Duration of antibody response following vaccination against feline immunodeficiency virus

Abstract

Objectives Recently, two point-of-care (PoC) feline immunodeficiency virus (FIV) antibody test kits (Witness and Anigen Rapid) were reported as being able to differentiate FIV-vaccinated from FIV-infected cats at a single time point, irrespective of the gap between testing and last vaccination (0-7 years). The aim of the current study was to investigate systematically anti-FIV antibody production over time in response to the recommended primary FIV vaccination series. Methods First, residual plasma from the original study was tested using a laboratory-based ELISA to determine whether negative results with PoC testing were due to reduced as opposed to absent antibodies to gp40. Second, a prospective study was performed using immunologically naive client-owned kittens and cats given a primary FIV vaccination series using a commercially available inactivated whole cell/inactivated whole virus vaccine (Fel-O-Vax FIV, three subcutaneous injections at 4 week intervals) and tested systematically (up to 11 times) over 6 months, using four commercially available PoC FIV antibody kits (SNAP FIV/FeLV Combo [detects antibodies to p15/p24], Witness FeLV/FIV [gp40], Anigen Rapid FIV/FeLV [p24/gp40] and VetScan FeLV/FIV Rapid [p24]). Results The laboratory-based ELISA showed cats from the original study vaccinated within the previous 0-15 months had detectable levels of antibodies to gp40, despite testing negative with two kits that use gp40 as a capture antigen (Witness and Anigen Rapid kits). The prospective study showed that antibody testing with SNAP Combo and VetScan Rapid was positive in all cats 2 weeks after the second primary FIV vaccination, and remained positive for the duration of the study (12/12 and 10/12 cats positive, respectively). Antibody testing with Witness and Anigen Rapid was also positive in a high proportion of cats 2 weeks after the second primary FIV vaccination (8/12 and 7/12, respectively), but antibody levels declined below the level of detection in most cats (10/12) by 1 month after the third (final) primary FIV vaccination. All cats tested negative using Witness and Anigen Rapid 6 months after the third primary FIV vaccination. Conclusions and relevance This study has shown that a primary course of FIV vaccination does not interfere with FIV antibody testing in cats using Witness and Anigen Rapid, provided primary vaccination has not occurred within the previous 6 months. Consequently, Witness and Anigen Rapid antibody test kits can be used reliably to determine FIV infection status at the time of annual booster FIV vaccination to help detect 'vaccine breakthroughs' and in cats that have not received a primary course of FIV vaccination within the preceding 6 months. The duration of antibody response following annual booster FIV vaccination and the resulting effect on antibody testing using PoC kits needs to be determined by further research. The mechanism(s) for the variation in FIV antibody test kit performance remains unclear.

Figure 1

Categorisation of feline immunodeficiency virus (FIV) uninfected cats from study 1 based on time (days) elapsed since last FIV vaccination (n = 114). Of the 114 FIV-uninfected cats, 114 had tested FIV positive with SNAP Combo, six with Witness and none with Anigen Rapid. The six FIV false-positive results obtained with Witness occurred at the following intervals after FIV vaccination: 0–30 days (n = 1), 121–150 days (n = 1), 181–210 days (n = 1), 241–270 days (n = 1) and 331–360 days (n = 2)

Figure 2

Results from ELISA testing for antibodies recognising feline immunodeficiency virus (FIV) gp40 peptide (study 1). Positive and negative controls are shown. The optical density (OD) is displayed on the y-axis. Mean and SEM bars are shown. (a) FIV-vaccinated/FIV-infected cats (n = 4), FIV-vaccinated/FIV-uninfected cats (FIV false positive with Witness, n = 6) and FIV-vaccinated/FIV-uninfected cats (FIV true negative with Witness, n = 108). *Significant difference (P <0.01) between groups of cats. (b) FIV-vaccinated/FIV-uninfected cats (n = 114) according to the time (days) elapsed since the last annual FIV vaccination. No significant effect was found (P = 0.42). (c) FIV-vaccinated/FIV-uninfected cats (n = 114) according to the age of cat at the time of sampling (years). No significant effect was found (P = 0.21). (d) FIV-vaccinated/FIV-uninfected cats (n = 114) according to number of annual booster FIV vaccinations received. No significant effect was found (P = 0.43). TM = transmembrane

Figure 3

Summary of feline immunodeficiency virus (FIV) antibody test results from the prospective study (study 2) at various time points (n = 12). The FIV target capture antigen(s) for each point-of-care antibody test kit is included in brackets. A primary FIV vaccination course was administered at 0, 4 and 8 weeks