Merck & Co Inc's animal health unit will buy privately held Harrisvaccines Inc to expand its portfolio of animal vaccines as the U.S. livestock industry seeks new defenses against diseases, the companies said on Thursday.

Financial details were not disclosed.

The deal, which is expected to close by year-end, follows rival Zoetis Inc's agreement, announced on Nov. 2, to buy Pharmaq, a market leader in vaccines for farmed fish, for $765 million.

Harrisvaccines, based in Ames, Iowa, is best known for developing a vaccine called "iPED" in 2013 to fight Porcine Epidemic Diarrhea virus, or PEDv.

It was one of the first U.S. companies to develop a vaccine to fight the disease, which wiped out an estimated 10 percent of the U.S. pig population over two years. It also received the first conditional license from the U.S. Department of Agriculture (USDA) to sell its vaccine to veterinarians and swine farmers.

Merck Animal Health's project to develop a PEDv vaccine is ongoing, the company said on Thursday.

Zoetis also has been granted a conditional license from the USDA to sell its PEDv vaccine.

In October, Harrisvaccines was one of two drug firms to be awarded a USDA contract to manufacture doses of avian influenza vaccine for poultry, in preparation for the potential return of the fast-spreading virus later this year.

(Reporting by P.J. Huffstutter in Chicago; Editing by Richard Chang)