Vanquishing the vaccine factor: Dependable in-clinic FIV diagnosis possible after all

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Why they did it

After the introduction of the feline immunodeficiency virus (FIV) vaccine in 2002, diagnosing FIV became more complicated. The available point-of-care tests were reported to be incapable of distinguishing between vaccine- and disease-associated antibodies to FIV. Determination of natural versus vaccine-induced FIV-positive testing status then shifted to alternative methods that were less convenient and more costly. Since then, several new point-of-care test kits have become available. The authors decided to review the assertion that point-of-care kits were incapable of determining vaccine-associated from disease-associated antibodies.

What they did

The authors recruited 119 cats with a verified history of FIV vaccination as well as 239 with no history of FIV vaccination. Cats with unclear vaccination histories were excluded, as were cats younger than 6 months and cats with an already known FIV status due to prior testing. The area in Australia where the cats were recruited was perceived to have a high prevalence of FIV. Cats were included in the vaccinated group if they had received one or more FIV vaccinations at any point in their life, regardless of whether the manufacture's labeling recommendations had been followed or not. Blood was collected from each of the cats and tested with three different kits: SNAP FIV/FeLV Combo (IDEXX), which is an ELISA test, and two immunochromatographic tests, Witness FeLV/FIV (Zoetis) and Anigen Rapid FIV Ab FeLV Ag (Bionote). A separate sample was also tested using the commercially available FIV RealPCR (IDEXX). For cats with discordant point-of-care and PCR results, virus isolation testing was used to determine FIV status. Virus isolation was also used to confirm FIV positive status in cats identified by these test results as both vaccinated and infected.

What they found

The authors found that the SNAP test was unable to distinguish between vaccine-induced and disease-induced antibodies but was useful for confirming a humoral response to FIV vaccination. Both the Witness and Anigen Rapid tests were able to reliably distinguish between the two. Both Witness and Anigen Rapid tests had 100% sensitivity and had 95% and 100% specificity, respectively.

The authors also noted that for the Witness test, 30% of positive results were recorded as faint positive. Of those faint positive results, most were found to be FIV negative by PCR and virus isolation, leading the authors to report a strong association between faint positive results and absence of FIV infection for the Witness test.

Based on the results, the authors proposed an algorithm for determining FIV status in an area where vaccination is practiced. The authors recommended testing with either the Witness or Anigen Rapid test and, if positive, confirming with the other test. Further testing such as PCR was only recommended if the two tests had discordant results. It should be noted that, in this study, no false negative results were found with any of the antibody kits.

Take-home points

According to this study, using point-of-care kits it is now possible to reliably determine FIV status in areas where FIV vaccination is practiced. Unfortunately, the Anigen Rapid kit is not currently available in the United States, making the application of the authors' algorithm unusable here for the time being. However, using the Witness test alone as opposed to the SNAP test would result in far fewer false positive results in areas where FIV vaccination is prevalent. This is especially applicable to shelter or other situations where further PCR testing is unavailable for financial reasons and decisions on euthanasia or adoptability may be made based on a point of care test alone.

Westman ME, Malik R, Hall E, et al. Determining the feline immunodeficiency virus (FIV) status of FIV-vaccinated cats using point-of-care antibody kits. Comp Immunol Microbiol Infectious Dis 2015;42:43-52.

Link to abstract: http://www.ncbi.nlm.nih.gov/pubmed/26459979

Michael Nappier, DVM, DABVP (canine/feline)

Department of Small Animal Clinical Sciences

Virginia-Maryland College of Veterinary Medicine

Blacksburg, VA 24061

Note: This article has been updated to correct a misleading statement implying that all three tests featured were ELISA tests. In fact, the SNAP FIV/FeLV Combo (IDEXX) is an ELISA test, while the Witness FeLV/FIV (Zoetis) and Anigen Rapid FIV Ab FeLV Ag (Bionote) are immunochromatographic tests. We apologize for any confusion this may have caused.