Rare human case of flesh-eating parasite New World screwworm identified in U.S.

Larvae of the screwworm fly, collected from infected cows, are observed at a sterile fly production plant in Panama in June. Enea Lebrun/Reuters via CNN Newsource

A rare case of travel-related New World screwworm was identified in Maryland in a patient who traveled from El Salvador, a spokesman from the U.S. Department of Health and Human Services said.

The U.S. Centers for Disease Control and Prevention and the Maryland Department of Health are investigating the case of the flesh-eating parasite, which was confirmed on Aug. 4, HHS spokesman Andrew Nixon said. No additional information about the patient’s condition was made available.

“This is the first human case of travel-associated New World screwworm myiasis (parasitic infestation of fly larvae) from an outbreak-affected country identified in the United States,” Nixon said in an email. “Currently, the risk to public health in the United States from this introduction is very low.”

An outbreak of New World screwworms — the larval form of a type of fly that’s known to nest in the wounds of warm-blooded animals and slowly eat them alive — has been spreading across Central America since early 2023, with infestations recorded in Belize, Costa Rica, El Salvador, Guatemala, Honduras, Mexico, Nicaragua and Panama. The fly reached southern Mexico late last year, sparking concern among U.S. agricultural industry officials and triggering closures of border-area cattle, horse and bison trading ports this year.

The parasite poses a significant threat to animal populations and the food supply. The United States mostly eradicated New World screwworm populations in the 1960s and 1970s by breeding sterilized males of the species and dispersing them from planes to mate with wild female flies. This month, the U.S. Department of Agriculture announced it would construct a fly production facility in Edinburg, Texas, at Moore Air Force Base, that could produce up to 300 million sterile flies per week. It will work with facilities in Panama and Mexico, USDA said.

“The construction of a domestic sterile fly production facility will ensure the United States continues to lead the way in combating this devastating pest,” USDA Secretary Brooke Rollins said this month. “If our ranchers are overrun by foreign pests, then we cannot feed ourselves.”

USDA said it will also provide up to $100 million for additional technologies, including traps and lures, therapeutics and techniques that could bolster fly production. USDA also said it would employ mounted patrol officers known as “Tick Riders” for surveillance and would train screwworm detection dogs to search for outbreaks among livestock.

This month, HHS also issued an emergency declaration that allowed the U.S. Food and Drug Administration more flexibility for animal drugs to treat and prevention New World screwworm infestations. There are no FDA-approved drugs for New World screwworm in the United States, but the declaration allows the agency to authorize drugs for emergency authorization.

New World screwworms are the parasitic larva of a metallic blue blow fly species called Cochliomyia hominivorax. Unlike all other blow flies native to the Western Hemisphere, the New World screwworm feeds on the flesh of living animals, rather than dead ones, Dr. Phillip Kaufman, a professor and head of the department of entomology at Texas A&M University, previously told CNN.

The flesh-eating maggots go for most warm-blooded animals, including horses and cows. It can also infect pets and humans.

“After mating, the female fly finds a living host, lands on its wound, and will lay up to 200 to 300 eggs,” Kaufman explained. “After 12 to 24 hours, those eggs all hatch, and they immediately start burrowing and feeding on the tissue of that animal, causing very, very large wounds to form.”

Treatment for infested cattle often involves cleaning, antiseptic treatment and coverings for wounds. If left untreated, the flies can kill an animal in a matter of one to two weeks and spread to others.

By Jamie Gumbrecht, CNN