Beware New World screwworm spread

The New World screwworm isn’t a pest most Midwestern cattle producers have ever needed to think about. But veterinarians say it’s worth keeping an eye on.

The parasite, once eradicated from the United States, has reappeared in Central and South America and is moving north. It’s not an immediate threat to the Midwest, but university Extension veterinarians say awareness is important.

Dr. Angela Varnum, livestock veterinarian with the University of Minnesota-Extension, has seen the parasite firsthand. Through her work with the nonprofit Equitarian Initiative, she recently spent time training with veterinarians and students in Guatemala who are responding to active screwworm cases.

The disease is spread by a fly whose larvae feed on live tissue, as opposed to feeding on dead tissue like what many are accustomed to seeing, Varnum said.

“That’s what makes it so dangerous,” she said.

A single fly can lay eggs on even a small wound, like a tick bite or ear-tag hole. Once the larvae hatch, they feed on living flesh and can kill an untreated animal within one or two weeks.

The United States first eliminated the parasite in the 1960s using sterile insects, which involves releasing sterilized male flies so females can’t reproduce. That same strategy is now being used in Central America and southern Mexico.

“There are two control methods we know work,” Varnum said. “The sterile-insect technique, and good wound-treatment practices among producers.”

Active cases are centered in southern Mexico and Guatemala as of early November, but a few infected animals have been transported north. Varnum said the risk to northern states is small but not zero.