Ovarian dynamics and injection site reactions associated with immunocontraceptive zona pellucida (ZP) and GnRH vaccination of domestic horse mares (Equus caballus)

Abstract

Determinants when utilising veterinary immunocontraceptive vaccines include ovarian function and injection site reactions. Ovarian dynamics and injection site changes were monitored following administration of zona pellucida (ZP) and GnRH immunocontraceptive vaccines in 41 domestic horse mares. Mares were assigned to five treatment groups, stratified by age, parity and body condition score. Groups 1-4 treatments incorporated Pet Gel A (6%) and Poly (I:C) (500 μg) adjuvant in sterile water, in a two or three inoculation protocol 5 weeks apart, incorporating the following specified antigens in 1 ml total volume: no antigen (Group 1, n=9); initial 100 μg pZP then 100 μg pZP booster (Group 2, n=8); initial 500 μg recombinant zona pellucida (reZP) then 500 μg reZP and finally 500 μg reZP booster (Group 3, n=8) and initial100 μg pZP then 500 μg reZP booster (Group 4, n=8). Group 5 (n=8) received 2 ml of 400 μg GnRF-protein conjugate initially and as booster. Mares were examined by trans-rectal palpation and ultrasound of the reproductive tract at each treatment period. Injection site reactions were assessed by inspection and palpation using a three point scale and rectal temperature was measured for 7 days following treatments. Comparing functional ovarian status one month after final treatments demonstrated strongly significant differences between treatment groups (P<0.0001). Notably, a high proportion of both Groups 3 and 5 had ceased ovarian activity. The occurrence of injection site reactions and elevated body temperatures across all groups were mild, resolving one month after treatments, without any difference between treatment groups following initial or subsequent booster treatments. Using reZP vaccine may be indicated in mares where suppression of ovarian activity is a desirable outcome following immunocontraception. Incorporation of a Pet Gel A and Poly (I: C) adjuvant formulation provides an effective alternative to currently-utilised Freund’s adjuvants reportedly associated with adverse injection site reactions.