The New World screwworm (NWS, Cochliomyia hominivorax) is a parasitic fly native to the Western Hemisphere. It lays eggs in the living tissue of fresh wounds in warm-blooded animals. The larvae (maggots) feed on the host’s flesh, causing severe wounds and often death if untreated.
The pest was eradicated in the U.S. in the 1960s. Since then, it occasionally reemerges and has recently resurfaced in Central America and Mexico. As of May 2025, renewed attention to this parasite is crucial, as it may pose future risks to U.S. livestock and wildlife.
New World Screwworm is controlled through the release of sterile males, known as the sterile insect technique (SIT). This approach, along with regular active surveillance and livestock inspections, has proven highly successful.
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, see NWS Pest Card.
Report mammals and birds with the following signs:Bloody or light-colored drainage from a cut or wound.
A cut or wound that rapidly enlarges for seemingly no reason.
White or cream-colored runny substance (the eggs) in and around a wound.
Presence of fly larvae (maggots) in wounds.
Irritated behavior and signs of pain.
Head shaking.
Reduced appetite.
Fever and other signs of secondary infection.
Foul odor or smell of decay.
How to Prevent This Disease
The best way to keep screwworm out of this country is to prevent introductions. NWS can infest mammals, including people, and birds. Here are steps you can take to avoid infestation:Be alert for NWS symptoms in pets and livestock.
Ensure that pets traveling internationally are inspected for screwworm.
Check your vehicle for screwworm flies if you are in an NWS-infested area.
If you think you have found a screwworm, report it immediately to your local veterinarian or the Missouri Department of Agriculture Animal Health Division at (573) 751-3377. This will allow agencies to respond quickly and remove the screwworms before a population becomes established.
How It Is Treated
Animals infested with NWS should be treated according to their veterinarian's recommendations.
Additional details can be found on the Missouri Dept. of Agriculture HERE.
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