proceedings, 2003.
• http://www.vin.com/Members/Proceedings/Proceedings.plx?CID=WSAVA2003&PID=pr06544&Print=1&O=VIN.  11/13/04
• Lappin, M.R. Chapter 102: Polysystemic Viral Diseases. In Nelson, R.W. and Couto, C.G. (Eds.).  Small Animal Internal Medicine, 3rd Ed.  Mosby, Inc.: St. Louis, 2003.
• Yin, S.A.  The Small Animal Veterinary Nerdbook, 2nd Ed.  CattleDog Publishing: Davis, CA, 1998.