They can weigh up to 300 pounds and are like snuffling, rooting machines.

Wild hogs roaming through at least 39 states are lately alarming wildlife officials by their population growth in Michigan, Iowa and other northern states.

"They destroy the natural habitat that's out there," said Angi Bruce, who heads the Iowa Department of Natural Resource's southwest wildlife bureau. "It basically looks like a plow has gone through the area."

Feral swine, numbering an estimated 4 million nationwide, can spread disease if they come in contact with domestic pigs. That's a big concern in Iowa, which boasts the top pork production in the nation.

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Before last fall's Michigan deer hunting season, the state departments of Agriculture and Natural Resources "encouraged hunters with a valid hunting license of any type to shoot feral swine ... in 23 Michigan counties."

"In states where feral swine have become established, they have caused crop damage, pose a serious threat to the health and welfare of the domestic swine, endanger humans, impact wildlife populations, and impact the environment by disrupting the ecosystem," the agencies said in a statement.

The rooting and wallowing activities of the creatures, which tear apart the earth to search for food, have also forced some farmers to repeatedly replant fields.

"They are an invader and we feel they should be controlled," said Sterling Liddell, with the Iowa Farm Bureau.

While feral swine have plagued warmer states for years, they are a recent problem in Iowa, with wildlife officials working to eradicate them since 2004.

About 100 of the animals are thought to be in southern Iowa, and officials said because they are such prolific breeders, 70 percent must be killed off each year just to keep that number in check.

The wild pigs are not native to the United States and are a combination of escaped or neglected swine and imported Eurasian wild boars, said Gail Keirn, a spokeswoman for the USDA's Animal and Plant Health Inspection Service.

Under a national wildlife disease management program, Keirn said researchers will collect and test wild pigs in up to 25 states to look for the presence of classical swine fever. The USDA is also studying how the animals might spread disease to domestic hogs and are developing ways to deliver vaccines in bait.

"Our goal is to safeguard the livestock industry as well as the environment from these unwelcome invaders," Michigan's state veterinarian, Steven Halstead, said in a statement.

People also can become infected with diseases from feral swine. Keith Massey, a crop farmer and hog producer from Columbus Junction in southeast Iowa, contracted the bacterial disease brucellosis from his sows that were exposed to wild pigs.

He experienced night sweats for weeks, took numerous shots and months of antibiotics, and was forced to have all his animals euthanized.

He now makes a point of warning other farmers.

"This is why you've got to be careful that these animals are under control," he said. "There's a myriad of diseases that these animals can spread, and if their concentration of numbers get worse, this can be a problem."