Understanding Feline Leukemia Virus (FeLV): Causes, Transmission, and Prevention

Guest Post by Lorie Huston, DVM

Feline leukemia is a frequent worry for cat owners, regardless of the cat’s breed. This is because feline leukemia, or FeLV as it sometimes known, is a contagious disease that can be passed from cat to cat.

What Causes Feline Leukemia

Feline leukemia is caused by a virus in the class of viruses known as retroviruses. It is passed from cat to cat by direct contact with body fluids. Normally, close contact is necessary for the disease to be passed. Activities such as grooming, sharing food dishes, water bowls or litter boxes, fighting or playing can all spread FeLV.

Feline leukemia can also be passed from a pregnant mother cat to her kittens. It is one of the leading causes of “fading kitten syndrome.”

How Is Feline Leukemia Detected or Diagnosed?

Many cats that test positive for feline leukemia appear perfectly healthy and it is impossible to diagnose FeLV by simply looking at a cat. The most common means of diagnosing feline leukemia is with a blood test that tests for antigen to the feline leukemia virus. This test is known as an ELISA test.

Though the ELISA test can be performed on saliva and on tears as well as blood, blood test results typically result in fewer inaccurate results.

Testing is recommended under several circumstances, according to the AAFP (American Association of Feline Practitioners) Retrovirus Guidelines.

• All kittens should be tested for feline leukemia, especially if their mother’s feline leukemia virus status is unknown.
• Any cat new to a household should be tested for feline leukemia before being introduced to cats already in the household.
• Any sick cat, regardless of previous feline leukemia virus status, should be tested.
• Any cat for which vaccination against feline leukemia is being considered should be tested prior to vaccination.
• Any cat exposed to feline leukemia should be tested.

What Happens if My Cat Tests Positive for Feline Leukemia?

One of three things can happen when a cat is tested for leukemia.

1. He can test negative, which is the ideal situation and indicates he is not likely to be infected with the virus.
2. He can test positive but be able to mount an immune response which is adequate to rid himself of the virus. In this case, a subsequent test will be negative.
3. He can test positive but be unable to clear the infection. Subsequent tests for feline leukemia will remain positive.

If your cat does test positive for feline leukemia, he should be retested in 6-10 weeks. If he still tests positive, there is a good chance he has a persistent infection. There is another test, the IFA test, which is recommended to confirm infection in this case. This is done to rule out the possibility of a false positive ELISA antigen test.

Under rarer circumstances, a cat can harbor the feline leukemia virus in the bone marrow, appear perfectly healthy and test negative on both tests (or test positive on an ELISA test and negative on the IFA test.) For these cats, under conditions of stress, the virus can become active again. This is the reason that some breeders recommend testing cats used in breeding programs every 6 months. It is also the reason for recommending that any sick cat be retested for FeLV, regardless of the previous FeLV status.

If your cat does test positive for the feline leukemia virus and is healthy otherwise, there are several precautions which should be taken.

• The cat should be housed indoors.
• Have your cat spayed or neutered.
• He should be housed separate from other cats or only with other cats that are positive for FeLV also.
• Keep your cat up-to-date on vaccinations.
• Keep your cat free of both internal and external parasites, such as fleas, ticks, intestinal parasites and heartworms.
• Feed your cat a high quality diet. Raw diets may not be appropriate.
• Monitor your cat closely for signs of illness. Any illness should be reported to your veterinarian immediately and should be treated aggressively.

Vaccinating Against Feline Leukemia

There is a vaccine available for feline leukemia which may be considered for your cat, depending on your cat’s lifestyle and risk of exposure.

• Only cats which have tested negative for feline leukemia should be vaccinated. The vaccine will not protect a FeLV positive cat.
• Only cats that are at risk of exposure to feline leukemia should be vaccinated. Cats that live indoors and whose risk is minimal do not need to be vaccinated. Vaccination may be considered for cats that spend time outdoors and are likely to socialize with other cats.
• Some veterinarians recommend vaccinating all kittens for feline leukemia because kittens tend to be more susceptible to the disease than mature cats.

Symptoms of FeLV in Cats

Many cats that test positive for feline leukemia will appear perfectly healthy. However, when disease does occur, it can mimic many other feline diseases. The feline leukemia virus can cause a form of cancer but most frequently, it acts by suppressing the immune system, leaving the infected cat susceptible to secondary infections. Essentially, feline leukemia should be considered a potential cause for any sick cat until the blood testing has ruled out FeLV as the cause. Respiratory symptoms (sneezing, coughing, runny eyes, runny nose, etc.) are common with feline leukemia, as are symptoms such as weight loss, lack of appetite, fever and other signs of generalized illness.

Cats that test positive for feline leukemia but are otherwise healthy may remain so, sometimes for long periods of time. However, once symptoms of feline leukemia surface, the disease is difficult to treat and is most often fatal. Treatment is usually symptomatic and based on clinical signs, although various medications to boost the function of the immune system have also been tried with varying degrees of success.