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USDA Takes Precautionary Action to Protect Against Northern Spread of New World Screwworm

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Sterile Fly Dispersal Area Shifted over Portions of South Texas

On January 30, 2026, the United States Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) announced a shift in New World screwworm (NWS) sterile fly dispersal efforts into Texas. To stop the northern spread of NWS, the new dispersal area, or polygon, will include operations about 50 miles into Texas, along the U.S. border with the state of Tamaulipas, Mexico. NWS have not been detected in Texas, but South Texans may see preventative sterile fly deployments in their area.

Sterile NWS flies in an area do not result in infestations and are a safe tool for reducing NWS egg production that may lead to infestations. While NWS have not been detected in the U.S. and the dispersal of sterile flies may not be noticed in day-to-day operations, targeted dispersal of sterile insects and other tools are critical components of an effective strategy to fight NWS.

To support a swift response if NWS reaches Texas, producers located on the southern border and travelers from NWS-affected areas should closely monitor animals for signs and promptly report suspected cases of NWS to the Texas Animal Health Commission (TAHC) or Texas Parks and Wildlife Department (TPWD).

The USDA-APHIS press release highlights details and important information on the current sterile fly dispersal, import requirements and protocols, and surveillance, monitoring, and reporting. According to the USDA, the sterile fly eradication technique is safe and poses no risk to wildlife, livestock, or people. General disease information and how to prevent the spread of NWS can be found through the TAHC websites and below.

WHAT IS NEW WORLD SCREWWORM?

New World screwworms (NWS) are larvae or maggots of the NWS fly (Cochliomyia hominivorax), that cause the painful condition NWS myiasis. NWS flies lay eggs in open wounds or orifices of live tissue. These eggs hatch into dangerous parasitic larvae, and the maggots burrow or screw into flesh with sharp mouth hooks. The wound can become larger, and an infestation can often cause serious, deadly damage. NWS primarily infest livestock and wildlife, but can also affect other mammals, including humans, and birds.

The parasite was last eradicated from the United States in 1966, through costly efforts by federal and state animal health officials, livestock producers, and veterinary practitioners. Eradication efforts have continued in Central America, but the pest is considered endemic in Cuba, Haiti, the Dominican Republic, and South America.

CLINICAL SIGNS

Producers should monitor animals closely and often for clinical signs associated with NWS and report suspicions to a veterinarian or the TAHC immediately. Clinical signs of NWS myiasis may include:

  • Irritated or depressed behavior
  • Loss of appetite
  • Head shaking
  • Smell of decaying flesh
  • Evidence of fly strike
  • Presence of fly larvae (maggots) in wounds
  • Isolation from other animals or people
REPORTING NEW WORLD SCREWWORM

Quick detection and reporting are essential to stopping the spread of NWS if they get to Texas. If you suspect NWS in your livestock or domestic animals, contact the TAHC immediately and do not move your animals. A representative will advise you on current collection protocols for submitting suspicious maggots. If you suspect wildlife with NWS infestations, contact your local Texas Parks and Wildlife Department (TPWD) wildlife biologist.

Producers, veterinarians, diagnostic laboratories, or anyone with suspicious livestock or domestic animals must make reports to a TAHC region officeor the 24-hour vet on-call line, 1-800-550-8242, within 24 hours of detection.

TRANSMISSION

NWS infestations begin when a female NWS fly is attracted by the odor of a wound or opening of a live warm-blooded animal to lay eggs. These openings can include wounds as small as a tick bite, nasal or eye openings, umbilicus of a newborn, or genitalia. One NWS female fly can lay 200-300 eggs at a time and may lay up to 3,000 eggs during her lifespan.

Eggs hatch into larvae (maggots) that burrow into a wound or other opening to feed. After feeding, larvae drop to the ground, burrow into the soil, and emerge as adult NWS flies. Adult NWS flies can fly long distances, and the movement of infested livestock or wildlife can lead to the spread of even longer distances.

DIAGNOSIS

There are several flies associated with wounds, but only NWS feed on living tissues, versus dead tissues and fluids. The identification of NWS is done by larvae collection and evaluation. NWS larvae have a series of backwardly protruding spines around a tapered body, giving a screw-like appearance, helping to identify the pest. Anyone who suspects suspicious wounds, maggots, or infestations should notify a veterinarian immediately.

PREVENTION

To avoid introduction of NWS, monitor pets and livestock closely for open wounds, and clean, treat, and cover any wounds immediately. Avoid creating unnecessary wounds on animals, establish an integrated pest management system to avoid insect bites, and discuss potential preventative products for livestock and pets with your private veterinarian. The TAHC has additional livestock producer guidance available.

TREATMENT AND ERADICATION

Animals exhibiting signs of myiasis, or NWS infestation, and any potential secondary infection should be immediately treated with an appropriate method by a veterinarian. Left untreated, animals may die within one week from complications associated with infestation.

Eradication of NWS infestations can be accomplished through the sterile insect technique. Sterilized male flies are released to mate in an area with an established NWS population. Females, only mating once in their lifetime, then lay nonviable eggs. The population decreases without the addition of new larvae and dies off naturally over a few lifecycles.

ADDITIONAL NEW WORLD SCREWWORM INFORMATION & RESOURCES
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