Fertility control potentially offers a humane management tool for wild animal populations. Generally there are two different types of immuno-contraception available. Firstly PZP (manufactured from pig ovaries) and secondly GnRH (GonaCon).

A trial of PZP on the Kaimanawa horses in the mid 1990’s regrettably failed. (Part of the cause was a faulty batch used.) However PZP has quite a number of unpleasant side effects both in terms of the health and well-being of the animals, but in addition has significant behavioural side effects. It has also proven to be unpractical and uneconomic.

GnRH is a naturally occurring hormone in all mammals; it works with other hormones to control the functioning of ovaries in females and testes in males. In humans GnRH is used in IVF, breast and prostate cancer treatment etc.

To use GnRH as a contraceptive it needs to be linked to a protein then combined with an adjuvant to create an immune response, this induces infertility by reducing reproductive hormones. None of this is new, and has been successfully used for years, but the problem has been to create a long-term effect which would enable the vaccine (GonaCon) to be used efficiently in wild animal populations. Recently a new adjuvant has been developed which could allow up to four years of infertility. The effects and durability of the vaccine reduce over the time period and it is believed that repeated vaccinations will not create issues with permanent infertility. To date it also appears safe to treat pregnant animals with no significant effects on behaviour, birth rates, survival rates or neonatal growth rates and fertility.

GonaCon is also proving safer in terms of side effects and behavioural changes and does not constitute a bio-hazard (unlike PZP). Unfortunately to date long-term trials on horses using repeated vaccinations have not yet been completed, but it has proven effective in quite a number of other species (including deer).

Recently we have been corresponding with Dan Baker, research biologist from Colorado State University. Dan is currently undertaking a long-term study (around eight years) to evaluate the use of GonaCon as an immuno-contraceptive vaccine in feral horses. Efficacy, durability of effects over time, behaviour and side-effects of treatment are being trialed in controlled experiments with captive, domestic horses and in a field application with wild horses at Theodore Roosevelt National Park in North Dakota.

There are many questions yet to be answered, and still many years before Dan’s study is completed, but the concept is gathering speed, albeit slowly at present. The Department of Conversation is showing cautious interest, and there is also some guarded optimism from their representatives at the New Zealand Veterinary Association. We also have on board Kevin Stafford and Erica Gee from the veterinary school at Massey University and Clare Veltman from the Department of Conservation. Kevin and Clare were part of the previous PZP study and along with Erica have a wealth of knowledge and resources to help make informed decisions. We believe there may be the option to trial the GnrH vaccine with the Kaimanawa horses perhaps in the future – all things going to plan of course! This would obviously require quite a lot of work to be done in terms of population studies and getting approval for the drug to be used in New Zealand, costings, Department of Conservation and the Kaimanawa Advisory Group approval etc.

We may have to look at assisting with the costs of this but if this enables us to reduce the slaughter of our Kaimanawa Heritage Horses we believe it will all be worth it.