How long does anti-FIV antibody production occur in response to a primary FIV vaccination series? Does this antibody production interfere with testing to differentiate between FIV-vaccinated and FIV-infected cats?
Recently, investigators have reported that two point-of-care (PoC) feline immunodeficiency virus (FIV) antibody test kits were able to differentiate between such cases of FIV-vaccinated and FIV-infected cats at a single time point. The differentiation was also noted to be irrespective of the time gap between testing and vaccination, 0-7 years. These studies have demonstrated the complexity of antibody responses following FIV vaccination.
With a natural infection, the antibody responses that occur show antibodies to p24 and gp40 detectable, with the use of Western Blot, within 3 weeks of infection and antibodies to p15 detectable within 4 weeks of infection. Antibody production in FIV-infected cats lasts for the life of the cat, although the antibody levels (in particular with p15 and p24) may wane in the terminal stage of infection.
Yet, with the use of three PoC antibody tests detecting antibodies to different target FIV antigens, it was demonstrated that FIV-vaccinated/FIV-uninfected cats consistently tested positive with SNAP FIV/FeLV Combo (p15 and p24) but FIV negative with Witness FeLV/FIV (gp40) and Anigen Rapid FIV/FeLV (p24 and gp40).
This current study had two aims:
- to determine if FIV-vaccinated cats produce antibodies to gp40 at levels below the detection threshold of Witness FeLV/FIV and Anigen Rapid FIV/FeLV kits by using a laboratory well-based ELISA (using residual plasma from the original study)
- to investigate the duration of antibody response to p15, p24 and gp40 in cats following a primary course of FIV vaccination using four PoC FIV antibody test kits (SNAP FIV/FeLV Combo, Witness FeLV/FIV, Anigen Rapid FIV/FeLV and VetScan FeLV/FIV Rapid). This was the prospective part of the complete study.
In regards to results among the sample population testing residual blood from FIV-vaccinated cats, 110/118 cats had received the initial FIV vaccination series of three vaccines, 2-4 weeks apart, and had also received three or more annual boosters (in most cases the last within the previous 0-15 months), laboratory-based ELISA demonstrated detectable levels of antibodies to gp40. These results were despite the samples testing negative with two kits that use gp40 as an antigen (Witness and Anigen Rapid kits).
In the second or prospective study using 12 cats, FIV antibodies were detected as early as 2 weeks after the first vaccination using SNAP Combo and Witness, and then as early as 4 weeks using Anigen Rapid and VetScan Rapid. At the end of the study, 6 months after the third vaccination (week 34), all cats (12/12) were FIV positive with SNAP combo, two cats had become negative with VetScan Rapid and all cats were FIV negative with Witness and Anigen Rapid.
The authors state their results confirm that care must be taken in the period immediately after primary FIV vaccination using Witness and Anigen Rapid tests since false-positive FIV results occur using both. They further recommend that antibody testing to detect FIV infection in FIV-vaccinated cats is reliable using these two tests, providing primary vaccination against FIV has not occurred within the preceding 6 months. If a positive FIV antibody result is obtained in a cat where a recent primary FIV vaccination is possible, submitting the seropositive samples for confirmatory FIV PCR is recommended. A negative FIV test with either Witness or Anigen Rapid is felt to be reliable by the authors and is their recommended screening test of choice, except in cases of recent infection, when repeat testing 2 months later is then recommended.
As part of the second study, the authors plan to monitor the cats for several years and determine their gp40 antibody response following booster FIV vaccination to determine if the antibody response following annual vaccination is less than their antibody response following initial (primary) vaccination (VT)
Westman ME, Malik R, Hall E, Norris JM. Diagnosing feline immunodeficiency virus (FIV) infection in FIV-vaccinated and FIV-unvaccinated cats using saliva. Comp Immunol Microbiol Infect Dis. 2016 Jun;46:66-72