When, in order to prevent the spread of any contagious, infectious, or communicable disease, it becomes necessary to slaughter any diseased or exposed animals, including poultry, and the purchase of such animals, including poultry, by the United States is authorized by law and an appropriation is available therefor, the value of the animals, including poultry shall be ascertained and compensation made therefor in accordance with the orders or regulations of the Secretary of Agriculture.