What is feline leukemia virus (FeLV)?
Feline leukemia virus (FeLV) is a virus that infects cats. It was first identified in cats that had a form of leukemia (cancer of the white blood cells), but it is now known to cause a range of serious health problems.
FeLV depresses the immune system, making infected cats more prone to infections, anemia, and certain cancers. Like all viruses, FeLV can only reproduce inside living cells. FeLV only infects members of the cat family and is not a risk to other animal species or humans.
How common is FeLV?
FeLV infection occurs worldwide. Around 1% to 3% of the cat population is persistently infected with this virus, and many more are exposed. The number of cats infected differs depending on geographical location, environment, and lifestyle. Infection is more common in colonies of cats where there is close contact between individuals.
What disease does the virus cause?
FeLV invades various cells of the cat's immune system and blood-forming tissues. This invasion causes the cell to die or creates a mutation (change) in the cell’s genetic code, which can make the cell potentially cancerous, although it may not happen for months to years after the infection. FeLV infection can lead to cancers in a variety of tissues, organs, and body sites. When it affects cells in the bone marrow, it can cause blood cancer like leukemia (elevated circulating white blood cells).
It can also decrease the production of red blood cells (causing anemia) or platelets (causing thrombocytopenia). When FeLV infects white blood cells in a body site, it can cause cancer in that site, such as lymphoma or lymphosarcoma. These tumors may occur at single or multiple sites in the body.
Although cancer is one outcome of FeLV infection, other diseases are more common. In many cats, FeLV suppresses the immune system, so the infected cat is less able to defend itself against a range of infections that would not normally be a problem. Affected cats may develop various clinical signs, and there is a progressive deterioration in their health over time.
FeLV-infected cats also commonly develop life-threatening anemia (low red blood cells). Other problems associated with FeLV infection include abortion, severe enteritis (intestinal inflammation), neurological (nerve) disease, and ocular (eye) disease. FeLV-related disease is usually fatal. Studies show that 80% to 90% of FeLV-infected cats will die within three to four years of initial diagnosis.
How is FeLV transmitted?
FeLV is usually transmitted by direct contact between cats. The virus is fragile and cannot survive longer than a few hours outside the cat. A cat with FeLV sheds the virus in its saliva, nasal secretions, urine, and feces.
However, FeLV is not a highly contagious virus, and transmission generally requires a prolonged period of close contact between infected and susceptible cats. Close contact activities include mating, mutual grooming, and sharing of litter trays and food bowls. A cat bite can readily transmit infection.
Infection can also occur when a pregnant cat infected with FeLV gives birth. In this situation, the kittens may be born with FeLV virus or, more likely, are infected when their mother grooms them. However, most mother cats infected with FeLV are infertile or the kittens die before they are born.
What happens when a cat is exposed to FeLV?
There are several possible outcomes once a cat is infected with FeLV.
Abortive infection: A cat is exposed to FeLV but does not develop persistent infection. Many infected cats have an effective immune response to the virus, and about 20% to 30% of cats successfully eliminate the virus before it can spread through the body.
Progressive infection: The cat’s immune system cannot get rid of the virus, so the virus spreads to lymph nodes and organs. These cats become permanently infected with the virus and are at the highest risk of developing FeLV-related disease. These permanently infected cats are primarily responsible for transmitting FeLV to other cats. Months or even years can pass between the initial infection and the onset of related disease problems. During this time, the cat may continuously shed virus particles in the saliva.
Regressive infection: The cat’s immune system is strong enough to prevent virus reproduction, but it cannot eliminate the virus before it spreads into organs or bone marrow. This type of infection occurs in up to 11% of infected cats. These cats are not normally infectious to other cats but can still develop tumors. If the cat’s immune system becomes suppressed, the virus may circulate again, causing a shift to progressive infection, becoming contagious, and potentially developing FeLV-related disease.
Focal Infection: The immune system doesn’t prevent the virus from entering tissues like the spleen or lymph nodes but does prevent it from spreading further. If the cat becomes immunosuppressed for another reason, the virus can become reactivated and spread further through the body. Focal infections are difficult to diagnose and believed to be rare.
How is FeLV infection diagnosed?
In cats with progressive infection, diagnosis of FeLV infection is relatively simple. A rapid blood test can detect parts of the virus in the blood of an infected cat. This test is very accurate and reliable, although false results may rarely occur.
Diagnosis is not as simple in cats with regressive infection. These cats may have inconsistent results (false negatives) due to fluctuating levels of virus in their blood. For this reason, sick cats that test negative for FeLV may still be tested again.
In cats with abortive infection, diagnosis varies. Some cats are positive on the initial blood test. Other cats may require another test, eight to twelve weeks after the first test, so the veterinarian can determine whether the infection is temporary or persistent. In some situations, additional blood testing at a specialized laboratory may be required to confirm infection.
Diagnosis of disease due to FeLV is more difficult because there may be a variety of signs and symptoms. It is common for other diseases or conditions to occur alongside FeLV infection, and it is difficult to determine whether FeLV is the cause.
Is there any treatment for FeLV infection or disease?
There is currently no specific treatment for FeLV-infected cats, and there is no treatment to eliminate the virus from the body. Most FeLV-infected cats eventually die or require euthanasia due to diseases related to their infection. However, many cats showing FeLV-related disease improve with symptomatic treatment, at least temporarily. For example, if FeLV is causing immunosuppression and the patient develops secondary infections, the secondary infections may be treatable, leading to clinical improvement.
Some newer treatments show promise, including antiviral medications like zidovudine (AZT) and raltegravir. Your veterinarian may try other treatments (e.g., interferons like human interferon alpha [IFN-α] and feline recombinant interferon omega [IFN-ω]), but their use is still in early stages and further studies are required.
How can infection be prevented?
Vaccines are available to protect cats against FeLV infection, and they are highly recommended for any adult cat that goes outside at any time. Vaccination is also recommended for all kittens, regardless of lifestyle, because kittens are highly susceptible to infection. As with other vaccines, an initial course of two injections is required, and regular boosters are necessary to maintain immunity. Your veterinarian will discuss the most appropriate vaccination options for your cat with you. All cats should be tested for FeLV prior to vaccination.
Although vaccination is helpful in preventing infection with FeLV and therefore controlling FeLV-related disease, no vaccine is 100% protective. Do not allow your cat to roam. When possible, do not allow your cat, particularly as a kitten, to come into close contact with known FeLV-infected cats or cats without a known history of proper vaccinations. In larger colonies of cats, it is possible to control FeLV infection through a combination of routine FeLV testing, quarantine, and vaccination programs. Fortunately, vaccinating a cat does not interfere with future blood testing for FeLV.
