19 CFR § 12.24 - Regulations of the Department of Agriculture.
(a) The importation into the United States of domestic animals, animal products, and animal feeding materials is subject to inspection and quarantine regulations of the Department of Agriculture, Customs officers and employees are authorized and directed to perform such functions as are necessary or proper on their part to carry out such regulations of the Department of Agriculture.
(b) Inspection by an inspector of the Animal and Plant Health Inspection Service, Veterinary Services is required for all horses, cattle, sheep, other ruminants, and swine as a prerequisite to their entry from any foreign country. Orders listing the ports designated as quarantine stations for the inspection and quarantine of animals will be issued by the Secretary of Agriculture, with the approval of the Secretary of the Treasury, whenever conditions warrant.
(c) The entry of domestic animals may be made, but shall not be required, before the expiration of the quarantine period. Such animals, if not entered at the time of arrival, shall be considered as under general order while under quarantine and shall not be released except upon notice from the port director that the importer has complied with all the requirements for entry.