New World Screwworm

New World Screwworm

Cause

New World Screwworm (NWS) (Cochliomyia hominivorax) is an insect pest in which the fly larvae (maggots) burrow into the flesh of living animals. Eggs are laid in open wounds before they develop into larvae. The larvae’s feeding behavior, and how the fly received its name, is to ‘screw’ into live flesh, feeding on the live tissue as it burrows with its sharp mouth hooks. The result is serious, often deadly, damage to the individual.

The flies are similar in size to the common housefly but with a different appearance. The eyes of the screwworm fly are orange, with a metallic blue or green body and three dark stripes along the back.

Species Affected

Several species of pets, livestock, and wildlife are susceptible to NWS maggot infestation. People are also susceptible to this pest if eggs are laid in wounds that are not cared for in a timely manner.

Clinical Signs

The result of infestation with New World Screwworm larvae is myiasis. In wildlife, including mammals and birds, this can cause the following symptoms and behaviors:

• Irritated behavior
• Head shaking
• The smell of decay
• Evidence of fly strike
• Presence of observable maggots in wounds of a live animal

Detection

Fly larvae must be identified by an accredited laboratory. Any potential detection of the fly or its larvae in animals or people needs to be promptly reported. 
Report maggots or flies that fit the above description in live wildlife, dead animals, or harvested game. Reports can be made to USDA Wildlife Services by calling 334-844-5670.

Report maggots or flies in livestock or domestic animals to APHIS Veterinary Services by emailing brandy.childress@usda.gov or calling 615-330-8179. For questions, contact the Alabama Department of Agriculture and Industries by calling their office at 334-240-7253.

If you are concerned that you are experiencing NWS myiasis, seek medical attention from your physician. For additional information, contact your local county health department.

Treatment

The only way to eradicate NWS is to release sterile male flies into the area. These sterile flies will then breed with reproductive female flies to reduce the population over time.

Transmission/Movement

The introduction of flies into new areas can happen through the movement of pets, livestock, or wildlife from an NWS-infested area. Introductions could also occur by the accidental movement of larvae or flies on travelers coming back from NWS-endemic areas. For more information on how to prevent the accidental introduction of NWS flies or their larvae and any current movement restrictions, visit www.screwworm.gov.

Management Implications

NWS started to be observed in the southwestern portion of the United States in 1842. However, it began to spread and become a problem in the Southeast starting in 1933. Producer losses were estimated to be $10-20 million annually. Wildlife, including white-tailed deer, also experienced increased mortality (up to 40-50%). For deer and other mammals, juveniles are particularly vulnerable to larvae infestation through the wound created by umbilical cord attachment, however, all ages are susceptible. Through intense management efforts involving sterile fly releases and development of effective insecticides in the 1950s and 1960s, eradication was declared successful in 1966. The result was the establishment of a NWS barrier zone that limited the fly to south of the Panama Canal (USDA APHIS 2025).

Despite the maintenance of this barrier zone, there have been localized outbreaks, having economic impacts on US producers. For example, a Texas outbreak in 1976 cost the Texas economy an estimated $1.8 billion before the outbreak was controlled (USDA APHIS 2025). Additionally, another outbreak in the Florida Keys in 2016 had significant impacts on wildlife, specifically Key deer, an endangered subspecies of the white-tailed deer. Roughly 15% of the Key deer population died before the outbreak was contained (MyFWC.com).

A New World Screwworm (NWS) lesion on a Key deer in Florida during the 2017 outbreak. Photo by FWC-FWRI-Fish and Wildlife Health

Distribution

NWS is endemic in Cuba, Haiti, the Dominican Republic, and several countries in South America. Since 2023, NWS cases have surged dramatically in Central America where there has been a long-standing biological barrier in eastern Panama’s Darien Province. The number of cases in Mexico has continued to climb as the outbreak has spread northward. The current distribution and status of NWS in the US, Mexico, and Central America can be viewed at www.screwworm.gov.

Resources

• Alabama Department of Agriculture & Industries: https://agi.alabama.gov/animalindustries/new-world-screwworm/
• Alabama Public Health: https://www.alabamapublichealth.gov/infectiousdiseases/zoonoses.html
• U.S. Department of Agriculture Animal and Plant Health Inspection Service (USDA APHIS). New World Screwworm Ready Reference Guide – Historical Economic Impact.
• January 2025. National Preparedness and Incident Coordination (NPIC)
• Florida Fish and Wildlife Conservation Commission. https://myfwc.com/research/wildlife/health/nws/#report
• The Centers for Disease Control and Prevention: https://www.cdc.gov/new-world-screwworm/about/index.html