Administration of an immunocontraceptice vaccine to Kaimanawa mares

A population of 29 bands of wild horses were identified. Bands were divided into three treatments (receive vaccine 9 bands with 26 mares; receive placebo 2 bands with 8 mares; control 18 bands with 63 mares). The initial vaccine and placebo was injected into 24 mares from 9 bands and 8 mares from 2 bands, respectively, during the muster in June 1995. A booster vaccine was successfully administered to at least 13 mares. The vaccine failed to prevent conception and all but one vaccinated mare had a foal in the spring of 1996. The reason why the vaccine failed is unknown. It is unlikely that Kaimanawa mares could not mount an immune response to the vaccine and we are confident that the vaccine was successfully administered to at least 13 horses. It is most likely that the vaccination programme failed because the vaccine was inactive for some reason or other. It is possible that the vaccine deteriorated during transit from the USA to the individual mare. Retrospectively the vaccine should have been tested in domestic mares before its use in wild mares.