What to Know about New World Screwworm

The return of the New World screwworm (NWS) may have younger or newer ranchers and farmers searching for information since the past few generations haven’t had to worry much about the potentially fatal parasitic pest. The confirmation this month of seven cases of NWS (6 in Texas and 1 in New Mexico) put the greatest focus on livestock and possibly pets.

The parasitic fly can cause severe – potentially fatal – infestations with its unique ability to target living issue rather than the dead bodies that some other parasites target. Its migration north through Mexico and American’s southern border meant a returning threat to the country’s livestock not seen since the 1960s.

Agricultural research universities across the country have been sharing information to help inform people searching for assistance with NWS.

New World screwworm looks for open wounds  

NWS is a creature that seeks opportunities, primarily in an animal’s wounds or orifice. 

Kansas State University Extension’s livestock entomologist Cassandra Olds said that the focus of the concerns for NWS is when the fly is in its larval stage.

"What we're concerned about is the maggot stage, or the larval stage of the New World screwworm fly," Olds said. "It needs to live inside a living organism, such as humans, cattle, dogs, cats or other."

The screwworm does not create its own wounds on an animal. The female flies find existing wounds to lay eggs. Eventually, the larva hatches and exists on the surrounding animal tissue.

"It's important to know that they can't initiate a wound," Olds said. "The only place where it can live is if a wound already exists."

Fed beef cattle futures market reaction to NWS

The fed beef cattle futures market experienced an immediate reaction to the official confirmation of an NWS infection in the United States, Texas A&M University AgriLife reported. Futures dropped, rose, and then dropped again.

“I think that’s what a lot of us expected, that once an announcement happened about screwworm being found in the United States, we’d see volatility in the future’s market. That’s just kind of how the futures market works,” said Dr. David Anderson, AgriLife Extension agricultural economist and professor in the Texas A & M College of Agriculture and Life Sciences Department of Agricultural Economics.

Quantity and quality of meat should not be impacted, Anderson said.

Cattle ranchers should watch their herds for NWS

Now is the time for cattle producers to watch closely to make sure that NWS isn’t latching on to their herds, according to advice from University of Illinois Urbana-Champaign’s College of Agricultural, Consumer, & Environmental Sciences. 

“OBSERVE. OBSERVE. OBSERVE,” emphasized Olivia Shike, University of Illinois Extension intern and student who works in beef production, research, and producer outreach.

“There is no substitute for eyes on cattle. You must inspect animals frequently to identify wounds. You should treat any wound quickly with insecticide and monitor closely,” Shike added.

She continued, “Fly control, now more than ever is essential. Minimizing wounds and careful management at calving, castration, and dehorning is key. When possible, move or plan to carry out these practices in colder temperatures. Finally, avoid unnecessary animal movement from risk areas across the country.”

USDA response to New World screwworm

Find the latest response to NWS from the USDA here.