What is Feline Leukemia?
Feline leukemia virus, also known as “FeLV,” is the cause of a variety of diseases. As a retrovirus, it replicates itself inside living cells, making it impossible to cure. Specifically, FeLV invades and replicates in cells of the cat’s immune system and tissues that create blood cells. FeLV is specific to cats and does not pose a risk to other animals or people.
What diseases does the virus cause?
In many cats, FeLV infection results in suppression of the immune system. This means that the cat is less able to defend him/her-self against a wide range of infections that would not normally cause a problem in healthy cats. A variety of symptoms could develop, and there is a progressive deterioration in their health over time. However, a FeLV cat can live for many years without illness.
FeLV can cause diseases such as enteritis (chronic diarrhea), eye disease or stomatitis (painful inflammation of the mouth). Occasionally the virus can cause cancer, but this is uncommon. Cancers can occur in a variety of tissues, organs and body sites, depending on the type and location of cells that have been infected with FeLV. The most common tumor associated with FeLV is that of lymphoid (immune) cells known as lymphoma. These tumors may occur at single or multiple sites in the body.
How common is FeLV?
FeLV infection is found worldwide in approximately 1-2% of the cat population. The proportion of cats infected differs according to the geographical location, environment and the lifestyle of the cat. Infection is more common in colonies of cats where there is close contact between individuals.
How is FeLV transmitted or spread?
A cat with FeLV sheds virus in bodily fluids such as saliva, urine and feces. FeLV is not a highly contagious virus, as the virus cannot survive longer than a few hours in the environment outside of the cat. This means that if you pet a cat with FeLV and then pet another cat, the risk of transmission is essentially zero.
Transmission requires a prolonged period of close contact between infected and susceptible cats. Susceptible cats are kittens and adult cats with weakened immune systems. Healthy adult cats are essentially immune to the disease. Cat bites by an infected cat can readily transmit infection. Close contact activities include mating, mutual grooming, and sharing of litter trays and food bowls. Because of this possibility, We does NOT recommend co-mingling FeLV positive cats with FeLV negative cats.
How can infection of FeLV be prevented?
- Keep cats indoors.
- Don’t allow your cats to interact with cats that are unknown to you, and could carry disease.
- Have your cats spayed and neutered.
- If your cat goes outside, consider vaccination. Speak with your veterinarian. (Although vaccination is helpful in preventing infection with FeLV, no vaccine is 100% protective.)
How long will my FeLV cat live? Although many FeLV cats live shorter lives, they will have several great years in their new home.
Sadly, 3 out of every 4 cats will need to be euthanized 3 years after diagnosis, but each cat is different. Eventually, when your cat becomes ill and there is no effective treatment, you will discuss with your veterinarian when it is time to say goodbye.
How do I keep my FeLV cat healthy?
- Keep cats indoors.
- Don’t allow your cats to interact with cats that are unknown to you, and could carry disease.
- Have your cats spayed and neutered.
- Get regular examinations every 6 months by a veterinarian.
- Keep your cat up-to-date on routine vaccinations and preventives (such as parasite medications). Bring your cat to the veterinarian as soon as you notice he/she is not feeling well.
- Feed a high quality diet.
How do I know if my cat is showing signs of FeLV?
There are many potential issues that FeLV cats can develop, so owners should be aware of their cat’s normal behaviors and bring them to the vet right away if anything changes. If your cat becomes ill, it does not mean that your cat cannot be treated – most secondary infections due to the virus are treatable. Some common symptoms of illness include:
- Lethargy, or a decrease in normal activity
- Decreased appetite or anorexia
- Frequent sneezing or coughing
- Green discharge from the eyes or nose
- Chronic diarrhea
- Lumps on or under the skin
How can FeLV infection be diagnosed?
A quick test can be performed which is able to detect the virus in the blood of an infected cat, also known as a “SNAP” test. This test is accurate and reliable; false results rarely occur.
If a cat is diagnosed positive on the SNAP test, a second test is sent to the lab called an “IFA”. This tests for cells infected by the virus in the bloodstream, not just the virus itself. If a cat is positive on both tests, there is no need to retest – the cat has FeLV and it is infecting their cells.
If a cat is positive on the one test but not the other, your veterinarian will recommend retesting in approximately 60 days. This is to ensure diagnosis. In rare cases a cat will test positive the first test, then negative the second test. This means they may have been exposed to FeLV but “cleared” the infection.
Is there any treatment for FeLV infection or disease?
There is currently no specific treatment for FeLV-infected cats and no cure. If FeLV is causing immunosuppression and the patient develops secondary infections, the secondary infections may be treatable, leading to improvement.
Making sure your FeLV+ cat really has Feline Leukemia
Idexx Snap/Elisa tests have false positives frequently. If you have run a Snap/Elisa test using whole blood, you first need to re-test using serum instead of whole blood. Blood simply needs to be spun down in a centrifuge and the serum used in a new Snap/Elisa test. Whole blood yields frequent false positives, serum does not. If you do not have a centrifuge, a vial of whole blood may be taped to a wall for 20 minutes and the blood will separate on its own in around 20 minutes, allowing you to gather only serum.”
If your cat is under six months of age and has serum tested positive, you must re-test again at six months. Kittens have an approximate 30% chance of fighting off the virus, which they were presumably exposed to at birth. No cat should be declared truly FeLV+ until tested at six months of age. These kittens should be isolated from other cats during this time.”
If your cat is an adult who has tested FeLV+ on an Elisa/Snap test, you still want to re-test again in 4 weeks. An Elisa/Snap tests measures exposure to the virus. Again, cats can, and many do, fight off the infection after initial exposure. If the Elisa/Snap tests positive on serum four weeks later, you can confirm the FeLV diagnosis.”
An IFA Laboratory test is a definitive test but is more expensive and must be sent to a lab, so it takes longer. The IFA test reveals true FeLV infection in the body, not just exposure to the virus, so it is considered conclusive. You may order an IFA after getting a positive serum Elisa test, if you choose, to be absolutely certain.
Caring for a Cat with FeLV
- Feed your cat a nutritionally balanced diet, one free of raw meat, eggs and unpasteurized dairy products, which can harbor bacteria and parasites and lead to infection.
- Provide a quiet, stress-free place for your cat to rest indoors and away from other cats who could promote disease.
- Bring your cat to the vet every six months—at the very least—for a wellness checkup and blood tests.
- It’s not advisable to introduce a new uninfected cat into the household, even one who has been properly vaccinated against FeLV.
- FeLV is contagious to other cats, but not to humans or other species. Other cats in the house can acquire the virus from an infected cat. Though the virus doesn’t live long outside of the body, and is easily inactivated with common disinfectants, it can be passed through mutual grooming, shared food and water as well as common litter boxes.
- Sadly, there is no cure for FeLV, and it is estimated that less than 20% of clinically infected cats survive more than three years of active infection. In the case of those cats who develop cancer, chemotherapy can help prolong life, but treatment often focuses on providing the best quality of life.
Taken from the ASPCA’s Website
Testing For Feline Leukemia
Currently available FeLV tests detect either antigens against the virus or the presence of viral nucleic acid, Dr. Little said. The POC ELISA test kits used in clinics detect soluble antigen (typically, p27 antigen) that circulates in the bloodstream. Referral laboratories also perform ELISA tests for FeLV, as well as follow-up testing using either an immunofluorescence assay (IFA) or PCR test—typically, to detect proviral DNA.
IFA Testing for FeLV
The IFA tests for the p27 cellular-associated viral antigen in infected neutrophils and platelets. The presence of viral antigen in these cells indicates that FeLV has infected the bone marrow, Dr. Little explained. This occurs later in infection, usually after 6 to 8 weeks, and represents a more progressive stage of FeLV infection. Consequently, she highlighted that 1 issue with IFA testing is that cats can be infected with FeLV but test negative using IFA if the infection has not yet reached the bone marrow. Another problem is that a low white blood cell count may prevent IFA from correctly detecting FeLV infection. “Remember that IFA is more important to use to determine whether FeLV infection has established in the bone marrow,” she emphasized.
PCR Testing for FeLV
PCR testing in a laboratory can also help veterinarians determine a cat’s FeLV status. For example, conducting PCR would be useful if a cat tested FeLV positive using a POC screening test but negative using IFA. “PCR turns positive earlier than any other test—as early as 2 weeks after infection,” Dr. Little said.
Veterinarians have a couple of options for retesting a cat that initially tests FeLV positive using a POC test kit. “You can retest the cat immediately using a different but reliable brand of POC test,” she said. However, many veterinarians go with the validated PCR testing option: “PCR has risen to top choice in such a scenario when you are trying to determine the cat’s true FeLV status after a positive screening test result.”
Testing Kittens for FeLV
As with FIV, there is no firm rule about the youngest age for FeLV testing in kittens, Dr. Little said, but FeLV can transmit more easily than FIV from queen to kitten. Nevertheless, a kitten that has been infected with FeLV from its mother may still test negative until it seroconverts and tests positive.
Dr. Little advised veterinarians on the importance of considering the FeLV-negative kitten’s living environment, because other in-contact animals may also need to be tested. The virus can be spread both vertically and horizontally. Although cats typically acquire FeLV via the oronasal route by mutual grooming, they can also acquire it through bites.Viremic cats also shed FeLV in body fluids, including saliva, nasal secretions, feces, milk, and urine.
Family testing can therefore be useful, Dr. Little said, especially for a young kitten that is still with its mother and/or litter mates. “If any of the cat’s family members test FeLV positive,” she said, “others will be suspect until we can sort out their individual FeLV status.”
How the Stages of FeLV Infection Affect Testing
FeLV testing can be somewhat more complex than FIV testing because of the different stages of infection, Dr. Little said.
Abortive Infection
After a cat is infected with FeLV, the virus initially replicates in the lymphoid tissue in the oropharynx. In some immunocompetent cats, the immune response eliminates the virus at this stage, before viremia occurs, and the infection is considered abortive. Cats with abortive FeLV infection thus test FeLV negative using all FeLV tests (Table). They show no evidence of viremia, and neither FeLV antigen nor proviral DNA can be detected in their blood.
Regressive Infection
If a cat fails to eliminate the virus, FeLV provirus can integrate into the DNA of the cat’s cells. After viral replication in the oropharyngeal lymphoid tissue, an initial wave of viremia occurs, and the virus infects lymphocytes and monocytes. These infected white blood cells circulate and transmit the virus throughout the body.
If the immune system can contain the FeLV infection at this point, it does not proceed and is considered regressive. Cats with regressive FeLV infection have a low risk of developing FeLV-related disease and do not transmit infection to other cats. However, viremia in these cats can be reactivated by concurrent disease or high-dose corticosteroid treatment, although this is considered uncommon.
Regressive infections may produce different patterns of positive and negative FeLV test results. Most commonly, these cats are FeLV negative using typical antigen-screening tests and just test positive using PCR (Table). “So, unless you have a reason to use the PCR test for FeLV, you might not know these regressively infected cats exist,” Dr. Little said. However, veterinarians should also be aware that the cat may have undergone PCR testing for FeLV during a period in which it was transiently antigen positive, she said. If retested at a later stage using an FeLV antigen- screening test, the cat will test negative.
Sometimes a regressively infected cat may transiently test FeLV positive using an antigen-screening test, Dr. Little said, which should also raise the suspicion of regressive FeLV infection. This scenario is more likely to occur in kittens older than 4 to 6 months because, as the immune system matures, they can eliminate or control FeLV infection more effectively.
Progressive Infection
If the cat’s immune system cannot contain the FeLV infection, the infection proceeds and extends to the bone marrow. A second wave of viremia occurs with viral shedding, and the infection is considered progressive. These cats become persistently viremic. “Progressively infected cats are the ones we are most concerned about because of their high risk of disease and ability to transmit infection to other cats,” Dr. Little emphasized.
Infection with FeLV early in life is the scenario that most likely will lead to progressive infection, Dr. Little said, because young kittens do not have robust immunity and are less likely to either eliminate the virus in the abortive stage of infection or become regressively infected by controlling it.
Progressively infected cats will test positive for FeLV using all tests (Table), she said, so they tend to be relatively easy to identify. Thus, if a cat tests FeLV positive using a POC antigen-screening test, veterinarians should at least consider that the cat might be progressively infected.
Conclusion
Regarding both retroviruses, Dr. Little emphasized that veterinarians may not be able to determine a cat’s infection status based on the results of any single test performed at a single time point. She also advised veterinarians not to pool blood samples from a group of cats for testing or select just 1 or 2 animals to sample from a litter or colony to reflect the overall FeLV or FIV status of the group.
Follow-up testing recommendations are changing for retroviruses, she said, with greater emphasis on validated PCR testing. The most recent version of the feline retrovirus management guidelines from the American Association of Feline Practitioners was published in 2008. However, updated guidelines will be published soon, she said, and will also address issues such as FIV testing in cats younger than age 6 months, core vaccinations in retrovirus-positive kittens, and retrovirus screening in shelter situations.
Dr. Parry, a board-certified veterinary pathologist, graduated from the University of Liverpool in 1997. After 13 years in academia, she founded Midwest Veterinary Pathology, LLC, where she now works as a private consultant. Dr. Parry writes regularly for veterinary organizations and publications.References
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