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Snap Decision: FIV/FeLV Testing

What you need to know BEFORE you test

January 31, 2013

Did you know that the widely-used ELISA test (also known as the Snap test) can't detect whether or not a cat actually has Feline Leukemia Virus (FeLV) or Feline Immunodeficiency Virus (FIV)? Did you know that false positive Snap test results are relatively common? Or that spaying and neutering ferals, rather than attempting to cull FIV/FeLV+ cats from colonies, is ultimately a more effective means of disease control?

The decision to test feral cats for FIV/FeLV can be costly - both economically and in terms of lives that may needlessly be lost. Before including the tests in your TNR program's protocol, please read Neighborhood Cats' article, FIV/FeLV Testing, below (excerpted from The Neighborhood Cats TNR Handbook, second edition*).


FIV/FeLV Testing

The vast majority of TNR programs do not perform FIV/FeLV testing on every feral cat, but only on those who are showing serious symptoms of ill health or who are candidates for adoption. There are several good reasons for this policy:

1. With limited resources available, spay/neuter should take priority

Studies show the prevalence of FIV (feline aids) and FeLV (feline leukemia) in the feral population is similar to that in the pet population - approximately 6% test positive for one or the other virus. On average then, testing 1,000 feral cats will result in the identification of 60 cats who test positive for either FIV or FeLV. Assuming even a low cost of $12 per test, that would mean $12,000 needs to be spent to identify 60 positive cats. That same $12,000, at an average spay/neuter cost of $50, could be put instead towards neutering 240 ferals. Because funds are limited and the primary goal of TNR is not disease control but to stem overpopulation, neutering 240 more cats should take priority over identifying 60 positives.

2. Spay/neuter can be a more effective means of disease control

Even if disease prevention is considered as important an objective of a TNR program as population reduction, investing in spay/neuter may have more benefits than trying to identify and cull positive cats. With respect to FIV, the primary mode of transmission is deep bite wounds. This type of injury is most often inflicted by male cats during fights provoked by mating behavior. Neutering males will thus eliminate the most prevalent form of FIV transmission. With respect to FeLV, experience has shown the greatest "at risk" population among ferals is kittens, probably because their immature immune systems are not capable of fighting off infection. Spay/neuter, of course, prevents the birth of kittens and hence the spread of the virus. Sexual activity between adult females and males can also spread FeLV, another route of transmission eliminated by spay/neuter.

3. The kind of tests normally used for ferals are not reliable enough to make life or death decisions

The FIV/FeLV "test" is really two separate tests - one for each virus. Depending on which virus is being tested for and which type of test is being used, the results may be unreliable.

Feline Immunodeficiency Virus (FIV)

When it comes to testing feral cats for the FIV virus, most veterinarians and clinics use the ELISA (Enzyme-Linked Immunosorbent Assay) test. Commonly referred to as a "snap" test, it produces a result within minutes. It detects whether antibodies to the FIV virus are present in the blood, not whether the virus itself is present. As a result, a positive test result does not necessarily mean the cat is infected. For cats less than six months of age, FIV antibodies may have been passed to them from their nursing mother, but not the virus. To confirm infection these cats must be re-tested when they are older than six months. Another complication arises from the advent of the FIV vaccine. Cats who have received the vaccine will test positive for FIV because the vaccine stimulated their immune system to form antibodies, not because they are infected.

Even putting aside the problem of kittens with their mother's antibodies and cats vaccinated against FIV, a positive ELISA test is not a definitive diagnosis. "With ELISA tests, the incidence of false positives is relatively high. Positive results, especially in asymptomatic cats, should be confirmed by another test such as a Western blot." (The Merck Veterinary Manual, Feline Immunodeficiency Virus, (2012) ). A Western blot or similar test must be performed in a laboratory and is usually considerably more expensive than the ELISA snap test. Because of the added time and expense involved, follow-up lab tests are rarely performed for feral cats.

Feline Leukemia Virus (FeLV)

For testing feline leukemia infection in ferals, the FeLV version of the ELISA snap test is also the most commonly administered. It detects whether FeLV antigen, a product of the virus, is present in the blood. A positive result indicates the cat was exposed to the virus, but does not necessarily mean he is permanently infected. He may have fought off the virus, leaving bits of antigen behind, or he might be in the process of fighting it off at the time of testing. Infection is not permanent until the virus enters the cat's white blood cells where it can replicate and spread. The ELISA test does not indicate whether this has occurred. Only a second test, such as the IFA (Immunoflouresence Assay), can determine whether the virus has entered the white blood cells.

Similar to follow-up tests for FIV, the FeLV IFA test must be performed at a laboratory and is rarely pursued in the context of TNR because it takes time to get results and is more expensive than the ELISA test. The failure to have an IFA test performed prior to euthanizing a supposedly FeLV positive cat can literally be a fatal mistake. In an article published in 2006, the National Veterinary Laboratory - a privately owned diagnostic lab founded by the inventor of the IFA test - found that over the course of three years, 32% of FeLV positive results from ELISA tests submitted to it for review were not confirmed by subsequent IFA tests. In other words, almost one of three positive ELISA results turned out to be a potentially false positive. The frequency of false positives with FeLV ELISA tests is why it is common knowledge in the veterinary field that, "Confirmation of positive results, especially in asymptomatic cats, should be pursued by testing for cell-associated antigen, eg, with an immunoflourescent antibody assay (IFA test)." (The Merck Veterinary Manual, Feline Immunodeficiency Virus, (2012) ). 

Another problem with the ELISA test for FeLV is it is sensitive and prone to false positives from mishandling. A classic example of mishandling occurs when the result is labeled a "weak positive." There is no such thing. Either the antigen is present in the blood or it's not. A "weak positive" finding almost always indicates some type of testing error.

To summarize, the common practice when feral cats are tested is to use the ELISA Snap tests with euthanasia the usual outcome when results are positive for FIV or FeLV. This practice is highly flawed as the ELISA tests are subject to error and best practices in the veterinary field call for more precise follow-up laboratory results to confirm any positive results, especially with asymptomatic cats. A TNR program that tests all cats and euthanizes based only on ELISA test results is thus not only incurring added expense, but could well end up euthanizing cats who are not infected.


4. FIV positive cats can lead relatively long lives

Cats infected with FIV have commonly been known to live for many years and some never get sick. While their immune systems are compromised, proper care and nutrition can compensate to at least some degree. Even in outdoor colony settings, FIV cats can live long lives. The caretaker does need to be alert for any symptoms of illness, which may require re-trapping and a visit to the veterinarian. By contrast, FeLV positive cats have a much higher mortality rate, with research showing 83% mortality within 3.5 years of full-blown infection. 2  Still, while they are alive, they can often live symptom-free until near the end if properly fed and sheltered.

5. Euthanizing positive cats is ineffective colony management

The theory often advanced by advocates of testing all cats is that positive ferals need to be identified so they can be removed from the colony and the remaining cats can be protected from the disease. In truth, removing the positive cat makes little difference. It's most likely the other cats in the colony have already been exposed to the virus and will have become infected or not. Furthermore, no matter how many ferals are removed because of positive test results, FIV and FeLV will remain in the environment and be an ongoing threat. New cats passing through or entering the colony could carry it and even colony members who tested negative might be harboring one of the viruses. While false negatives occur less frequently than false positives with the ELISA tests, they are known to happen. For example, a cat may have been exposed too recently before the test for antigens (FeLV) or antibodies (FIV) to appear in the blood. Or, in the case of FeLV, the virus might be hiding in the cat's bone marrow.

In our experience, the primary cause of illness in ferals, including FIV and FeLV, has more to do with proper colony management than the presence of any virus. Colonies with lots of sick cats are invariably ones that receive poor or insufficient nutrition, have inadequate shelter from cold and rain, and are unneutered and reproducing. These conditions lead to weakened immune systems and susceptibility to disease. The best way to prevent the spread of disease is not by testing and removing individual cats, but by improving the quality of food, making sure the cats have warm, dry shelter and getting them altered.


6. The life of a FIV or FeLV positive cat has value, too

At Neighborhood Cats we euthanize cats only if they are actively ill, suffering and terminal. We believe all lives, including positive cats, have value and should be respected, so if a feral cat was to test FIV or FeLV positive, but showed no signs of illness, we would return him to his colony, anyway. As a result, there's no point in our doing the testing as a matter of routine. We only test if the cat will be placed for adoption or is sick and the results are needed for diagnosis, treatment or determining if euthanasia is appropriate.

There have been a few instances where we knowingly returned FIV or FeLV positive but asymptomatic cats back to their colonies. These colonies were neutered and well managed. We have yet to observe other cats falling ill as a consequence. Cats who are known to be positive do need to be closely observed for signs the terminal stage of the illness has taken hold. Loss of weight, persistent upper respiratory infections, drooling or difficulty eating indicates the cat should be re-trapped and examined by a veterinarian.

If your veterinarian insists on testing anyway

Despite these factors weighing against the testing of ferals as part of a standard protocol, your veterinarian may insist upon it and not give you any choice in the matter. Or the agency providing funds for spay/neuter funding may require testing and be inflexible. If that's the case, then find out before the cat is tested what the policy is for handling positive cats. If they will automatically euthanize the animal, which unfortunately is often the case, then we recommend you do not work with them if at all possible. As the caretaker of the colony, as the one who watches over, worries and feeds the cats on a daily basis, it is your right to decide their fate. The veterinarian or agency giving support is there to help and provide information, not make life-or-death decisions for you. Feral cats are not second-class animals and should receive the same consideration as a pet cat. A veterinarian would never presume to decide for the guardian of a pet cat that the animal is going to be euthanized, nor should he do so for a feral simply because he's providing his services to you free or for a discount.

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1 National Veterinary Laboratory Newsletter, Current Feline Leukemia Virus Research Supports: Confirm All In-Hospital FeLV ELISA Positive Tests by IFA (2006), Vol. 5, No. 4.
2 Beatty, J., Markers of Feline Leukemia Virus Infection or Exposure in Cats from a Region of Low Seroprevalence, Journal of Feline Medicine and Surgery (2011) 13:927.



*The Neighborhood Cats TNR Handbook, second edition, is due for release in February 2013. The Handbook will be available for free download on Neighborhood Cats' website. The original Handbook is currently available; download a pdf here.

For a pdf of the article FIV/FeLV Testing
click here.