But those same precautions have not worked everywhere. A Central Indiana farm that Dr. Ackerman also works with was among the first to lose piglets to the virus in May. “If it gets in, you can’t stop it,” Dr. Ackerman said. “We filled wheelbarrows with dead pigs.”

The porcine epidemic diarrhea virus, which is deadly only to young pigs and poses no food safety risks or danger to humans, appeared in the United States for the first time last spring in Ohio and within weeks had spread to four other states.

The outbreak led to a flurry of lab testing and a survey of the industry to determine how the virus had entered the country, comparing supplies and feeds in an effort to find a smoking gun. Farmers are cross-referencing vaccine and semen distributors, even the brands of plastic pipettes they use to inseminate sows, desperate to contain a threat that has made the industry feel increasingly vulnerable.

“It’s anybody’s guess at this point,” said Lisa Becton, director of swine health information and research at the National Pork Board, which is spending $800,000 for research into the virus.

First surfacing in Britain more than 40 years ago, the virus has spread throughout Europe and Asia. It has caused problems most recently among pork producers in China, where a 2012 strand of the disease was 99.4 percent similar to cases now found in the United States, according to researchers.