The United States has been waging a little-known war in Latin America against flesh-eating worms, or screwworms, for several decades. Although these parasites were eradicated from the U.S. more than 40 years ago, anxiety about their return has increased since a small resurgence in Florida in 2017.
A new case of a screwworm infected person who traveled to the U.S. from Central America was registered on Sunday, Aug. 24.
Historically, screwworms were found across the U.S. from Florida to California and posed a threat to all warm-blooded animals, including cattle, deer, pets, and even humans. Now, the U.S. is trying to contain the spread of flesh-eating worms in Latin America, where they remain well-established.
What are the screwworms or ‘flesh-eating worms’?
The scientific name of the screwworm is Cochliomyia hominivorax, commonly known as the “man eater.” As the worms feed on the flesh of their host, they invade and destroy both the wound and the surrounding tissue.
The biological process begins when a female screwworm fly lays up to 500 eggs in an open wound. Within hours, the eggs hatch into larvae, which then feed on the flesh, causing a condition known as myiasis, a parasitic infection. After feeding, the larvae drop to the ground, where they burrow and pupate. Two days after birth, they emerge as adult flies, copulate and the cycle begins again, as the females lay more eggs.
Efforts to produce and release sterile flies at the Panama-Colombia border
Since it is nearly impossible to eliminate screwworms using pesticides without harming the ecosystems, the United States Department of Agriculture (USDA) maintains an international biological screwworm barrier along the Panama-Colombia border, in the Darien region, to prevent these parasites from re-entering to North America.
The Panama-United States Commission for the Eradication and Prevention of Screwworms (COPEG) manages this effort by producing and releasing sterile flies, which they sterilize through radiation, on a weekly basis. This technique disrupts the screwworms’ biological cycle and helps reduce their population.
Each week, COPEG releases approximately 14.7 million sterile flies through 18 flights. According to Entomology Today, this program helps the United States avoid US$1.3 billion in livestock damage annually.
Although the screwworm-rearing facility in Panama, responsible for sterilizing these flies, was not operating at full capacity for a period, it increased its production last year from 20 million to 90 million sterile pupae per week. COPEG disperse these pupae across Panama, Costa Rica, and Nicaragua. Moreover, the program requests permission from Colombia each month to release sterile flies within its borders to bolster the intercontinental barrier.
This cooperative effort began with an agreement signed in 1994 between Panama’s Ministry of Agricultural Development and USDA. It was formalized into law in 1999, and is now recognized as an international mission.
Flesh-eating worms causing alarm in Central America
In 1991, Mexico declared itself free of this parasite. Belize and Guatemala followed in 1994, El Salvador in 1996, Nicaragua in 1998, and Costa Rica in 2000. Despite these successes, the disease remained endemic in South America and the Caribbean, with the Darien Gap serving as a containment barrier. The eradication efforts in the United States and Mexico decades ago cost approximately US$955 million, while in Central America, the cost was around US$341 million.
However, in 2023, the resurgence of this parasite led to economic losses exceeding US$100 million annually in the southern United States and Mexico. Additionally, Panama reported a rise in cases not only in the Darien region, but also along its border with Costa Rica, prompting the latter to declare a national emergency in response. According to COPEG, by Aug. 10, 2024, there had been 13,989 cases reported in Panama; 5,029 in Costa Rica; and 1,717 in Nicaragua.