Merck & Co. Inc.'s animal health unit will buy Ames-based privately held Harrisvaccines Inc. to expand its portfolio of animal vaccines as the U.S. livestock industry seeks new defenses against diseases, Reuters reported.

The companies did not report the terms of the sale, which is expected to close by the end of the year.

Harrisvaccines gained fame for developing a vaccine to fight porcine epidemic diarrhea virus.

In October, the firm signed a contract with the U.S. Department of Agriculture to supply avian influenza vaccine for poultry. The USDA is preparing for another potential outbreak of the virus later this year.