A swine virus deadly to young pigs, and never before seen in North America, has spiked to 199 sites in 13 states - nearly double the number of farms and other locations from earlier this month.
Iowa, the largest U.S. hog producer, has the most sites testing positive for Porcine Epidemic Diarrhea Virus: 102 sites, as of June 10. The state raises on average 30 million hogs each year, according to the Iowa Pork Producers Association.
PEDV, most often fatal to very young pigs, causes diarrhea, vomiting and dehydration. It also sickens older hogs, though their survival rate tends to be high.
The total number of pig deaths from the outbreak since the first cases were confirmed May 17 is not known.
Researchers at veterinarian diagnostic labs, who are testing samples as part of a broad investigation into the outbreak, have seen a substantial increase in positive cases since early June, when data on the PEDV outbreak showed it at some 103 sites nationwide.
The data was compiled and released last week by Iowa State University, University of Minnesota, Kansas State University and South Dakota State University.
The virus does not pose a health risk to humans or other animals and the meat from PEDV-infected pigs is safe for people to eat, according to federal officials and livestock economists.
But the virus, which is spreading rapidly across the United States, is proving harder to control than previously believed. In addition to Iowa, Oklahoma has 38 positive sites, Minnesota has 19 and Indiana has 10, according to the data.
PEDV has also been diagnosed in Arkansas, Colorado, Kansas, Illinois, Michigan, Minnesota, Missouri, Ohio, Pennsylvania and South Dakota.
Swine veterinarians, investigators with the U.S. Agriculture Department and others are trying to determine how the virus is spreading from farm to farm and state to state. Currently the focus is on the nation's livestock transportation system.
PEDV is spread most commonly by pigs ingesting contaminated feces. Investigators are studying physical transmission, such as truck trailers marred with contaminated feces, or a person wearing dirty boots or with dirty nails.
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 PEDV 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, according to the U.S. researchers.
After PEDV was first diagnosed in China in 2010, it overran southern China and killed more than 1 million piglets, according to the Centers for Disease Control and Prevention's Emerging Infectious Diseases Journal.
No direct connection has been found between the U.S. outbreak and previously identified outbreaks in Asia and Europe, say scientists and researchers.