Horses
The challenge: Feral horse populations in the U.S. can grow at 20% or more annually. The U.S. Bureau of Land Management (BLM) manages free-roaming equids by removing animals from the range and occasionally applying fertility control measures. The majority of horses that are removed are sent to long-term holding facilities. In March 2026, approximately 58,274 horses and burros were held in off-range corrals and pastures by the BLM and cost $101 million USD to maintain (66% of the total program’s FY2024 budget). As of March 1, 2026, the nationwide on-range wild horse and burro population estimate was 85,466—over three times the mandated appropriate management level of 25,592.
Published research:
- 12 captive mares: given a single injection of SpayVac had pregnancy rates of 0%, 17%, 17%, and 17% for years 1-4 post-treatment compared to 75%, 75%, 88%, and 100%, respectively, in controls. [6]
- 14 captive mares: 93% of the mares stopped reproductive cycling 3-4 months post-vaccination, while control mares continued to cycle. [5]
- 30 feral mares in holding facilities: after a single injection of SpayVac, fertility dropped to 13%, 47%, and 43% (vs. 100%, 98%, and 100% in controls) for years 1-3, respectively. [4]
Ongoing trials:
- BLM is funding a 5-year study in captive mares to determine differences in immune response and contraceptive efficacy based on injection site.
Deer
The challenge: Too many deer can contribute to animal-vehicle collisions, spread tick-borne diseases, decimate ornamental plants, and cause extensive damage to agricultural crops.
Published research:
- Fallow deer (Dama dama): 41 free-ranging deer were vaccinated with a single injection—0% became pregnant for 3 years post-treatment compared to 97% annually in controls. [11]
- White-tailed deer (Odocoileus virginianus): 38 free-ranging deer were given a single injection of SpayVac—0% became pregnant for 2 years post-vaccination compared to 78% annually in controls. [9,10]
- White-tailed deer (Odocoileus virginianus): 5 captive deer—0% became pregnant for 3 years compared to 100% in controls. [12]
- Red deer (Cervus elaphus): 20 free-roaming deer in Germany, which were followed for one year, were vaccinated with one SpayVac injection—fertility was reduced to 11% compared to 86% in controls. [13]
Elephants
The challenge: Too many elephants living near people can destroy crops, raid food stores, damage infrastructure, and cause injury and loss of human life.
Published research:
- African elephants (Loxodonta africana): 6 captive elephants given a single injection of SpayVac maintained elevated antibody titers for over 7 years post-vaccination. [14]
Ongoing trials:
- Asian elephants (Elephas maximus): A trial is underway in Thailand testing contraceptive efficacy of SpayVac in captive and free-ranging animals.
Seals
The challenge: Overabundant seal populations can negatively impact marine fish population dynamics.
Published research:
- Grey seals (Halichoerus grypus): 101 wild seals were vaccinated with a single injection of SpayVac shortly after pupping, which resulted in a 80-85% decrease in fertility for more than 10 years. [2,18,19]
