An in-depth look at screwworms and the USDA response to keep the problem from spreading in the U.S.

An adult female screwworm fly can lay up to 300 eggs
at a time. (USDA photo via Offrange)

 

Hollywood horror flicks can't hold a candle to the gruesomely parasitic screwworm. The flesh-eating larvae were eradicated in the U.S. nearly 60 years ago, only to reappear in South America and then migrate back into Mexico, where the Department of Agriculture has played hardball at the border, trying to keep the insidious pest from re-entering the states.

Luckily for cattle and other mammals, the screwworm, which is a type of blowfly's larva, isn't even a particularly effective parasite because it inevitably kills its host.

Female Cochliomyia hominivorax fixate on the smell of blood from any wound as a place to "lay their eggs — up to 300 at a time. These hatch out spined maggots that anchor themselves into flesh, then proceed to eat their way through it, sometimes deep into muscle," reports Lela Nargi of Offrange. "If it’s deep enough, the host animal cannot survive."

The USDA continues to use a multi-pronged approach to control screwworms:

• In May, U.S. Secretary of Agriculture Brooke L. Rollins suspended the import of live cattle, horses, and bison through U.S. ports of entry along the Mexican border, aiming to curtail entry possibilities while providing time to prepare and counter any screwworm infestations. While Rollins announced a phased reopening of the southern ports this July, the plan comes with heightened and constant monitoring.
• The plan includes the incremental release of thousands of sterile male screwworm producers with heavy monitoring by U.S. and Mexican scientists. The mass numbers of sterile screwworm-producing blowflies work to create a buffer by out-competing the fertile male flies. Since a female screwworm blowfly can only mate once in a lifetime, when she mates with a sterile male blowfly it insures that her eggs will never hatch -- no matter how many eggs she lays.
• Experts can also trap and measure the ratio of sterile to fertile male screwworm producers in different areas. Sterile releases aim to overwhelm the wild population, which scientists can also measure through traps.
• The USDA also employs teams affectionately known as "tick cowboys" who patrol borders on horseback, looking for any signs of illness in herds. By scouting for any cattle that appear distressed, the USDA can quickly detect, contain, and report any screwworm (or other) infestation.

An active screwworm infestation can be treated and cured, but like the worms themselves, it's "disgusting," Nargi writes. "The maggots have to be removed before the wound is cleaned and sutured." 

Screwworms do have a natural enemy -- fire ants. Nargi writes, "Unsurprisingly, no one is talking about dispersing more fire ants to eat the screwworms."

Experts are already hard at work helping livestock owners get prepared. Philip Kaufman, an entomologist at Texas A&M, "helped compile a new screwworm fact sheet that’s being distributed to ranchers," Nargi reports. "He isn’t prone to screwworm pessimism." He told her, "We are concerned, but we are not in a crisis. People are getting trained up and we’re gonna tackle this and solve it again."

Sharing any suspicious findings is key. Kaufman told Nargi, "Reporting is the number one thing people can do. If you ignore this, that fly population is just going to get bigger and bigger and cause more of a problem for everybody else. This is a community issue.”