Understanding The Feline Leukemia And Feline AIDS Tests
Does My Cat Really Need Them?
Ron Hines DVM PhD
In North America feline leukemia (FeLV) is not as common a disease as it once was. That is probably due to the very effective vaccines that veterinarians now have at their disposal to administer to your young cat. Little if anything has changed regarding the prevalence of the feline immunodeficiency virus (FIV) for which no acceptably-effective vaccines exist. You can read about both those diseases through the links at the top of this page. For a closer look at the test itself, expand the first cat and kitten’s image as well.
Both FeLV and FIV are retrovirus that target your cat’s immune system – specifically your cat’s CD4+ T cells. The two virus only differ clinically in the speed with which they affect your cat’s health and in the likelihood that your cat will succeed in destroying them. Feline leukemia is by far the worst. Because the symptoms of both of these diseases are non-specific, the only way your veterinarian has to confirm the presence of one or the other or both is by confirming the genetic signature of the FeLV virus itself or, for FIV, confirming the presence of antibodies your cat produces in response to the presence of that virus. Veterinary clinics throughout the world now have in-office (“point-of-care”) tests to rapidly do that. The most common in-office tests used in North America today are manufactured by Idexx Laboratories of Westbrook, Maine and Zoetis Inc. of Parsippany, New Jersey. Both are patented variations of the immunochromatography and ELISA test protocols. Both of these feline viruses are very distant cousins of the HIV-1 retrovirus that jumped from chimpanzees to humans in the 1930s. (read here) Although neither FeLV nor FIV virus pose a threat to you, the effects of all three of them on immune systems are very similar.
How Common Are These Two Cat Viruses?
Studies report between 3-10% of cats in America being infected with one of these two viruses with the percentage of positive cats varying substantially depending on the population of cats being sampled. Estimates are that only ~40% of cats in the United States ever visit a veterinary hospital for post-neuter care. The majority of homeless and feral cats never do. (read here) So the estimates tend to be falsely low among the general cat population, high among feral cat colonies where transmission risk is greater and highest among cats presented to veterinarians because of undiagnosed general health issues.
How Reliable Are These Two Tests? Is One Company’s Test Better Than The Other?
To date, no one without a dog (or cat) in the race (without a monetary interest in the results obtained) has performed a comparison. The studies paid for by Idexx found that their SNAP FIV/FeLV Combo Test was undeniably better than Zoetis’ WITNESS® FeLV-FIV Test. (read here) While the studies paid for by Zoetis found that their WITNESS® FeLV-FIV Test was undeniably better than Idexx’s SNAP FIV/FeLV Combo. (read here) Would you trust Ford to unbiasedly rate a Chevrolet or vice versa?
However we know that neither of these two feline tests are 100% accurate. Much depends on how long the virus has been present in your cat and the expertise of the individual performing the test and interpreting the results. That is no different regarding the in-office tests used to confirm the presence of another retrovirus, HIV, in people. Only 50% of all HIV infections are accurately identified with point-of- care tests during the first 26-37 days of infection and it was found that the expertise of the person performing the test was considerably more important than the brand of test being used.
So if you require more certainty, a positive in-office FeLV/FIV test ought to be followed up by an IFA or PCR test performed at a large veterinary diagnostic laboratory to confirm your cat’s initial in-office results. False-positive in-office tests are more common that false-negative tests because many more cats in the general population are free of both viruses. Unlike the situation with prior FIV vaccinations, prior feline leukemia vaccinations are not thought to result in false positive tests. Even central lab-run IFA tests have been know to yield false negatives in FIV+ and FeLV+ cats early in the disease when the virus has not yet established itself in sufficient numbers in the cat’s bone marrow. ( ask me for Westman2019.pdf) Theoretically, PCR tests should never return false positives but even PCR tests have their limitations. (read here)
When Should One Of These Two In-Office Tests Be Performed?
Cats are commonly tested for Feline Leukemia Virus and Feline Immunodeficiency Virus:
When they are showing signs of illness that your veterinarian cannot readily attribute to a specific disease – particularly general declines in their health.
When they relapse despite what should have been effective treatment.
When a new cat is being considered for a family member.
Sixty days after your cat was lost and a substantial time passed before it was found or returned to you or after any potential FIV exposure.
When another FIV+/FeLV cat has been identified in your household.
Prior to release for adoption from an animal shelter or cat rescue. Insist on it. You agree to cover the test’s cost and be sure that an experienced veterinary nurse performs the test.
All kittens born to FIV/FeLV-positive queens. Before adoption and again at 4 month of age. Four month of age will catch most infected cats but not every last one of them. A test at 6 month of age usually does. That is because blood virus and/or antibody level must reach substantial levels to trigger positive test results. Kittens can also test false-positive due to antibodies passed on through their mother’s first milk (colostrum). Many of those false-positive kittens return negative test results sixty days later. Some shelters only test the mother (when she is available) and then test the infants should she turn out to be positive. Others operating on a shoestring might just test one kitten out of a litter. Still others might pool the blood sample from the entire litter of kittens and use that in one of the two in-office tests (really not a good idea). Administering feline leukemia-containing vaccines to kittens at recommended times are not thought to influence in-office test results (read here)
When doubt persists, immunofluorescent antibody (IFA) tests and PCR tests performed at a central lab are the most accurate way to determine your kitten or cat’s true FIV/FeLV status. At some point the ELISA, IFA and/or PCR tests should all agree.
A few FeLV+ young cats will return negative results on subsequent tests for the virus. Current thinking is that these cats represent abortive infections in which the cat’s immune system actually destroyed all of the pathogenic FeLV virus in its body. Read more about Abortive FeLV infection here.
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