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New World Screwworm

New World Screwworm

Description

New World screwworm (NWS) is a parasitic fly, Cochliomyia homnivorax, that completes part of its lifecycle by feeding on the tissue of live, warm-blooded animals and people.  NWS flies lay their eggs in wounds and body openings like the nose, eyes, ears, mouth and umbilicus of newborn animals. The eggs hatch into maggots (larvae) that eat live tissue and burrow into the flesh causing extensive tissue damage and foul-smelling, often painful wounds.

Distribution

NWS was eradicated from the U.S. in the 1980s, and from Mexico and Central America in the mid-2000s; however, it persisted in the Caribbean and South America. Since 2023, the fly’s distribution has moved steadily back northward from southern Panama, extending into Mexico in late 2024. For the first time in several decades, cases were detected in Texas and New Mexico in June 2026. Substantial federal and state resources have been invested in management and surveillance techniques to minimize further spread and re-eradicate NWS from North America.

Therehave been no detections of New World screwworm in Michigan.

NWS can infest any warm-blooded animal (including people), but livestock are most commonly affected. Birds can be infested, but this is relatively uncommon. When NWS was endemic in Texas, NWS was extensively documented in white-tailed deer and had significant effects on the Texas deer population. Cottontail rabbits, raccoons and opossums were also documented to have been infested.

NWS flies thrive in environments from 77 to 86 oF and 30-70% humidity. Temperatures consistently below 46 oF kill the pupae and disrupt the life cycle.

Transmission

NWS spreads as female flies lay eggs on open wounds or other parts of the body in live, warm-blooded animals. Wounds as small as a tick bite or deer fly strike can attract a female fly to lay eggs and one female can lay 200-300 eggs at a time. 

Eggs hatch into maggots that burrow into a wound or body cavity and feed on living flesh.  Over about 7 days, those maggots mature through different larval stages, then drop to the ground and burrow into the soil where they pupate and then emerge as adult screwworm flies. The entire life cycle can take as little as 21 days.

Clinical Signs

Animals with NWS have maggots moving in a wound, ears, nose, eyes or mouth. Affected animals may be agitated, lethargic or distressed. A foul-smelling odor often comes from the infested site, and open wounds may actively bleed.

As NWS infests the wound, it causes additional damage, attracting more flies to lay more eggs and causing even more damage. This can result in very large areas of tissue destruction. Untreated cases of NWS are often fatal.

Diagnosis

NWS is diagnosed by identifying characteristic lesions, including larvae damaging living tissue. White egg masses may also be found near the nose, eyes, ears, genitalia or umbilicus of newborn animals. Not all maggot infestations in tissue are caused by NWS, and NWS maggots and adult flies look similar to other species of botflies and blowflies. Diagnosis must be performed by a professional trained on NWS identification.

Reporting

NWS is a reportable disease. If you suspect that a wild animal in Michigan has signs of screwworm infestation, report it to the DNR.

Report screwworm online

  • Or call the DNR Wildlife Disease Laboratory at 517-336-5030.
  • Or call the main Wildlife Division switchboard at 517-284-9453.

For suspected infestation of domestic animals, report it immediately to the Michigan Department of Agriculture and Rural Development (MDARD):

  • 800-292-3939 (daytime)
  • 517-373-0440 (after-hours)

Additional resources can be found at Michigan.gov/Screwworm.

For suspect infestation of a human case, please call the Michigan Department of Health and Human Services at 517-335-8165.

Treatment and Control

New World screwworm is eradicated using a sterile insect technique, where NWS flies are captively propagated and then sterilized with radiation. These sterilized flies are released to mate with wild flies. Because female NWS only mate once, they are unable to produce fertile eggs after mating with sterile males. Release of hundreds of millions of sterile flies results in attrition of NWS populations and eradication of NWS over time. This treatment has been applied in Texas in 2026.

In addition, massive surveillance and awareness campaigns are being conducted to rapidly identify NWS in domestic animals, wildlife and people. Transport and movement restrictions are in place to avoid transporting NWS to other locations.

Domestic animals diagnosed with screwworm are quarantined and treated until the infestation is resolved. The US Food and Drug Administration has issued emergency use authorizations for several drugs to support the treatment of NWS. Affected producers and owners should work with their veterinarians to determine the appropriate treatment. There are a few treatment options for temporarily captive wildlife, such as wildlife in rehabilitation, available from USDA.

Significance

NWS poses a serious threat to domestic animals and wildlife as it causes significant tissue damage and can lead to death. Historically in Texas, NWS caused serious issues in white-tailed deer populations.

Although NWS can be detrimental to animal health, it is not a food safety issue. NWS is not transmitted directly from animal to animal or from animals to people.

Resources