New World screwworm (NWS)

A rapidly changing geographic distribution

Once widespread throughout the Americas, NWS was eradicated from the U.S., Mexico, and most of Central America between 1966 and 2006 through sterile insect release programs (Sterile Insect Technique) and other coordinated control efforts.

Today, the parasite remains endemic in parts of the Caribbean and South America. To prevent NWS from moving back northward, the U.S. Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) has collaborated with Panama to maintain a biological barrier zone in eastern Panama. Unfortunately, that barrier was breached in 2023, with cases of NWS now confirmed in every Central American country and Mexico, heightening the risk of reintroduction to the U.S.

Veterinarians are frontline defenders in recognizing NWS and responding to NWS cases.

Adult flies are similar in size to common house flies or may be slightly larger. They can be distinguished by these characteristics:

• Orange eyes
• Metallic blue or green body
• Three dark stripes along their backs

Signs of NWS myiasis include:

• Larvae visible in wounds or orifices (e.g., nose, ears, genitalia, umbilicus)
• Egg masses in or around wounds
• Wounds with bloody or purulent discharge and an odor of decay
• Wounds that enlarge or deepen over time
• Behavioral signs of irritation, like head shaking or rubbing against trees or other structures
• Signs of pain, like depression, irritability, inappetence, or isolating from other animals or people

By three days after initial infestation, hundreds or thousands of larvae may be present. They typically are positioned head-down, with their posterior ends visible at the surface of the wound. In deep or narrow wounds, subtle movement may be the only visible sign.

Should NWS breach the Mexico-U.S. border, veterinarians will play a critical role in mobilizing a response.

If you suspect NWS, follow USDA-APHIS standard operating procedures for animals. Similar guidance is available for dogs specifically.

• Immediately contact your state animal health official and USDA-APHIS area veterinarian in charge. The after-hours reporting line is (866) 536-7593.
• If directed to do so:
  • Collect larvae from several sites within the wound—including the deepest part—using forceps and disposable gloves.
  • Place the larvae in a tight-closing vial/test tube containing 70% alcohol.
  • Submit samples to the National Veterinary Services Laboratories with a completed Parasite Submission Form.
• Quarantine the animal(s) to prevent larvae from spreading.

Note that affected animals must be held in quarantine under the supervision of a USDA-accredited veterinarian and/or an official (federal or state) veterinarian until parasite identification has been officially confirmed, effective treatment has been implemented, and the official veterinarian has confirmed the animal(s) to be free of screwworm. APHIS and state animal health officials will determine appropriate quarantine premises, depending on location, overall risk, and availability of suitable facilities with effective vector control.

If NWS is confirmed, the affected animal(s) and their environment must be treated in accordance with standard operating procedures, under the supervision of the official veterinarian. Veterinarians are urged to wear disposable gloves during these procedures to protect themselves.

Animal treatment

• Remove all visible eggs and larvae in and around the wound.
• Provide wound care, including analgesia, as appropriate.
• Use antiseptics to prevent secondary infections.
• Administer approved topical or systemic antiparasitics with demonstrated effectiveness in eliminating screwworm larvae from the affected species.
• Reexamine the animal(s) 24 hours after treatment to confirm no live larvae remain in the wound. If live larvae do remain, remove them and retreat.

Environmental decontamination

• Collect any larvae expelled into the environment and safely discard them by collecting them into tightly sealed tubes with alcohol (to kill the larvae) and placing the tubes in a Ziploc bag before throwing in the trash.
• Spray surfaces of holding/quarantine areas with approved larvicidal insecticide.
• Wash sprayed surfaces with warm soap and water to prevent animal exposure to insecticide residue.
• Monitor the premises for screwworm flies at least weekly for a full month after the quarantined animal(s) have been released.

Stay alert, stay prepared

Veterinarians play an essential role in early detection and rapid response. With NWS cases on the rise in nearby regions, vigilance—especially in southern states—and prompt action are key to protecting animal health and welfare and helping prevent reestablishment in the U.S.