Testing for FIV and FeLV – What the Tests Can and Cannot Tell Us

Why testing is not the end-all-be-all of retrovirus management in cats.

by Brittany Roth, Calvin’s Paws Medical Care Coordinator and FIV/Felv Advisor

It seems simple enough – you go to the vet with your new furry friend, and the vet recommends testing her for the two common retroviruses in cats – FIV and FeLV (for the difference between the two viruses and more information, read our previous blog post here). You get the test results back, and they’re either positive or negative, and that’s that, right? Well, not really.

Why not?

A positive test result may indicate that your cat has one of the two viruses…but it might not. How can this be? There are two main factors that play into this: the age of the cat or kitten and the virus that they’re testing positive for.

Kittens

As we explained in our previous blog post, My Kitten has FIV, any kitten younger than 6 months testing positive for FIV most likely does NOT actually have the virus. The test for FIV looks for antibodies to the virus, not the actual virus itself. Kittens get antibodies from their mother while they are nursing, so if the mom has antibodies to FIV, she can pass those on to the kittens. Once the maternal antibodies clear out of the kitten’s system, the kitten will test negative.

The test for FeLV looks for an antigen (protein on the surface of the virus) of FeLV. A kitten testing positive for FeLV is a little trickier, but still not a sure thing. Yes, mothers can pass the virus on to their kittens in the womb or after birth, but just because a kitten gets a few virus particles in their system does not mean it will definitely become persistently infected. Kittens born to positive mothers should be separated from mom as soon as safely possible (6 weeks or so) and retested at 3-6 months of age. We have had several cases (including one just this year!) where kittens testing faint FeLV+ later cleared it from their system and tested negative within about 2 months.

FIV+ versus FeLV+

Adults testing positive are a somewhat different story, depending on which virus they’re testing positive for. Adults testing positive for FIV are usually actually infected, but a second test should ALWAYS be performed to rule out test error. If a cat tests positive with Calvin’s Paws, we always immediately do a second test, and then retest the cat in a month. If all tests agree that the cat is positive, then the cat is probably positive. But here’s the kicker – as mentioned above, tests for FIV look for antibodies to the virus, and current tests can’t tell the difference between an infected cat and a cat that was vaccinated for FIV! So even if all tests agree that your adult cat is positive for FIV, there is still no way to determine if he actually has the virus itself.

Adult Cats

Adults testing positive for FeLV simply cannot be diagnosed with FeLV based on one test. FeLV is a very complex virus and is still not well understood. When a cat is exposed to FeLV, there are 3 possible outcomes:

1. Some cats will not be infected due to inadequate exposure and a good immune response. These cats may initially test positive for FeLV after exposure, but will test negative once they clear the virus from their system.
2. Some cats will develop a latent or regressive infection; these cats will not be able to destroy all of the virus, but will be able to hold it in check. The virus integrates into the cat’s own DNA but is not active. These cats show no signs of infection and usually do not shed virus in their saliva or other body secretions. However, the infection can later become active again, especially if the cat becomes stressed or immunocompromised. These cats may or may not ever test positive, depending on when they are tested.
3. Some cats will become persistently infected; these cats will not develop an adequate immune response and will remain permanently infected with FeLV. This is called a progressive infection. These cats will shed large amounts of virus in their saliva and often develop FeLV-associated diseases within a few years. These cats should always test positive.

If a cat is tested only once for FeLV and comes up positive, it doesn’t tell you much. One positive test does not tell you which of the 3 outcomes above that the cat will have – they might be able to clear the virus completely, they might suppress it and never have symptoms, or yes, they might actually be persistently infected. To actually diagnose a cat with FeLV, you must do more testing. When an adult cat tests FeLV+ in Calvin’s Paws, we immediately repeat the in-house test to rule out test error. If that is also positive, we then take blood and send it off for an IFA lab test – this gives us a better picture of the stage of the disease. If the IFA test is also positive, it means that outcome #1, clearing the virus completely, is extremely unlikely, and we can officially diagnose the cat with FeLV. If the IFA test is negative, it means that the cat is still in the stage of infection where they might be able to fight the virus off completely. In this case, we isolate the cat and retest them in one month. Theoretically, you keep retesting until the test results agree, but it is possible for a cat to have an atypical infection and consistently have discordant results on the in-house test and IFA test. This is the case with one of our FeLV cats, Ice Dream, who has been with us since 2007 and STILL tests positive on the in-house test and negative on the IFA test.

But if my cat is negative and has ALWAYS been negative, then he’s negative, right?

Not necessarily. Remember outcome #2, where cats can have a latent or regressive infection? If cats have a regressive infection, their immune system has cleared all traces of the virus from the blood, but the virus’ DNA has integrated into the cat’s own DNA, which the immune system can’t do anything about. These cats will test negative on the in-house tests AND the IFA lab test! The only way to identify them is through a PCR test, which looks for FeLV DNA and magnifies any fragments it finds until it’s at a detectable level. Because PCR technology is relatively new, there is still a lot of ongoing research on what the results of it mean in terms of the health of a cat. We do know that regressive infections can become active again if the cat is stressed or immunosuppressed, but there is very little data on how often this happens. The most current research has found that somewhere between 5% and 10% of cats that test negative for FeLV actually test positive on a PCR test, indicating a possible regressive infection.

Aside from all of this, tests cannot detect infections immediately after exposure. If a cat was recently exposed to either virus (less than a month or so before testing), they may test negative simply because they were tested too soon after exposure. Then when you take into account the fact that the tests themselves do have a known error rate for both false positives and false negatives…well, you can see why interpreting tests gets tricky.

In conclusion, a positive test result does not always mean the cat has the virus, and a negative test result does not always mean the cat does not have the virus. This doesn’t mean that testing isn’t worth doing – it just means that testing should be a starting point, not the one and only step of retrovirus management. Unfortunately, many shelters (and even vets!) treat each individual test as undeniable fact, and the results of that test are literally life-or-death.

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Calvin’s Paws is a 501(c)(3) rescue. We work through a network of foster homes throughout the Triangle area to save homeless cats and dogs. We are a dedicated group of volunteers with common goals: rescuing animals (both positives and non-infected felines), finding the best fitting homes for each animal, and educating the public on animal health and responsible pet ownership.
www.calvinspaws.com