Hog farmers in the Midwest are watching closely as African swine fever spreads in China and into Western Europe.

There’s no evidence the virus is present in the U.S., but the global outbreak has already affected farmers in Minnesota and Iowa by lifting U.S. hog prices.

Lean hog prices bottomed out in August at about $40 below break-even per pig. Then, news emerged that African swine fever was spreading in China, and prices have risen to just above break-even in a little over a month. As the breadth of China’s outbreak emerges and China looks to import more pork in coming months, prices could keep rising.

That, and renewed trade talks with Japan, have brought relief to farmers buffeted by a glut of hog production and the effects of a trade war with China and renegotiation of NAFTA with Mexico and Canada.

But now farmers are worried the virus will spread to America. Infected hogs were discovered in Belgium and Japan in September, demonstrating the fever has now spread well beyond Russia, Eastern Europe and Africa, where it originated.

“It’s a real concern for everybody in the U.S.,” said Montserrat Torremorell, a professor in the College of Veterinary Medicine at the University of Minnesota. “For many years, it’s been restricted. It seemed like the virus was going nowhere. But then in the past five years there have been these new introductions.”

The virus is not a threat to humans, according to the World Organization for Animal Health, but it causes internal hemorrhaging and organ failure in pigs. The disease resides in warthogs without any sign of sickness and can be spread to domesticated pigs by ticks.

The few animals in an infected herd that survive become carriers of the disease. Only a minuscule amount of the virus can cause infection, and the virus can travel in animal feed, in meat that isn’t cooked thoroughly, or even on shoes, clothing or farm equipment. There is no effective vaccine.

“People have tried to produce a vaccine for a long time, but the vaccines don’t work very well,” Torremorell said.

While hog confinements in the U.S. generally practice rigorous biosecurity — workers must shower and change clothes at the entry to sow barns, for instance — there are still multiple weak points in the nation’s defenses to the disease, said Chris Hodges, a consultant and former chief executive of the National Pork Board in Des Moines.

Smithfield Pork, the largest hog farmer in the country and one with a meatpacking facility in Sioux Falls, S.D., is owned by a Chinese company called the WH Group. Many hog farms in China are owned by American firms. So the risk of someone carrying the disease from a Chinese farm to a U.S. farm on a pair of boots or a sweater is not insignificant.

“We’ve never had such a globalized industry as we have now in pork,” Hodges said.

Hog farmers buying feed on the global market could unknowingly introduce the virus to their pigs, and infected pork could be brought to the U.S. from other countries, missed by customs officials, and somehow transferred from human to pig.

“In most instances it will not make it to the pigs,” Torremorell said. “But it’s still a risk.”

The last major disease outbreak among pigs was a major boon for hog prices. When porcine epidemic diarrhea virus hit the U.S. in 2013 and 2014, prices skyrocketed to their highest level ever.

The same would not be true of African swine fever, said Dave Preisler, executive director of the Minnesota Pork Producers Association, because it would halt pork exports. U.S. hog farmers sell one in four pigs internationally. “It would be awful,” he said. “Our exports would basically go to zero. It would have a huge negative impact.”

Farmers are being encouraged to step up their already tight biosecurity measures, Preisler said.