By Mary Hightower
U of Arkansas System Division of Agriculture
FAYETTEVILLE, Ark. — The New World screwworm is the stuff of nightmares.
Named for its habit of screwing into the living flesh of warm-blooded animals with its razor-sharp mouth hooks, they are maggots — the larvae of Cochliomyia hominivorax, a fly with large orange eyes, a dark blue or green metallic body bearing three dark stripes.
Adult New World screwworm fly, characterized by its large orange eyes and bluish or greenish metallic body and three back stripes. (Image by Judy Gallagher (lCreative Commons Attribution 2.0 Generic license)
All but disappeared from the United States since the 1960s, the New World screwworm has prompted fresh concerns from the U.S. Department of Agriculture and the livestock industry after a case was reported in southern Mexico last fall.
This prompted the USDA’s Animal and Plant Health Inspection Service, or APHIS, to put in effect a temporary ban on importation of live cattle, bison, horses and other livestock from Mexico.
“The United States Department of Agriculture had successfully eradicated New World screwworm from the U.S. in 1966,” said Kirsten Midkiff, extension animal health and wellbeing specialist for the University of Arkansas System Division of Agriculture. “The last case that we had in the U.S. was in October 2016 in Florida, and swift action was taken to eliminate the presence by March 2017.
“Because of this new case in the southern part of Mexico, APHIS announced on Dec. 13 that there will be a release of $165 million in emergency funding to protect U.S. livestock, pets, and wildlife,” Midkiff said.
Setting up a barrier
1944 bulletin from Florida Extension Service features discussion on managing screwworm before the pest was eradicated in the U.S. in 1966 (U of A System Division of Agriculture image by Jon Zawislak)
New World screwworms are endemic to Cuba, Haiti, the Dominican Republic, and countries in South America. Panama was considered the border for screwworm prevalence, but recently the screwworm has trended north into Costa Rica, Nicaragua, Honduras, Guatemala, and the southernmost areas of Mexico.
In 1972, USDA and Mexico started a screwworm eradication program to create a biological barrier distributing sterile flies which disrupt reproduction. The sterile flies were raised in a facility in Mexico and dispersed using specially equipped planes. In 2006, a new sterile fly production facility was built in Panama, with flies distributed along the border between Panama and Columbia, pushing the barrier further south. According to the U.S. Embassy in Costa Rica, an average of 60 million sterile flies were dispersed weekly.
Midkiff said the emergency funds are being used to support surveillance efforts, animal health checkpoints, and collaborate with Mexico and Central American countries to establish a new barrier on the Isthmus of Tehuantepec and re-establish the Panamanian barrier.
Precaution and prevention
Maggie Justice, extension beef cattle specialist for the Division of Agriculture, said she’s been getting questions from ranchers. However, she wants to emphasize that “currently, there is no outbreak in the U.S., we have imposed restrictions on live cattle imports from Mexico out of caution.
“This could cause disruption of the overall cattle supply, especially in regions such as Texas and an overall reduced supply of cattle in the market. This also can cause uncertainty in the market,” Justice said. However, “prevention of the screwworm entering the U.S. is imperative, as there could be major economic losses associated with the pest with devastating consequences for animal health and welfare.”
Economic concerns
The United States imported 1.23 million head of feeder cattle from Mexico in 2024. Mexico is second only to Canada in terms of supplying beef to the U.S.
However, during “the last five weeks of the year, we imported zero,” said James Mitchell, extension livestock economist for the University of Arkansas System Division of Agriculture. “This is important because we typically observe a seasonal high in Mexican cattle imports during December. These cattle are fed in U.S. feedlots before being processed into beef in preparation for grilling season, which spans from Memorial Day to Labor Day.”
Mitchell said the import ban increased cattle prices, particularly in Texas and other border states.
“However, it also had negative effects on feedlots already struggling to maintain capacity due to tighter domestic cattle supplies,” he said.
The potential cost of a New World screwworm outbreak in domestic herds could be substantial, depending on its severity. Implementing an import ban is a logical step to reduce the risk of such an outbreak. Prior to its eradication in 1966, New World screwworms cost the U.S. livestock industry more than $100 million annually, according to a 1991 presentation by James E. Novy, assistant chief of program evaluations and planning of veterinary services for USDA.
What to look for
New World screwworm flies lay eggs on an open wound or around an orifice, and after a few hours the eggs will hatch and larvae burrow into the wound to feed. The maggots are unique in that they have sharp mouth hooks that cause extensive damage, tearing into the host tissue. Unlike other species, these maggots will only feed on living tissue.
“Being vigilant about New World screwworm is the key to keeping it out of the United States,” Midkiff said.
Early signs for the screwworm are enlarged or draining wounds, discomfort, and the presence of eggs or larvae near superficial injuries, according to APHIS. Other signs in animals — both mammals and birds — may be irritated behavior, head shaking, the smell of decay, evidence of fly strike, or the presence of maggots in a wound.
“If screwworms are diagnosed, then affected animals will be quarantined until daily wound care and treatments of infected animals are successful,” Midkiff said. “If the USDA investigates cases and it is deemed necessary, then additional control measurements including insecticidal treatment of the environment or sterile screwworm fly release may occur.”
If you suspect a screwworm infestation, contact your veterinarian or state veterinarian.
To learn about extension programs in Arkansas, contact your local Cooperative Extension Service agent or visit www.uaex.uada.edu. Follow us on X and Instagram at @AR_Extension. To learn more about Division of Agriculture research, visit the Arkansas Agricultural Experiment Station website: https://aaes.uada.edu. Follow on X at @ArkAgResearch. To learn more about the Division of Agriculture, visit https://uada.edu/. Follow us on X at @AgInArk.