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Cats Feline Leukemia Virus (FeLV)







Feline leukemia virus infection was, until recently, the most common fatal disease of cats. Because we can now protect cats with a leukemia virus vaccine, we are seeing fewer cases of the disease. However, it still remains a major cause of death in cats.


"Leukemia" means cancer of the white blood cells. This was the first disease associated with the feline leukemia virus (FeLV), a retrovirus and, thus, the source of its name. We often use the term "leukemia" rather loosely to include all of the diseases associated with the virus, even though most are not cancers of the blood. This virus causes many other fatal diseases, in addition to leukemia.


What diseases are caused by the FeLV?

There are three major disease categories associated with the FeLV:

  1. The Leukemias are cancers of the white blood cells.

  2. Lymphosarcoma is a cancer of many different organs but it begins in lymphoid tissue, such as a lymph node. Almost any tissue may be affected; organs commonly involved include lymph nodes, intestinal tract, kidneys, liver, spleen, spinal cord, brain, bone marrow and blood.

  3. The Non-Cancerous Diseases include a variety of somewhat unrelated diseases: anemia, abortion, arthritis, and immune suppression are examples. When the immune system is suppressed, the cat becomes susceptible to many diseases cats would ordinarily resist and mild diseases, such as respiratory infections, may become fatal.

How is the virus transmitted?

The main means of transmitting the virus is through cat fights. Because large quantities of the FeLV are shed in cat saliva, puncture wounds associated with fighting result in injection of the virus into other cats. Low levels of virus are found in the urine and feces. Other less frequent routes of viral spread include sneezing, sharing food and water bowls, cats grooming each other, and transmission from mother to kittens before birth.


What is a "leukemia test"?

The "leukemia test" is used to determine if a cat harbors the virus. Any of three different tests may be used to detect a particular virus protein (antigen) in the blood, white blood cells, or tears/saliva.

  1. The blood ELISA test (preferred screening test) is performed on a blood sample and detects the FeLV at any stage of infection. This test turns positive within a few days of infection and, in some cases, may later turn negative if the cat's immune system eliminates the infection.

  2. The IFA test (a confirmatory test) is performed on a blood smear and turns positive only after the FeLV infection has progressed to a late stage of infection. Once positive, the IFA test usually means that the cat has a permanent infection. A cat who tests IFA positive is only rarely able to successfully eliminate the virus.

  3. The tears/saliva ELISA test is performed on a sample of tears or saliva. It turns positive only in a late stage of infection; therefore, it may yield a false negative result in cats who are in the early stage of FeLV infection. It also has been associated with some false positive results due to inherent errors in the way the test is performed. Therefore, this test is not used routinely.

We recommend if a positive or equivocal result occurs, the test should be repeated, before declaring the cat positive.





What can happen if a cat is infected with the FeLV?

When we are exposed to a virus, such as a flu virus, there are two possible outcomes. Either our immune system responds to the challenge and protects us, or it is unable to respond successfully, and we develop the flu. A number of factors determine which outcome occurs and whether or not we will get sick:

  • The amount of the virus

  • The strain of the virus

  • The status of our immune system

  • Age (the very young are more likely to become infected)

  • The presence of other infections, which might cause debilitation

The behavior of the feline leukemia virus in the cat's body is not so black or white. Instead of the two possible outcomes described above (that is, we get sick or we get well), there are four possible outcomes for cats with FeLV. Understanding these allows one to more fully comprehend some of the unusual situations which may arise in cats.


Outcome 1: Immunity

The cat mounts an immune response, eliminating the infection. This is the most desired outcome because it means that the cat will not become persistently infected with the virus. During this period of virus challenge, the cat may actually develop a mild form of illness.


Fever, poor appetite, lethargy, and swollen glands (lymph nodes) in the neck may develop and last for 3 to 10 days. Outcome 1 occurs about 40% of the time after a cat is challenged by the FeLV. Immunity to the virus is more likely to develop in the adult cat than in the kitten.


Outcome 2: Infection

The cat's immune system is overwhelmed by the virus. This is the least desired outcome because the cat is persistently infected with FeLV. All three of the FeLV tests will become positive and remain positive for the rest of the cat's life. Although the cat may be sick for a few days initially (as described above), the cat usually recovers and appears normal for weeks, months, or years.


Ultimately, most of these cats die of FeLV-related disease, but as many as 50% will still be healthy after 2-3 years and 15% after 4 years. Vaccination of these cats will not cause any problems but doesn't help the cat, either.


Outcome 2 occurs about 30% of the time after a cat is challenged by FeLV. Although infection is more likely to occur in the kitten, many cats are persistently infected as adults. Although the main mode of viral transmission is through bite wounds (saliva), direct daily contact with an FeLV infected cat will often result in transmission of the virus. Non-infected exposed cats are at risk and should be vaccinated, although daily viral contact will result in vaccination failure of some cats.


Outcome 3: Latency

The cat harbors the virus, but we cannot easily detect it. Unlike other viruses, the FeLV does not directly kill the cat's cells or make them become cancerous. Instead, it inserts a copy of its own genetic material (called DNA) into the cat's cells; these cells may later be transformed into cancer cells or cells which will no longer function normally.


In Outcome 3, the genetic change in the cat's cells will remain undetected for an average of 2 1/2 years, during which time the cat will appear completely normal.


In the early stages of infection, the blood ELISA test will be positive, but it will turn negative about 2-4 weeks later. Following that, the blood ELISA and the IFA tests will remain consistently negative. The PCR test, a recently available diagnostic tool, will detect the latent infection. However, this test is expensive and not widely available so it is not used for routine testing.


The prospect of latent infection presents us with a frustrating and precarious situation. Latency is estimated to occur about 30% of the time.


Some cats will ultimately reject the abnormal cells, and the state of latency will be terminated. In other cats these abnormal cells will result in the production of new FeLV which will result in Outcome 2. Outcome 2 generally leads to death due to an FeLV disease.


Latency is the state that explains the following situations:

  • Latently infected cats will test negative on any FeLV test. If they are vaccinated, they will not be protected. They may develop a fatal FeLV-related disease later, especially following some form of stress or the administration of steroids.

  • Stressors that may activate latent infections include pregnancy and nursing, overcrowding, movement to a new environment, territorial conflicts, poor nutrition, and other diseases. Steroids are used commonly in cats because they are very beneficial for many feline diseases.

  • Lymphosarcoma is the form of cancer normally caused by the FeLV. Cats with some forms of lymphosarcoma normally test positive with any FeLV test. Latently infected cats may have lymphosarcoma and test negative on the FeLV tests. It is also thought that some cats successfully eliminate the virus but not before malignant transformation of cells has already occurred. This may be another explanation for FeLV-negative cats with lymphosarcoma.

  • Latently infected pregnant cats may test FeLV negative (and even be vaccinated) but pass the FeLV to their kittens through nursing. These kittens often experience Outcome 2.

Outcome 4: Immune carrier

The cat becomes an immune carrier. The FeLV becomes hidden in some of the cat's epithelial cells. Although the FeLV is multiplying, it is not able to get out of these cells because the cat is producing antibodies against the virus. The cat will appear normal in every way except for the tests results. The immune carrier will have a positive blood ELISA test and a negative IFA test.


This situation is unlikely to happen; it is estimated to occur 1-2% of the time. These cats may revert to an active FeLV infection (Outcome 2) or may develop a latent infection (Outcome 3). The main reason for understanding this situation is that it explains conflicting FeLV test results. Otherwise, there is not a specific test to detect it.


How are cats with leukemia treated?

Some forms of leukemia (blood cancer) are unresponsive to all available forms of cancer treatment. Other types of leukemias may respond to chemotherapy, though many of these have an average survival time of less than one year. Because the virus is not affected by treatment, the cat will always remain infected with FeLV. Also, relapse of leukemia is possible (and expected).


What should I do to disinfect my house?

The FeLV lives, at most, only a few hours outside the cat if the environment is dry. Therefore, extensive environmental disinfection is not necessary. If you wait even 2 days to get a new cat, you can be assured that none of the virus from a previous cat will remain in your house.


I have a healthy cat who is infected with the virus. What does that mean?

Healthy infected cats may remain apparently unaffected by the virus for several years. However, such cats should be considered infectious and potentially dangerous to other cats. Such cats should be isolated from non-infected cats to prevent spread of infection. To be fair to other people's cats, your cat should be kept indoors and not allowed to meet any other cat.


Is there any danger to my family?

Extensive tests have been conducted for over 15 years to determine if the FeLV can be transmitted to humans. Thus far, no evidence has shown any FeLV-related disease in humans or other animal species, including the dog. However, persons with compromised immune systems for example, newborn babies, persons on chemotherapy, AIDS patients, or transplant recipients on anti-rejection drugs should probably not be unnecessarily exposed to this or any other virus.


Can I protect my other cats?

A vaccine is available to protect cats from the FeLV. Although not 100% of cats are totally protected, the vaccine is strongly recommended for cats who are exposed to open populations of cats (that is, outdoor cats). We have seen a definite decline in the incidence of feline leukemia virus infection and related diseases since vaccine use became widespread. We strongly recommend it. Two vaccines 3 weeks apart can be given to kittens 10 weeks and older, and yearly boosters are needed thereafter. If your cat stays indoors at all times and is not in contact with another cat that goes outdoors, the need for the vaccine is minimal. Cats who are already infected with the FeLV will not be helped by the vaccine. (They will not be hurt by it, either).


We recommend pre-vaccination testing for the FeLV for:

  • Cats with a history of cat fights or fight wounds (abscesses)

  • Cats exposed to FeLV-infected cats

  • Cats from unknown backgrounds (particularly animal shelters, humane societies, or pet shops)

  • Routine health care, especially in multicat households

We recommend all kittens and cats who test negative on the first blood ELISA test, and with suspected exposure to FELV, be retested in 90 days.


Will vaccinating my cat with the FeLV vaccine cause the leukemia test to be positive?

No. The vaccine will not cause a cat to test positive for the virus. While the history of vaccination is important to know, it does not alter the ability to interpret the feline leukemia virus (antigen) test.


Note: If you live in Israel, the incidence of FeLV in your area might be so low that a vaccination against FeLV would be unnecessary, even for an outdoor cat. Please discuss this with your veterinarian.