Fly Maggots: Return of the Flesh-Eating Insect
- New World screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae infest the wounds of warm-blooded animals, including livestock, pets, and occasionally humans.
- For decades, the New World screwworm was considered eradicated from the United States and most of Central america thanks to a highly prosperous sterile insect technique (SIT) program.
- However, beginning in 2016, screwworm cases began to reappear in livestock in several Mexican states.
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New World Screwworm: A Resurgent Threat to Livestock and Public Health
What is New World screwworm?
New World screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae infest the wounds of warm-blooded animals, including livestock, pets, and occasionally humans. the larvae feed on living tissue, causing myiasis – a possibly fatal condition characterized by severe tissue damage, secondary infections, and economic losses in agricultural sectors. Unlike Old World screwworm, which affects Africa and Asia, the New World screwworm historically plagued the Americas, from the southern United States to Argentina.
A History of Eradication and Recent Resurgence
For decades, the New World screwworm was considered eradicated from the United States and most of Central america thanks to a highly prosperous sterile insect technique (SIT) program. This program, pioneered by the U.S. Department of Agriculture and international partners, involved releasing millions of sterilized male flies to mate with wild females, preventing the production of viable eggs. The last confirmed U.S. case was in 1985, and Mexico declared eradication in 1995.
However, beginning in 2016, screwworm cases began to reappear in livestock in several Mexican states. This resurgence has since spread throughout much of Mexico and into Central America, notably affecting cattle, and raising concerns about a potential northward expansion. as of late 2023 and early 2024, reports indicate the infestation is continuing to spread, prompting renewed eradication efforts.
Where is the Screwworm Now?
Currently, the most affected areas are concentrated in Mexico, particularly the states of Chiapas, Oaxaca, guerrero, and Veracruz. Significant outbreaks have also been reported in Guatemala, Honduras, Nicaragua, and El Salvador. There have been isolated reports of cases in Belize. The northward creep of the screwworm is a major concern for the United States, especially in border states like Texas and Arizona.
impact and Risks
The economic impact of screwworm infestations is substantial. Livestock producers face losses due to reduced weight gain, decreased milk production, treatment costs, and even animal mortality. The cost of treating individual animals can be significant, and widespread outbreaks can disrupt trade and agricultural economies.
While human cases are rare, they can occur, particularly in individuals with open wounds or poor hygiene. Screwworm myiasis in humans can cause severe pain, inflammation, and secondary infections. The risk is highest for those working with livestock or living in areas with high fly populations.
| Country | Reported Cases (2023-2024) |
|---|---|
| Mexico | Thousands (ongoing) |
| Guatemala | Hundreds |
| Honduras | Dozens |
| Nicaragua | Dozens |
| El Salvador | Reported, numbers limited |