Nebraska has become the latest U.S. state to be hit by a deadly pig virus, bringing the total number of states affected to 20, the U.S. Department of Agriculture said.

The Porcine Epidemic Diarrhea virus (PEDv) had never been reported in North America until May, when it was discovered in the United States.

The virus has fueled market concerns that U.S. hog supplies will decline steeply next spring and summer.

PEDv causes diarrhea, vomiting and severe dehydration. Hog epidemiologists have found that a large number of very young piglets infected with the virus die.

While the disease has tended not to kill older pigs, mortality among very young pigs infected on U.S. farms is commonly 50 percent, and can be as high as 100 percent, according to veterinarians and scientists studying the outbreak.

To date, more than 1,500 cases, each of which could represent thousands of infected animals, have been reported in 20 states across the Hog Belt. The states include such major pork producers as Iowa, North Carolina, Minnesota and Oklahoma.

As defined by the USDA, each diagnostic case could represent multiple animals at either a single farm site or several locations. The USDA's National Animal Health Laboratory Network released its latest pig virus data on Wednesday.

Nebraska, the sixth-largest pork production state, had 1.35 million hogs spread over 2,200 operations as of September 1, according to USDA data.

The spread of the disease has heightened scrutiny of the U.S. trucking industry, as livestock transportation vehicles have been targeted as a possible means of transmission.

The National Pork Board has issued biosecurity guidelines urging transporters to clean, disinfect and dry vehicles that are used to transport pigs and hogs.

The guidelines also include stricter standards for handling of manure by producers and commercial haulers.