Diagnostic Tests: FeLV/FIV

July 1, 2019

Two particularly serious infectious diseases we are worried about in cats are feline leukemia virus (FeLV) & feline immunodeficiency virus (FIV). These viruses infect approximately 2-3% of cats.

These retrovirusus can be a bit tricky to predict and diagnose.  There is often confusion and miscommunication regarding feline leukemia and FIV testing.  Here are some reasons why these retroviruses often cause uncertainty and a diagnostic challenge:

• It Takes Time – If your cat was infected with a virus a couple of weeks prior to testing, the test may not indicate your cat has the virus.  It takes time for the body to make antibodies against FIV or for FeLV to replicate in the body so that a blood test can detect the presence of FIV antibodies or the FeLV virus.

• Hide & Seek – FeLV invades cells, inserts itself into your cat’s DNA and begins making new viruses.  Sometimes, FeLV enters the bone marrow and remains dormant.  In this circumstance, blood tests cannot detect the virus.

• It’s Complicated – FeLV is a complex disease with different disease stages.  Sometimes the immune system mounts a sufficient response to eliminate the virus from the body, sometimes the virus wins.  FeLV has a latent phase where it is “hidden” in the body and undetectable from routine blood tests. Depending on the course of the disease, a lab test may or may not indicate a current infection with this virus.

• Years Go By – Both viruses can enter the body, do nothing for years, and then start to replicate and cause disease.  Cats positive for FIV can live many years without any signs of disease.  Most FeLV cats succumb to disease after only 1-3 years, but some live longer without any signs.

• Vaccines – If a cat is vaccinated against FIV, the screening tests cannot distinguish between a FIV vaccinated cat and a true FIV positive cat.  This is one of the reasons why the FIV vaccine is generally not recommended.

• Antibodies – Cats less than 6 months may carry FIV antibodies from their mother.  Until these maternal antibodies wane, your cat may test positive for FIV at a young age and not be infected.

Testing and re-testing is strongly recommended.

Unfortunately, many cats continue through life without ever being tested. One of the reasons this occurs is some organizations give false, misleading information regarding these viruses. Some false claims on the internet include:

False Claim #1: “Testing is not needed because the percentage of infected cats is low.” I would argue this is why we should be testing all cats and testing often. The goal should be to find positive cats and prevent the spread of these viruses to other cats. Even though there is a low 2-3% infection rate, when your cat is infected with a virus that potentially threatens their life, that low percentage rate does not matter.

False Claim #2: “Testing is unreliable and false positives can occur.” There is no laboratory test that is 100% accurate. False positive and false negative results can occur with any test. If we used this standard to determine whether we should test or not, there would be very few laboratory tests we would use. Part of medical training is to understand the pros and cons of a diagnostic test and interpret a test result in light of known risk factors, demographics, patient clinical signs, and in association with other lab tests. If a lab test result “doesn’t fit” with a patient’s clinical signs or expectations, confirmatory tests exist, or a doctor can take a different approach to confirm or dispute an unexpected test result. It is very dangerous to say that since a test is not 100% accurate it should not be used at all. False negative tests are also a concern since infected cats that have “tested negative” could potentially transmit the virus to other cats unknowingly. Fortunately, negative FeLV/FIV tests are very reliable.

False Claim #3: “Spaying or neutering prevents the spread of these viruses.” This statement is misleading. It is true that FIV is generally transmitted through bite wounds, and cats are less likely to fight if they are spayed or neutered. However, this fact cannot be used as a blanket statement to say transmission of both viruses is 100% prevented by spaying or neutering. Spaying or neutering will not prevent all fighting. FeLV can be transmitted from mutual grooming. Spaying or neutering will not prevent this mode of transmission.

False Claim #4: “Cats are more resistant to FeLV later in life.” True, as cats age they develop greater resistance to a FeLV infection. However, this statement is a bit misleading as many cats are infected early in life and the virus remains dormant for some time to come out later in life. Having a stronger resistance later in life means nothing if your cat is infected at an early age.

False Claim #5: “Positive cats will be euthanized in a rescue organization when they could have lived a long, healthy life.” Unfortunately, I only have experience from the veterinary private practice side. It is absolutely devastating to have to tell an owner of a recently adopted, unwell cat that their cat tested positive for FeLV. As a clinician, having more medical information on an individual cat is always better, especially when adopting. I understand the arguments that a few cats will potentially be unnecessarily euthanized and testing all cats will add extra strain on the rescue organization (cats will not be adopted quickly if positive, money is better spent spaying or neutering through trap-neuter-release programs than on screening, it costs money to keep a cat for 2-3 months for retesting, etc.). I would counter by saying that a negative test is very accurate and will overwhelmingly be the most common result. Yes, there will be some false positives, especially on healthy cats. However, I feel the cat population will be better served by finding and addressing these positive cats, whether they turn out to be a true positive or a false positive. Essentially, the argument boils down to “I don’t want to run a test because I don’t want any bad news.” Bad news is a part of life. Furthermore, there are many organizations that take FeLV positive cats in and allow them to live out their lives. FIV positive cats generally live long lives and this information should be known and shared with potential adopters. Euthanasia for positive cats does not have to be the only option.

There is no reason to forego FeLV/FIV testing in a cat.

I hope we have convinced you that FeLV/FIV testing is important. So, when should we test? Here are the guidelines outlined by the AAFP (American Association of Feline Practitioners):

• Every kitten should be tested before introducing into a new household.

• Every kitten/cat should be tested twice, at a minimum of 60 days apart between tests.

• All cats should be tested at regular intervals based on their individual risk for coming in contact with these viruses. For example, outdoor cats who tend to get into fights should be tested 60 days after a fight. Annual testing is recommended if your cat is indoor/outdoor despite their vaccination status.

• Any cat that is unwell should be tested, even if your cat has tested negative or has been vaccinated previously. The viruses may “hide” dormant in the body (i.e. in the bone marrow) and “come out” at a later date.

• Cats should be tested prior to vaccinating against FeLV.

• All cats should be tested to confirm a previously positive test.

Here’s the take home message: It may not be possible to determine your cat’s retroviral status with a single test at a single point in time.

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