Antibody Responses in Cats Following Primary and Annual Vaccination against Feline Immunodeficiency Virus (FIV) with an Inactivated Whole-Virus Vaccine (Fel-O-Vax® FIV)

Abstract

Although the antibody response induced by primary vaccination with Fel-O-Vax^® FIV (three doses, 2-4 weeks apart) is well described, the antibody response induced by annual vaccination with Fel-O-Vax^® FIV (single dose every 12 months after primary vaccination) and how it compares to the primary antibody response has not been studied. Residual blood samples from a primary FIV vaccination study (n = 11), and blood samples from cats given an annual FIV vaccination (n = 10), were utilized. Samples from all 21 cats were tested with a commercially available PCR assay (FIV RealPCR^TM), an anti-p24 microsphere immunoassay (MIA), an anti-FIV transmembrane (TM; gp40) peptide ELISA, and a range of commercially available point-of-care (PoC) FIV antibody kits. PCR testing confirmed all 21 cats to be FIV-uninfected for the duration of this study. Results from MIA and ELISA testing showed that both vaccination regimes induced significant antibody responses against p24 and gp40, and both anti-p24 and anti-gp40 antibodies were variably present 12 months after FIV vaccination. The magnitude of the antibody response against both p24 and gp40 was significantly higher in the primary FIV vaccination group than in the annual FIV vaccination group. The differences in prime versus recall post-vaccinal antibody levels correlated with FIV PoC kit performance. Two FIV PoC kits that detect antibodies against gp40, namely Witness^® and Anigen Rapid^®, showed 100% specificity in cats recently administered an annual FIV vaccination, demonstrating that they can be used to accurately distinguish vaccination and infection in annually vaccinated cats. A third FIV PoC kit, SNAP^® Combo, had 0% specificity in annually FIV-vaccinated cats, and should not be used in any cat with a possible history of FIV vaccination. This study outlines the antibody response to inactivated Fel-O-Vax^® FIV whole-virus vaccine, and demonstrates how best to diagnose FIV infection in jurisdictions where FIV vaccination is practiced.

Keywords: Australia; FIV; capsid protein; diagnosis; gp40; immunity; lentivirus; p24; transmembrane glycoprotein; veterinary science.

Figure 1

Anti-p24 antibody response from primary FIV vaccination was higher than annual FIV re-vaccination. Microsphere immunoassay (MIA) detected increases in anti-p24 antibodies over time in both primary and annual vaccination groups (Primary: p < 0.0001; Annual: p = 0.012; blue and red bars below the box plots, respectively; mixed-effects analysis). Anti-p24 antibodies differed significantly over time between groups (interaction, p < 0.0001) and at individual time points following vaccination (p < 0.0001, where the black bars joining the box plots indicate the analysis of the comparative time points), exhibited by increased antibody levels in primary vaccinated cats compared to annual vaccinated cats (mixed-effects analysis). Samples from days 0, 14 *, and 28 ^† post-final vaccination were compared (Primary: days 0, 70 * and 84 ^†; Annual: days 0, 14 *, and 28 ^†). Negative fold (NF) = (sample MFI)/(2 × negative control MFI). OD = optical density. * = p < 0.05, **** = p < 0.0001.

Figure 2

Primary FIV vaccination also induced a higher anti-gp40 antibody response than annual FIV re-vaccination. Anti-gp40 antibodies increased significantly over time in both primary and annual vaccination groups (Primary: p = 0.024; Annual: p = 0.0022; blue and red bars below the box plots, respectively; mixed-effects analysis). Anti-gp40 antibodies differed significantly over time between groups (interaction, p < 0.0001) and at individual time points following vaccination (day 14 *: p < 0.001; day 28 ^†: p = 0.012, where the black bars joining the box plots indicate the analysis of the comparative time points), with primary vaccination inducing higher levels of antibodies compared to annual vaccination (mixed-effects analysis). Samples from days 0, 14 *, and 28 ^† post-final vaccination were compared (Primary: days 0, 70 * and 84 ^†; Annual: days 0, 14 *, and 28 ^†). NF = (sample OD)/(2 × negative control OD). OD = optical density. * = p < 0.05, *** = p < 0.001.