Cochliomyia Hominivorax (Coquerel, 1858)

''Cochliomyia hominivorax'', the New World screwworm fly, or simply screwworm or screw-worm, is a species of parasitic fly that is well known for the way in which its larvae (maggots) eat the living tissue of warm-blooded animals. It is present in the New World tropics. Of the five 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 technically called 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 .

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 North and Central America, as well as parts of the Caribbean since the 1950s. It is still widespread in tropical and subtropical parts of the Caribbean and South America, and import of infected animals from endemic areas risks reintroducing the fly to areas where it has been eradicated.

Life cycle

New World screwworm flies are dependent on a host (obligate parasitism), as their larvae require living tissue for food. This lifestyle distinguishes it from the common facultative myiasis ( facultative parasitism) that is common within similar species, which primarily feed on necrotic tissue or excrement. Facultative parasites can be attracted by screwworm fly myiasis and are considered secondary myiasis pathogens, such as ''Cochliomyia macellaria''. This species is also called "secondary screwworm fly" because, after the New World screwworm fly, it is the most common cause of myiasis.

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 and domestic animals, and less frequently birds and humans. 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 are capable of causing severe tissue damage or even death to the host. About three to seven days after hatching, the larvae fall to the ground to pupate. The pupae reach the adult stage about seven days later. Female screwworm flies mate four to five days after hatching.

The entire life cycle is around 20 days. A female can lay up to 3,000 eggs and fly up to 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 US, Mexico, Central America, and the northern states of South America up to Uruguay and northern Argentina.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 up 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 winter months, they remain stable in the tropics. Before its elimination in the US, the screwworm fly could spread as far as the Canadian border in favorable years. Dispersals occur at a rate of about 80 to 160 kilometers 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. The Florida government instituted control measures including mandatory inspections of all animals leaving the area; the outbreak was declared as neutralized in March 2017. 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, which is trying to push the parasite south of the narrow Isthmus of Panama. However, in December 2024, the reemergence of the screwworm was reported in Southern Mexico, indicating the protective barrier in Panama had been breached. Consequently, the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) pledge
emergency funding
to help prevent the northern migration of this parasitic fly. The US government also ordered th
closure of livestockimportations
from Mexico in May and again July 2025 to prevent infected animals from entering the USA as the movement of wild and domestic animals drives disease dispersion.

Attempts at eradication

Due to the parasitic nature of the species, its danger to livestock and, less prominently, humans, the New World screwworm has been the forefront of many attempts at elimination within regions.

In 1954, the first tests using the Sterile Insect Technique (SIT) were conducted on Curaçao, whereupon 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 United States, followed by the southwestern United States. Since 1966, the species has been considered extinct throughout the United States.

Eradication efforts continued in Mexico (1972–1991), the Virgin Islands (1971–1972), Puerto Rico (1975), and in 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. The goal is to create a permanent barrier against New World screwworm fly invasions in Panama. 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, treated with ionizing radiation, and thus sterilized.

It is important that the bred insects do not suffer any impairment in competition with wild, fertile insects. Researchers are investigating the possibilities and methods for producing genetically sterilized males in the future. However, this method is not yet practical.

Etymology

From the Greek ''kochlias'' (snail with a spiral shell) + ''myia'' (fly) and the Latin (man) + (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.

References

Further reading

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* James, Maurice T. (1947) The Flies That Cause Myiasis in Man. USDA Miscellaneous Publication No. 631.
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External links

Species Profile - Screwworm (''Cochliomyia hominivorax'')
National Invasive Species Information Center, United States National Agricultural Library
STOP Screwworms: Selections from the Screwworm Eradication Collection
– United States National Agricultural Library
Invasive Species Compendium
CAB International

{{Taxonbar, from=Q526003

Calliphoridae
Diptera of South America
Veterinary entomology
Parasitic flies
Insects described in 1858
Taxa named by Charles Coquerel