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New World Screwworm

New World screwworm (NWS) is a serious pest of livestock and wildlife. USDA is leading a coordinated One Health response to combat NWS. This is not a food safety issue—the U.S. food supply remains safe.


While the current risk to animals and people in Wyoming is very low, it's important to keep an eye out for this pest.

New World screwworm (NWS), or Cochliomyia hominivorax, is species of parasitic fly that completes part of its lifecycle by feeding on the tissue or flesh of warm-blooded animals and people. NWS flies are attracted to wounds and body openings like the nose, eyes, ears, and mouth, where they lay eggs. The eggs hatch into maggots (larvae) that eat live tissue, causing a worsening, often painful and foul-smelling wound.


NWS is typically found in South America and parts of the Caribbean. However, the fly has steadily moved northward from South America through every country in Central America and Mexico since 2023.


USDA has put together a comprehensive website with information about the current status, latest news, what people need to know, information about the response, and a way to report suspected cases.


CLICK HERE TO VISIT SCREWWORM.GOV


new world screwworm fly

What to Look For


The name screwworm refers to the maggots' feeding behavior as they burrow (screw) into the wound, feeding as they go like a screw being driven into wood. Maggots cause extensive damage by tearing at the hosts’ tissue with sharp mouth hooks. The wound can become larger and deepen as more maggots hatch and feed on living tissue. As a result, NWS can cause serious, often deadly damage to the animal.


Adult screwworm flies are about the size of a common housefly (or slightly larger). They have orange eyes, a metallic blue or green body, and three dark stripes along their backs.


Report mammals and birds with the following signs:

  • Irritated behavior
  • Head shaking
  • The smell of decay
  • Presence of fly larvae (maggots) in wounds


How to prevent this pest

Protecting the United States From Screwworm


The best way to keep screwworms out of the United States is to prevent their introduction. NWS can infest warm blooded animals, including livestock, pets, wildlife, people, and even birds.


Some steps you can take to prevent infestation:


  • Watch for signs of NWS in pets and livestock.
  • Make sure pets traveling internationally are inspected for screwworm.
  • Handle livestock carefully and inspect pens and equipment for sharp objects that can cause wounds.
  • Treat the umbilical cords of newborn animals and all wounds immediately with an approved insecticide.
  • Protect pets and livestock from other wound-causing parasites such as ticks.


In the event of an NWS detection, APHIS recommends additional measures to prevent the spread including:


  • Inspect your vehicle for screwworm flies when leaving an NWS-infested area.
  • Postpone or avoid procedures that create wounds such as dehorning, branding, shearing, ear notching, tail docking, and castration if you are in an NWS-infested area.


If you think you have found a screwworm, report it immediately to your State animal health official and APHIS office. This will allow APHIS and partner agencies to respond quickly and remove the screwworms before a population becomes established.


How It Is Treated


Eradicating NWS is only possible through sterile insect technique (237.54 KB). With this method, sterile male flies are released into an area where a known population has become established. The sterile male flies mate with wild female NWS flies, which then lay unfertilized eggs. Because female NWS flies mate just once in their lifespan, the NWS population progressively reduces and ultimately dies out.


Animals infested with NWS should be treated according to their veterinarian's recommendations.


The U.S. Food and Drug Administration (FDA) provides information to support veterinarians with the identification of FDA-regulated products to prevent and/or treat NWS myiasis. This includes FDA-approved animal drugs labeled for indications other than NWS myasis that may be effective in preventing or treating NWS myasis. 


There are pesticides to potentially use against NWS. View list.


People who suspect they are infested with NWS should seek immediate medical treatment following the U.S. Centers for Disease Control and Prevention (CDC) guidelines. For information on how screwworms affect people or to report human disease, visit CDC at About New World Screwworm Myiasis  and Clinical Overview of New World Screwworm Myiasis.