Cochliomyia hominivorax

Cochliomyia hominivorax, the New World screwworm fly, or simply screwworm or screw-worm, is a species of parasitic blowfly whose larvae (maggots) eat the living tissue of warm-blooded animals.^[1] It is present in the New World tropics. Of the four species of Cochliomyia, only C. hominivorax is parasitic; a single parasitic species of Old World screwworm fly is placed in a different genus (Chrysomya bezziana). Infestation of a live vertebrate animal by a maggot is scientifically termed myiasis. While the maggots of many fly species eat dead flesh, and may occasionally infest an old and putrid wound, screwworm maggots are unusual because they attack healthy tissue. This increases the chances of infection, and damaged tissue also attracts more of these flies.

Cochliomyia hominivorax
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Diptera
Family:	Calliphoridae
Genus:	Cochliomyia
Species:	C. hominivorax
Binomial name
Cochliomyia hominivorax (Coquerel, 1858)

Larva

The New World screwworm fly was the first species upon which the sterile insect technique was tested and then applied in a natural environment, resulting in the control and systematic eradication of this species from the United States, Central America, and parts of the Caribbean beginning in the 1950s.^[2] By the early 2000s, it was considered eradicated from North America. However, in 2024 and 2025, the New World screwworm was once again detected in Mexico,^[3] leading to renewed efforts to prevent its re-emergence as a threat to agriculture.^[4] Meanwhile, the fly is still widespread in tropical and subtropical parts of the Caribbean and South America, so that animals imported from these areas to non-endemic regions must be inspected or treated to prevent the pest's reintroduction.^[5]

Life cycle

New World screwworm flies are obligate parasites, as their larvae require living tissue for food. This lifestyle distinguishes them from other members of the genus, whose larvae primarily feed on necrotic tissue or excrement. These other Cochliomyia flies, such as Cochliomyia macellaria, are facultative parasites, in that they sometimes lay eggs on lesions already infested with C. hominivorax to produce secondary myiasis. Thus they are considered "secondary" screwworm flies, while hominivorax is "primary".^[6]

The New World screwworm fly prefers to lay its eggs near wounds, such as those caused by dehorning or castration, but also near the navel of newborns or on insect bites. Healthy areas with soft tissue or body orifices can also be infested, such as the inner corner of the eye (medial canthus) or the perineum. The New World screwworm fly infests wild^[7] and domestic animals, and less frequently birds and humans.^[8][9] Cold-blooded (poikilothermic) animals or carrion cannot be infested.

Screwworm females lay 250–500 eggs in the exposed flesh of warm-blooded animals. The larvae hatch and burrow into the surrounding tissue as they feed. Should the wound be disturbed during this time, the larvae burrow or "screw" deeper into the flesh, hence the larva's common name. The maggots can cause severe tissue damage or even death to the host. About three to seven days after hatching, the larvae fall to the ground to pupate, reaching the adult stage about seven days later. Female screwworm flies mate four to five days after hatching.^[10]

The entire life cycle is around 20 days. A female can lay up to 3,000 eggs and fly up to 200 km (120 mi) during her life. Males, on the other hand, mate up to ten times. They behave relatively isolated and hide in vegetation until they spot a female. Mating occurs on vegetation. Females copulate only once and retain the male's sperm for life, which has been exploited in eradication programs using the sterile insect technique.

Epidemiology

Historically, the original distribution area was the southern states of the United States, Mexico, Central America, and the northern states of South America south to Uruguay and northern Argentina.^[2] It was also present on some Caribbean islands, such as Cuba, Trinidad, Hispaniola, Jamaica, and Tobago. Currently, due to eradication programs, the species is distributed exclusively in South and Central America to the Panama Canal and some Caribbean islands. The populations in southern Argentina are dynamic. They depend primarily on temperature; in winter and early spring no infestations are recorded in the southern half of Argentina. While the extreme southern and former northern populations collapse during the winters, they remain stable in the tropics. Before its elimination in the US,^[11][12] the screwworm fly could spread as far as the Canadian border in favorable years. Dispersals occur at a rate of about 80 to 160 km (50 to 99 mi) per generation.

The United States officially eliminated the screwworm in 1982 using the sterile insect technique, although an isolated outbreak occurred in the Florida Keys, Monroe County, Florida, in autumn of 2016.^[11] The Florida government instituted control measures including mandatory inspections of all animals leaving the area; the outbreak was declared as neutralized in March 2017.^[12] The screwworm was eliminated in Guatemala and Belize in 1994, El Salvador in 1995, and Honduras in 1996. Campaigns against the flies continue in Mexico, Nicaragua, Costa Rica, Panama, and Jamaica with financial assistance from the United States Department of Agriculture (USDA), which is trying to push the parasite south of the narrow Isthmus of Panama.^[13] However, in December 2024, the screwworm was reported to have re-emerged^[3] in Southern Mexico, indicating that the protective barrier in Panama had been breached. Consequently, the USDA's Animal and Plant Health Inspection Service pledged emergency funding to help prevent the northern migration of this parasitic fly.^[14] The US government also stopped livestock imports from Mexico in May 2025 and again in July 2025^[15] to prevent infected animals from entering the US as the movement of wild and domestic animals drives disease dispersion.^[16]

As a cost benchmark, the 2016–2017 Florida Keys eradication effort was later reported at about US$3.2 million, covering sterile-fly releases, surveillance, and interagency operations.^[17]

Attempts at eradication

Range of the New World screwworm

Due to the danger to livestock and to a lesser extent humans, the New World screwworm has been at the forefront of many attempts at elimination within regions. In 1954, the first tests using the sterile insect technique were conducted on Curaçao; the New World screwworm fly was successfully eliminated from the island within four months. From 1958 to 1960, the species was eliminated from the Southeastern and then the Southwestern United States. Since 1966, the species has been considered extinct throughout the US. The US and Mexico bred and released more than 94 billion sterile flies from 1962 to 1975.^[18]

Eradication efforts continued in Mexico (1972–1991), the Virgin Islands (1971–1972), Puerto Rico (1975), and Central America from Guatemala (1988–1994) to Belize (1988–1994), El Salvador (1991–1995), Honduras (1991–1995), Nicaragua (1992–1999), and Costa Rica (1995–2000). In 1998, the first sterile flies were released in Panama with the goal of creating a permanent barrier zone at the Darién Gap against New World screwworm fly invasions from South to Central America.^[2] This is achieved through weekly sterile New World screwworm fly releases of up to 50 million insects. The insects are bred en masse in factories and sterilized by ionizing radiation.^[2]

The bred insects must not suffer any impairment in competition with wild, fertile insects. Researchers are investigating the possibilities and methods^[19] for producing genetically sterilized males in the future, but this method is not yet practical.^[2]

Screwworm flies were again found in southern Mexico in late 2024, and the United States was preparing to breed billions of sterile flies in Mexico and dump them over Mexico and southern Texas in 2026 to protect the country against invasion.^[18] The USDA estimated in 2025 that an outbreak in Texas (the biggest cattle-producing US state) could cost US$1.8 billion.^[20]

Etymology

Presumed to be from the Greek kochlias (snail with a spiral shell) + myia (fly) and the Latin hominis (man) + vorax (consuming), Cochliomyia hominivorax, or the New World screwworm fly (formerly Callitroga (Greek kallos, (beautiful), + trogein, (to gnaw), Americana), was first described by French entomologist Charles Coquerel in 1858.^[21]