By PAYTON MEDFORD
In August 2025, warning flags were raised as a sudden spike of New World Screwworm (NWS) cases in Mexico appeared and spread to approximately 150 miles from the U.S. border. Having been eradicated once before, many livestock owners, pet owners, and others who might encounter wildlife in affected areas may not understand the implications of this outbreak.
What Is the New World Screwworm?
The New World Screwworm is a parasitic fly whose larvae exclusively consume living flesh from any warm-blooded animal, including humans. Unlike most fly larvae which feed on dead or decomposing matter.
These flies lay eggs in open wounds, and while significantly larger than common houseflies they don’t swarm. This makes detection challenging as only microscopic examination can truly verify if a larva or adult fly is a screwworm.
Life Cycle and Reproduction
The screwworm’s reproductive capacity makes it particularly dangerous. Female flies mate only once but can lay up to 3,000 eggs during their approximately 21-day lifespan, which can extend to 30 days under favorable conditions. The flies prefer warmer weather and can be either seasonal or active year-round depending on climate.
After being laid in an open wound, eggs hatch within 24 hours, larvae feed on living tissue for five to seven days before dropping to the soil to pupate for another five to seven days, and then emerge as adults for seven to ten days of reproductive activity.
Signs and Dangers
These parasites can lay eggs in wounds as small as tick bites which can lead to a fatal outcome. Animals affected often emit a putrid odor as flesh is eaten and exposed to the environment. Multiple female flies may lay eggs in the same wound, creating several generations of larvae in a single location and increasing the likelihood of infection and death. The larvae will also feed on mucous membranes and tissue if the conditions are favorable.
While the screwworm infestation itself is serious, animals typically die from secondary bacterial infections rather than the direct tissue damage, though larvae can consume brain and other vital organ tissue in severe cases.
Treatment and Prevention
Early detection is critical for successful treatment. If you suspect screwworm infestation, take these steps immediately:
- Photograph the wound before any treatment
- Collect multiple larvae and preserve them in 70% alcohol or less
- Report the incident to veterinary or agricultural authorities immediately
Several medications can help prevent infection, including some avermectins, nitenpyram, isoxazolines, and most flea and tick medications. However, moxidectin and epsiprantel should not be used for treatment or prevention. Prior to any treatment or prevention you should first consult a veterinarian.
For deceased animals, controlled burning is the preferred disposal method. If burning isn’t possible, douse the carcass with alcohol, pesticides, or hydrated lime, or freeze until proper disposal can be arranged.
Staying Vigilant
The best defense against screwworm infestation involves three key practices: vigilant observation of animals for wounds and unusual odors, prompt wound treatment and mitigation, and limited travel during outbreak periods.
States and countries have specific laws regarding health records when crossing borders, especially during outbreaks, so check current requirements before traveling with animals.
