New World screwworm

New World screwworm (NWS, Cochliomyia hominivorax) is a devastating pest. When NWS fly larvae (maggots) burrow into the flesh of a living animal, they cause serious, often deadly damage. NWS can infest livestock, pets, wildlife, occasionally birds, and in rare cases, people.

Photos Courtesy: USDA

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

Report Signs of Animal Disease:

Find an accredited veterinarian
State Veterinarian’s Office: 615-837-5120

How To Prevent This Disease

The best way to keep screwworms out of the United States is to prevent their introduction. NWS can infest warmblooded 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 in the United States, APHIS will recommend additional measures to prevent 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.