One virus causing a worldwide infection in cats is feline immunodeficiency virus (FIV). The virus creates an immune dysfunction that can lead to cancer, chronic health problems or both scenarios. Prevalence in the United States of FIV is approximately 2.5% with a higher prevalence in cats allowed outdoors (4.3%), cats visiting veterinarians with clinical illness (20.6%) and cats located in animal hoarding sanctuary situations (8.8%).
The American Association of Feline Practitioners advise in their retrovirus guidelines FIV testing for any cat in the following scenarios: 1) new to a household, 2) showing signs of clinical illness, 3) FIV- uninfected and living in a high-risk environment (outdoor cats, dynamic household, or living with FIV-infected cats), or 4) FIV-uninfected and recovering from a cat fight. Initial screening for FIV is commonly by commercial point-of-care (POC) ELISA tests. The guidelines recommend performing confirmatory testing for positive POC test results. Additional testing is also recommended if there is a high suspicion of disease when the test result is negative. The most commonly used form of additional testing is through polymerase chain reaction (PCR) tests, though diagnostic accuracy can vary between PCR tests. This can make understanding and interpretation of results difficult. Other testing methods such as Western blot and immunofluorescent antibody (IFA) tests are available, yet it is stated that sensitivity and specificity are lower than POC ELISA test results.
The objective of the study was to show there is good agreement between the results of a POC ELISA test and a commercially available PCR test for FIV from results obtained from owned and shelter-housed cats. The samples came from 168 cats, 110 male cats and 58 spayed females.
When the results were paired, they showed very good agreement between the testing methods, ELISA and PCR. Eleven test pairings had discordant ELISA/PCR results: 7 ELISA+/PCR- and 4 ELISA-/PCR+. The researchers used virus isolation (VI) as the reference standard with the following noted: 4 false-positive PCR results, 5 false positive ELISA results, and 1 false negative PCR result (1 cat was lost to follow-up).
The results showed that both the POC ELISA and PCR tests used in the study had very similar numbers of false-positive results. The authors state these findings demonstrate the importance of being cautious when interpreting the results of FIV test results. If a cat is strongly suspected of being infected with FIV, a PCR test should still be considered as an additional diagnostic if there is a negative ELISA test result. The study results suggest that when a cat has a negative ELISA test result and a positive PCR test result, the cat should have follow-up PCR, VI testing, or both to confirm FIV infection. Sequencing of the PCR product should also be requested.
There can be multiple reasons for some cats to test positive on an ELISA test and have a negative PCR test. Viral load can fluctuate during infection. There also could be nucleic acid degradation. The diversity of FIV subtypes could also be a factor in affecting discordancy in results. It would be expected to have positive ELISA test results when not infected with FIV as the result of maternal antibodies in young kittens or FIV vaccine-induced antibodies.
Importantly, in the end, the authors state FIV infection should not be made based on the result of any single test of a sample taken at a single time point. They advise following the AAFP retroviral testing guidelines. (VT)