In the late 20th century, some ponies previously sold at auction were returned to Assateague Island when population numbers threatened to drop below the targeted numbers due to large numbers of deaths from storms or other issues. Since 1990, the ponies from the Virginia herd have been rounded up biannually for veterinary treatment, including deworming and vaccinations for diseases such as rabies, tetanus and Eastern and Western encephalitis, although they make the swim to Chincoteague only once per year. In addition, continual monitoring and basic first aid for any minor injuries is performed by a committee from the fire department. Such intervention is needed because many of the ponies will be brought into the general horse population through the auction and purchase by private buyers. During the veterinary visits, they are also tested for EIA.