Submitted by Lisa Labrecque, DVM
Feline Leukemia Virus (FeLV) and Feline Immunodeficiency Virus (FIV) affect cats of all ages worldwide. The percentage of cats that are infected with these viruses varies according to geographic region, the environment, and the lifestyle of the cat, but estimates tend to be quite low, with 2-4% of cats infected in the U.S.1,2 and a similar percentage here in Hawaii.
There has been much debate over the past few years in the animal welfare community about whether to routinely test cats for FeLV and FIV. For many years, shelters have felt an obligation to test all cats for FeLV and FIV prior to adoption. The hope was that testing would ensure that shelters were placing healthy cats up for adoption and that the viruses might be eradicated through shelter testing efforts. However, new information about the tests and the diseases has led to the recommendation that shelters discontinue routine testing of cats for FeLV and FIV.3
New research has shown that a negative or positive result at one point in time is not definitive by itself. The diseases have an overall low prevalence in our cat population, and the available tests are not 100% accurate.4
Furthermore, we now know that cats may experience a variability of infection, so we can no longer make conclusions about the results of a single test. For cats that test negative, we can only conclude that they are negative at the time they were tested. It takes at least 30 days from the time of infection with FeLV to test positive, and it can take up to 60 days for a cat infected with FIV to develop enough antibodies to cause a positive result. Therefore, if a cat was infected just prior to intake at the shelter, they would test negative but could have the disease.
A negative test can also give adopters a false sense of security. The concept of incubation periods and exposure can be difficult to convey during adoption when people are already overloaded with information and excitement about their new pet. Although chances are that a cat testing negative is truly negative (due to low prevalence of the disease), some cats may have been exposed prior to intake and could develop FeLV or FIV after adoption.
On the other hand, a positive test in a healthy cat who is at low-risk for infection is more likely to be a false-positive than a positive result in a sick cat or one at higher risk. In typical cat populations where there is low prevalence of disease, a majority of positive results will be false positives.4 For example, if there is 2% disease prevalence in a population, and a test with 98% sensitivity is used to test 1,000 cats, there will 39 positive tests:
19 true positives (0.02 x .98 = 0.096, 0.0196 x 1,000)
20 false positives (0.02 x 1,000)
Note that there will be slightly more false positives than true positives. Compare this to the same testing performed in a population with 4% disease prevalence:
39 true positives (0.04 x .98 = 0.0392, 0.0392 x 1,000)
20 false positives (0.02 x 1,000)
In this population with higher disease prevalence, there are many more true positives than false, making the test that much more reliable.
Positive FIV test results may also be caused by circulating maternal antibodies that can be present in a kitten up to 6 months of age. And some cats who are exposed to FeLV can contain the virus early on; if this occurs, they no longer shed the virus and cannot infect other cats but may still test positive.2
At the Hawaiian Humane Society, we no longer test healthy cats or kittens. We still test sick cats, those who have clinical signs consistent with infection, and at-risk cats. Based on our newer understanding of the variability of disease, we believe that FeLV/FIV testing is best done in collaboration with a cat’s primary care veterinarian where they receive comprehensive care and recommendations based on their lifestyle.
1. Cornell Feline Health Center, Cornell University, College of Veterinary Medicine, “Testing for FeLV and FIV”
2. 2020 American Association of Feline Practitioners’ Feline Retrovirus Testing and Management Guidelines
3. Shelter Medicine Program, University of Wisconsin-Madison, School of Veterinary Medicine, “Why are some shelters no longer testing all cats for FeLV and FIV?”
4. Journal of Veterinary Internal Medicine, Performance of 4 Point‐of‐Care Screening Tests for Feline Leukemia Virus and Feline Immunodeficiency Virus.