The New World screwworm (Cochliomyia hominivorax) is not currently circulating in the United States, and there is no active outbreak of the parasite in the country. While the screwworm was historically a significant threat to livestock, the United States officially eradicated the pest in 1966 through the release of sterile male flies, according to the U.S. Department of Agriculture (USDA).
What is the New World Screwworm?
The New World screwworm is the larva of a parasitic fly that infests the living tissue of warm-blooded animals. Unlike common blowflies that lay eggs on decaying carcasses, female screwworm flies are attracted to open wounds and mucous membranes. Once the eggs hatch, the larvae burrow into the host’s flesh, feeding on living tissue. This process causes severe, painful lesions and can lead to secondary infections or death if left untreated. The USDA describes the feeding behavior as similar to a screw being driven into wood, which gives the parasite its name.
Historical Eradication and Current Status
The United States maintained a screwworm-free status for decades following a massive eradication program. The USDA utilized the Sterile Insect Technique (SIT), which involves rearing millions of male flies, sterilizing them with radiation, and releasing them into the wild. These sterile males mate with wild females, who then lay infertile eggs, effectively crashing the population.
While the parasite remains endemic in parts of South and Central America, the USDA maintains a buffer zone in Panama to prevent the northward migration of the pest. The Centers for Disease Control and Prevention (CDC) notes that while occasional travel-related cases in humans or animals may occur when individuals return from endemic regions, these do not constitute a domestic outbreak or a threat to the U.S. food supply.
Why the Confusion About Recent Outbreaks?
Public concern regarding a "flesh-eating" parasite occasionally resurfaces due to misinterpretations of news reports involving localized infestations in other countries or isolated, travel-associated cases. There is no evidence from the USDA or state agricultural departments of a current, widespread screwworm infestation in Texas, New Mexico, or any other U.S. state as of 2024.
The U.S. food supply remains secure. Because screwworm is not a foodborne pathogen, even in regions where it is endemic, it does not pose a risk to consumers who purchase beef or other livestock products.
How Does the Screwworm Affect Humans?
Human infestation, known as myiasis, is extremely rare in the United States. When it does occur, it is almost exclusively linked to travel in tropical regions where the fly is active. According to the National Library of Medicine, risk factors for humans include:
- Traveling to endemic areas in the Caribbean or Central and South America.
- Having open, untreated wounds while in an environment where the flies are present.
- Living in close proximity to infested livestock without proper veterinary intervention.
Key Takeaways
- Status: The New World screwworm is eradicated in the U.S.
- Safety: The U.S. food supply is safe; screwworm is not a foodborne illness.
- Prevention: The USDA continues to partner with international agencies to maintain the sterile fly barrier in Panama, preventing the pest from moving north.
- Medical Risk: Human infection is rare and typically associated with international travel to tropical climates.
If you suspect an animal has a maggot-infested wound, contact a local veterinarian or your state animal health official immediately. Proper wound care and animal hygiene remain the most effective methods for preventing any form of myiasis in pets and livestock.