Living in Wyoming and Colorado for the last ten years has increased my appreciation for wild horses. I’ve always been a bit "horse crazy," and I love my paint gelding Atticus, but there is something extra special about seeing a horse that is beholden to no person gallop across western grasslands.

The Bureau of Land Management (BLM) estimates that approximately 37,300 wild horses and burros (about 31,500 horses and 5,800 burros) are roaming on BLM-managed rangelands in ten Western states. The estimated current free-roaming population exceeds by nearly 11,000 the number that the BLM has determined can exist in balance with other public rangeland resources and uses.

Current management options are limited, with the majority of actions involving the removal of horses and burros from the range and either offering them for adoption or holding them indefinitely in captivity. The BLM estimates there are more than 49,000 wild horses and burros off of BLM-managed lands that are fed and cared for at short-term corrals and long-term pastures.

Nobody thinks this situation is ideal, so I was excited to see that the U.S. Environmental Protection Agency (EPA) has granted regulatory approval for the use of an equine immunocontraceptive vaccine (GonaCon) in adult female wild or feral horses and burros.

GonaCon stimulates the production of antibodies against gonadotropin releasing hormone (GnRH).

GnRH is normally responsible for stimulating the production and release of sex hormones. When GnRH is inactivated by the antibodies produced in response to this vaccine, the levels of estrogen and progesterone drop in a female’s body and sexual activity ceases as long as sufficient levels of these antibodies remain present. The vaccine can be delivered by hand injection, jab stick, or darting, and tends to last for several years.

Scientists with the U.S. Department of Agriculture's Wildlife Services' National Wildlife Research Center (NWRC) developed GonaCon, which was initially used to control populations of white tailed deer. The vaccine is currently manufactured by NWRC; however, the goal is to license the vaccine to a private manufacturer. Future NWRC research with GonaCon will likely involve studies to support expanded registration to other species (e.g., prairie dogs and feral dogs) and aid in preventing the transmission of wildlife diseases.

A study published in 2011 even looked into whether or not GonaCon could be used to control feral cat populations. It found that:

A total of 93% of vaccinated cats remained infertile for the first year following vaccination, whereas 73, 53, and 40% were infertile for 2, 3, and 4 y, respectively. At study termination (5 y after a single GnRH vaccine was administered), four cats (27%) remained infertile.

The five cats in the study that did not receive the vaccine were all pregnant within one month.

Hopefully GonaCon will soon be in wide use keeping the numbers of wild horses within sustainable limits and eliminating the need for long term holding facilities or "adoptions" to people with questionable motives.

Dr. Jennifer Coates

Image: ilFaber / via Shutterstock