The Granby Animal Clinic, Inc.




At one time feline leukemia virus infection was one of the most common fatal diseases 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) and, thus, the source of its name.  However, this virus causes many other fatal diseases including lymphoma, anemia and life-threatening infections due to immune suppression.  


The virus is primarily transmitted through saliva; however, it can also be transmitted through nasal secretions, blood, milk, and feces. The most common source of infection is social interactions including mutual grooming and sharing food and water bowls and litter boxes. Cats can also become infected via cat fights and infected mother cats can pass the virus to their kittens before and after birth.


FeLV infection is diagnosed by a blood test. Although there are several different types of tests, the most common test is an ELISA test which can often be run during an office visit. It detects the FeLV at any stage of infection, but the test can take up to four weeks after exposure to become positive. Due to the serious nature of the disease, it is recommended to confirm a positive test with a second test. This is usually an IFA test which is performed at a laboratory.  


When a cat is exposed to the virus, it may be able to eliminate the virus or it may become infected. If the cat is able to eliminate the virus it will usually develop immunity. During this period of virus challenge, the cat may actually develop a mild form of illness.  Fever, poor appetite, lethargy, and enlarged lymph nodes in the neck may develop and last for 3 to 10 days.  About 40% of exposed cats can eliminate the virus. Immunity to the virus is more likely to develop in the adult cat than in the kitten.


About 30% of the cats exposed to the FeLV become persistently infected. Although these cats may be sick for a few days initially (as described above), they usually recover and appear normal for weeks, months, or years. Ultimately, most of these cats die of FeLV related diseases within a few years. Vaccination of infected cats will not cause any problems, but doesn't prevent the cat from developing illness.


Some cats exposed to FeLV will develop a latent infection. In latent infections, the cats have the virus in their bone marrow, but they do not have the virus in their blood stream nor are they contagious to other cats. Latent infections are difficult to diagnose and often require PCR testing on the bone marrow. These cats can develop cancer or myelodysplasia.


Once persistently infected with the virus, cats cannot be cured.  Keeping the cat indoors and isolated from other cats helps to protect the cat from contagious diseases and prevents the cat from infecting other cats. Providing an infected cat with good nutrition and routine medical care, can help extend its life. Once sick, there are multiple treatments that can help the cat feel better and extend its life, but they will not cure the disease or prevent it from progressing.


The feline leukemia virus lives, at most, only a few hours outside the cat if the environment is dry. The virus is sensitive to most disinfectants. Therefore, extensive environmental disinfection is not necessary.  It is generally recommended to wait a couple of days before bringing a new cat into a household where there was previously a FeLV infected cat.  


The feline leukemia virus only infects cats, but FeLV infected cats can harbor other infections that can be transmitted to people. Therefore, people whose immune system is compromised should discuss with their doctor if they need to take special precautions around FeLV infected cats.


If you have other cats, it is best to vaccinate them with the feline leukemia vaccine. However, no vaccine is 100% and it is recommended that FeLV infected cats be kept separate from FeLV negative cats.


Cats that are already infected with the feline leukemia virus will not be helped by the vaccine.  Prior to being vaccinated with the FeLV vaccine, cats should be tested to ensure that they are not already harboring the disease. It is also recommended to test any newly acquired cats, and cats with a history of being exposed to FeLV infected cats or with a history of cat fights regardless if they have received the vaccine or not.


The feline leukemia virus vaccine cannot cause a cat to become infected and it will not cause the feline leukemia test to become positive. The vaccine is considered safe, but as with all vaccines there is always a chance of side-effects.  The most common side effects are mild and include lethargy and fever.  On rare occasions more serious reactions can occur, including anaphylaxis (a severe potentially life threatening reaction) or fibrosarcoma (a form of cancer that will arise at the injection site).  However, in most at risk cats, the risk of contracting a feline leukemia virus disease and dying of it is considered far greater than the risk for developing a serious vaccine-related side effects.


Vaccination for feline leukemia is recommend for all kittens due to their increased susceptibility of becoming infected and for adult cats considered to be at risk for infection. At risk cats are those who go outside or have exposure to FeLV infected cats.


2003-2014 Granby Animal Clinic, Inc. All rights reserved