Return of the New World Screwworm

In the heat of the afternoon, a glistening blue-green fly hovers over a calf with a small tick bite. It’s drawn to the animal’s tiny wound. To the untrained eye, it looks like any other blow fly. But when her eggs hatch, the larvae won’t wait for decay. Instead, it will burrow into the animal, consuming living flesh, perhaps even to the point of death. This is the New World screwworm (NWS) fly, Cochliomyia hominivorax. With new detections across the southern border, a pest once eradicated from the U.S. is back in the spotlight.

NOT ALL MAGGOTS ARE THE SAME.

To understand the threat of the New World screwworm (NWS), it is essential first to understand myiasis. Myiasis is the infestation of living animals by fly larvae (maggots). It is often classified into three types: facultative, accidental and obligatory.

Facultative myiasis occurs with species that usually develop in carrion (dead, decaying animal flesh). These fly larvae are sometimes used in forensic investigations to estimate time of death. However, occasionally the adult female will opportunistically lay her young on wounds or necrotic areas on a living host. From a public health standpoint, a concern of these flies is their ability to be mechanical vectors of pathogens.

Accidental myiasis usually occurs when an animal eats something that was contaminated with fly eggs or larvae. The fly species involved are not normally parasitic but can cause gastrointestinal illness when unintentionally ingested. Since the larvae are only opportunistically parasitic, some sources group accidental myiasis underfacultative myiasis. Importantly though, neither involves flies that require a living host to complete development.

In contrast, for a few flies, such as bot flies and screwworms, it is necessary for the larvae to feed on living tissue for their development. This is termed obligatory myiasis. These flies pose the most serious direct medical threat as they deliberately invade and destroy living tissue.

Currently, in the U.S., there are no established fly species that commonly cause obligatory myiasis in humans or pets. Rare cases of human myiasis from bot fly species, which more typically infest wildlife, have been reported in rural settings (Goddard, 2013). However, the only significant historical threat of obligatory myiasis in both people and pets was the NWS.

A PARASITE ONCE DEFEATED.

In the 1960s, the U.S. mounted a successful pest eradication campaign, defeating the flies by releasing sterile males. Successful eradication programs also took place in Mexico, the Caribbean and Central America. However, over the last decade, NWS has been found in the Florida Keys, Costa Rica, Nicaragua, Honduras, Guatemala, Belize and Mexico, where the population has been documented moving northward (USDA, 2025). Additionally, in August this year, the CDC reported a human case in Maryland in an individual who had recently traveled to El Salvador.

A significant concern is that these pests can cause enormous economic losses in the livestock industry. For example, in Brazil, where NWS is endemic, annual costs are estimated at $337 million (Grisi et al., 2014). However, it is not just an economic threat. In areas where they are established, NWS also has the potential to infest pets, wildlife and even people. It remains a serious public health threat throughout much of South America (McGarry, 2014).

Recent modeling suggests that if the NWS were to spread northward, the regions of the U.S. most at risk are Texas and Florida (Valdez-Espinoza et al., 2025). Those states have both a climate suitable for the pest and areas with high livestock density. Importantly, the authors also noted that climate change is likely to expand the NWS geographic range even farther north, as has been seen with other public health pests such as ticks and kissing bugs.

BIOLOGY AND IDENTIFICATION.

Female NWS flies are drawn to wounds and mucous membranes (e.g., nose, eyes, ears, umbilicus) on warm-blooded mammals, where they deposit large batches of eggs along edges. Eggs hatch within 24 hours. Larvae then burrow deep into the living tissue, causing significant destruction. With constant body heat of a live host, larvae can reach maturity as quickly as four days. The entire life cycle usually completes in about three weeks.

Adults are 8-10 mm long and are generally a metallic blue to bluish green in color. They appear very similar to their close relatives, green and blue bottle flies. Three distinct dark longitudinal stripes on the back (thorax) of the NWS help to distinguish them from these more common blow flies. Screwworm flies (Cochliomyia spp.) also lack prominent bristles in this area, unlike the blow fly species of Lucilia and Calliphora (Service, 2018).

The real challenge comes when trying to distinguish between the Cochliomyia hominivorax and Cochliomyia macellaria, the secondary screwworm. The latter species is widespread in the U.S. and can cause facultative myiasis. Although there are subtle differences in the striping on the thorax, accurate identification is best left to trained entomologists. For PMPs, the best way to make a distinction is this: if larvae actively feeding on healthy, living tissue are encountered, it should be treated as a red flag for NWS. Suspicious cases should be reported immediately. For more information on reporting, visit the USDA NWS website.

WHAT PMPS CAN DO.

The USDA recently announced a broad new plan including additional expenditures, working with Mexico and building a sterile insect dispersal facility (USDA, 2025). With these additional efforts in place, it’s unlikely that PMPs will encounter NWS. Still, there are some things that PMPs can do.

Stay alert and report. Watch for infestations. PMPs working around livestock, pets, or even wildlife habitats may spot or be asked about wounds where flies are invading. Report all suspected NWS cases immediately.

Educate clients. Help livestock owners, kennels, zoos and similar customers recognize the difference between other maggot infestations and screwworms.

Assist in Integrated Pest Management (IPM). Promote fly control around barns, corrals, kennels and animal facilities to reduce fly populations and risk.

While eradication efforts are largely in the hands of federal agencies, PMPs still have an important role to play. Staying observant in the field, advising clients, and promoting sound fly management practices all help ensure that potential screwworm cases are recognized and reported quickly, so that we can continue to avoid this devastating insect pest.

The author is an urban entomologist focused on public health pests.

References

Goddard, J. (2013). Public Health Entomology. CRC Press.

Grisi, L., Leite, R. C., Martins, J. R. de S., Barros, A. T. M. de, Andreotti, R., Cançado, P. H. D., Pérez de León, A. A., Pereira, J. B., & Villela, H. S. (2014). Reassessment of the potential economic impact of cattle parasites in Brazil. Revista Brasileira de Parasitologia Veterinária, 23(2), 150–156. https://doi.org/10.1590/S1984-29612014042.

McGarry, J. W. (2014). Tropical myiases: Neglected and well travelled. The Lancet Infectious Diseases, 14(8), 672–674. https://doi.org/10.1016/S1473-3099(14)70830-8.

Service, M. (2018). Flies and myiasis. In Medical entomology for students (pp. 157–175). Cambridge University Press.

USDA. (2025, June 18). Secretary Rollins announces bold plan to combat New World screwworm’s northward spread. https://www.usda.gov/about-usda/news/press-releases/2025/06/18/secretary-rollins-announces-bold-plan-combat-new-world-screwworms-northward-spread.

Valdez-Espinoza, U. M., Fadda, L. A., Marques, R., Osorio-Olvera, L., Jiménez-García, D., & Lira-Noriega, A. (2025). The reemergence of the New World screwworm and its potential distribution in North America. Scientific Reports, 15, Article 23819. https://doi.org/10.1038/s41598-025-04804-9.