New World screwworm can be devastating to animals, people

COLLEGE STATION, Texas — Although the New World screwworm has been eradicated from the United States for many years, the recent movement of the pest through Central America and Mexico is a concern for Americans.

“The New World screwworm is native to the Americas, and prior to its eradication, it was found throughout the southwestern parts of the U.S,” said Sonja Swiger, professor and entomologist with Texas A&M AgriLife Extension.

“The females only mate a single time in their life, so by utilizing the sterile insect technique that led to the eradication of this fly from the U.S.,” said Swiger during a webinar hosted by the Council for Agricultural Science and Technology.

However, during late 2022 and into 2023, the entomologist said, there were increasing cases in Panama and other countries including Costa Rica, Nicaragua, Honduras and Guatemala.

“By the end of 2024, it had entered into Mexico and in 2025 it began to expand throughout the southern parts of Mexico,” Swiger said. “That led to the borders being shut to the movement of animals.”

The New World screwworm is unique because it infests living tissue of a living animal. If this fly were to return to the United States, there is concern for any warm-blooded animal including livestock, wildlife and pets, as well as humans.

“Cattle are high targets for these species, but we see them in other hosts,” Swiger said.

“The flies have a tendency to be found in more forested areas over wide-open pasture areas, but they will frequent those areas where animals are found in order to have a host to lay their eggs on,” she said.

Once the female flies have mated, they will find an animal with a wound to put their eggs on.

“Those eggs will hatch into larvae which will go through three life stages,” Swiger said. “Those three larval stages last from five to seven days where they will be found feeding in the tissue of the host.”

After they finish feeding, the larvae will leave the body of the animal and pupate on the ground.

“The life cycle will restart after the pupation stage which lasts about one week in most climates,” Swiger said.

A female fly can lay hundreds of eggs in her lifetime.

“She will generally lay 200 to 300 of them at a time near a wound or mucus membrane,” Swiger said. “Those eggs will hatch after 10 to 12 hours.”

The flies will target certain areas of the body, but they can attack any body part.

“They will target ears from the usage of identification tags, navels are highly targeted after the birth of an animal, as well as castration and dehorning sites,” the entomologist said.

The adult stage of the New World screwworm is very difficult to find in nature, Swiger said.

“They are a little bigger than a housefly,” she said. “It is a blue to greenish, metallic in color fly with red or orange eyes.”

Eradication Important

“They have the ability for multiple infestations, so if there is one, quite often there will be multiple and they can do some really horrific things to animals if they are left untreated, but it is not contagious from animal to animal,” said Chris Needham, deputy director for the APHIS New World Screwworm Directorate.

“It can be a small wound — even a tick bite is enough for an entry point,” he said. “As the larvae go in, they create pockets under the skin and really destroy tissue.”

If left untreated, animals can die in seven to 14 days, Needham said.

“They often affect newborns and if that is untreated, it is nearly 100% fatal,” he said.

Human Cases

“There have been over 1,000 human cases reported in Mexico and Central America, with six deaths,” said Megin Nichols, director of the CDC’s National Center for Emerging and Zoonotic Infectious Diseases. “However, we believe underreporting is likely where some may not seek health care.”

New World screwworm myiasis in humans, Nichols said, causes painful lesions often accompanied by a sensation of movement and foul-smelling, bloody discharge associated with the wound.

“Sometimes there will be visible larvae around the wound and extensive evasion and destruction,” she said.

“It can affect any of the mucous membranes or body openings including eyes, ears, nose, mouth, genitals, rectum or wounds,” Nichols said.

“The symptoms vary based on the infestation site and there can be secondary bacterial infections that cause fever, chills and severe illness,” she said.

Anyone with a suspected infestation of New World screwworm is advised to seek health care immediately.

“The primary treatment is physical removal of all larvae and wound care of any lesions,” Nichols said. “Pain management and antibiotics may be needed to treat any secondary bacterial infection.”

Prevention Measures

“We know that if we can attack the flies and eradicate them, that is the key to solving this,” Needham said. “It is something we have done in the past and we’re quite confident we can do it again.”

The likelihood of identifying a New World screwworm in the United States is really low, the deputy director said.

“We have deployed about 120 screwworm traps all along the southern border, predominately in Texas,” he said. “We have inspected about 30,000 flies from those traps with zero detections.”

Sterile flies are the key to eradicating the New World screwworm, Needham said.

“Since the female flies only mate once in their lifetime, if we can get them to mate with a sterile male, over time that will drop the population numbers and eventually eradicate them,” he said.

Currently sterile flies are only produced in one Panama location.

“We produce 100 million sterile flies per week there and disperse them mostly in southern Mexico,” Needham said.

“One hundred million flies are not really enough so we’re working on renovating a facility in Metapa, Mexico, jointly with Mexico,” he said. “That will produce an additional 100 million sterile flies and hopefully that will be running by the middle of next summer.”

In Texas, a production facility is under development with a two-phase approach.

“The first phase will hopefully be completed in early 2027, which will give us an additional 100 million flies and soon after at full capacity which will be 300 million flies,” Needham said.

The USDA is also working to disperse sterile flies throughout Texas.

“We are building a dispersal facility at Moore Air Base that should be operational early next year,” Needham said. “It’s a hatching facility where we get the sterile pupae from other facilities, grow them out and then release them as flies.”

For more information about the New World screwworm, go to www.screwworm.gov.