Factsheet on BLM's Wild Horse and Burro Fertility Control Program
Updated as of June 5, 2009
The Bureau of Land Management has supported the development of an effective contraceptive agent for wild horses since 1978. The most promising agent is known as Porcine Zona Pellucida (PZP), a fertility-control vaccine that was developed in the 1990s but is not commercially available.
PZP is used by the BLM under an investigational exemption issued by the Food and Drug Administration (FDA) that is held by the Humane Society of the United States (HSUS).
A short-term, liquid form of conventional PZP, considered safe and highly effective, must be administered annually. However, it is not feasible to gather wild horse herds every year to administer this liquid vaccine; moreover, it is very difficult to approach most wild horses and burros on Western public rangelands closely enough to allow for injections by hand or darting.
Instead, the BLM has been using a 22-month, time-release pellet that must be administered to mares after they have been captured. This means more mares need to be captured and released than would normally be gathered and simply removed from the affected Herd Management Area.
Since 2004, the BLM has administered more than 2,000 doses of the pelleted form of PZP to wild horses in 52 Herd Management Areas. So far, the pelleted form appears to be safe, but it is unclear how effective it will be in controlling population growth rates when applied in the typical four- to five-year gather (round-up) cycle.
Early data from the Clan Alpine Herd Management Area in Nevada shows that untreated mares foaled at a rate of 54 percent, while mares treated with pelleted PZP foaled at a rate of six percent in the first year following treatment and 14 percent in the second year. More recent data from treatments using a different pelleted form of PZP at the McCullough Peaks Herd Management Area in Wyoming indicate somewhat lower efficacy, but a significant effect nonetheless (a 65 to 80 percent foaling rate in untreated mares as compared to a 30 to 40 percent foaling rate in treated mares).
These results are encouraging, but it remains challenging to demonstrate large impacts on overall population growth rates because of difficulties associated with capturing and treating enough mares, monitoring herd growth rates using aerial population estimates, and the factor of movements of horses between treated and untreated HMAs. Work to develop a longer-lasting agent and demonstrate its effects continues as the BLM applies fertility control on the range in an ongoing effort to suppress population growth rates.
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