The Bureau of Land Management (BLM) is working with a veterinarian to determine why 22 foals became sick and died last week at the agency’s Litchfield Corrals, near Susanville, California.

Wranglers at the corrals first noticed signs of flu-like illness in a pen of 4- to 5-month-old foals on Oct. 2, according to a written statement from the BLM. The clinical signs spread rapidly through the pen and foals began dying later that week.

The illness has only affected foals isolated in a single pen. The agency halted foal adoptions as soon as the illness was detected, the statement said.

Alan Bittner, manager for the BLM’s Northern California District, said the agency’s contract veterinarian collected blood and tissue samples which are now being analyzed