A swine virus deadly to young pigs, one never before seen in North America, is spreading rapidly across the United States and proving harder to control than previously believed.

The virus now has spread to 13 states - with more than 100 positive cases to date - since it was first diagnosed in the United States last month, said Montserrat Torremorell, the Allen D. Leman Chair in Swine Health and Productivity at the University of Minnesota's College of Veterinary Medicine.

While the virus has not tended to kill older pigs, mortality among very young pigs infected in U.S. farms is commonly 50 percent, and can be as high at 100 percent, say veterinarians and scientists who are studying the outbreak.

The strain of the virus, known as Porcine Epidemic Diarrhea Virus, that is making its way across the nation's hog farms and slaughterhouses is 99.4 percent similar in genetic structure to the PEDV that hit China's herds last year, the researchers say. After it was first diagnosed in China in 2010, PEDV overran southern China and killed more than 1 million piglets, according to the Centers for Disease Control and Prevention's Emerging Infectious Diseases Journal.

The virus does not pose any health risk to humans or other animals. The meat from PEDV-infected pigs is safe for people to eat, according to federal officials and livestock economists.

No direct connection has been found between the U.S. outbreak and previously identified outbreaks in Asia and Europe, say scientists and researchers.

Industry fears

The U.S. pork industry had hoped the virus' spread would slow - or at least plateau - as summer approached and the weather grew warm. But Tom Burkgren, executive director of the American Association of Swine Veterinarians, said PEDV has proven far more tolerant of heat than a more common malady, transmissible gastroenteritis.

PEDV was diagnosed earlier this month for the first time in Arkansas, Kansas and Pennsylvania. Previously, the virus had been found in barns in Colorado, Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, Oklahoma and South Dakota.

It has been found in baby pigs, adult sows and in other hogs being fattened for slaughter in the United States, say researchers and veterinarians who are investigating the outbreak. No known cases have been reported in Canada or Mexico.

When and how PEDV arrived in the United States remains a mystery. The total number of pig deaths from the outbreak is not known, and the uncertainty is fueling fears among traders, meat processors and farmers about the potential impact on pork supplies later in the year.

The outbreak comes as U.S. hog and wholesale pork prices in the large hog-raising states of Iowa and Minnesota have surged to nearly two-year highs. Supermarkets are racing to fill meat cases for the summer grilling season even as supplies tighten, analysts said. Hogs supplies were already tight after last summer's historic drought drove up feed-grain costs, which prompted a higher-than-normal slaughter rate last summer.

The first U.S. case of PEDV was reported on May 17. But researchers at the National Veterinary Services Laboratory in Ames, Iowa, and other diagnostic labs have since discovered that PEDV arrived as early as April 16, according to the American Association of Swine Veterinarians. The labs have begun testing older samples taken from seemingly unrelated cases in an effort to track the virus' first appearance in the U.S.

Investigators with the U.S. Agriculture Department and others are hunting for clues to the widening outbreak and focusing on the nation's livestock transportation system.

PEDV is spread most commonly by pigs ingesting contaminated feces. Investigators are focused on physical transmission, perhaps equipment marred with feces, or a person wearing dirty boots or with dirty nails.

"It could happen at the slaughterhouse, where you have a trailer unloading a truck of pigs that was positive," said Torremorell, who noted that diagnostic researchers at the University of Minnesota and elsewhere have tested hundreds of samples in recent weeks.

"If the person doesn't clean the trailer correctly, and then goes to load up another load of pigs that were negative for PEDV, that person could end up delivering a truck of pigs to an uninfected farm," she said.