What cattle producers need to know about New World Screwworm

New World Screwworm (NWS) is a devastating pest of livestock and other mammals. Screwworms are fly larvae, or maggots, that burrow into the flesh of living animals causing serious, and even deadly, damage to the animal. NWS can infest livestock, pets, wildlife, occasionally birds, and in rare cases, people.¹

Identifying New World Screwworm

Adult screwworm flies have orange eyes, a metallic blue or green body and three dark stripes along their backs. The center stripe begins partway down the backside and appears shorter than the outer stripes.²

Screwworm larvae burrow into a wound, feeding as they go like a screw driving into wood. The maggots cause extensive damage by tearing at the hosts’ tissue with sharp mouth hooks. The wound becomes deeper and larger as more maggots hatch and feed on living tissue.²

Producers should monitor livestock and other warm-blooded animals regularly for signs of NWS including irritated behavior, such as head shaking, smell of decay and presence of maggots in a wound.² Egg masses may be found around or in the wound and larvae may be visible by the third day of infestation.³ NWS infestations are painful for the host, so animals may become depressed, stop eating and separate themselves.³

Economic impact of New World Screwworm

Although the United States Department of Agriculture (USDA) eradicated NWS from the United States in 1966 using sterile insect technique, there is a constant risk of re-introduction into the United States.¹ During the 20th century, the presence of NWS cost the U.S. livestock industry more than $100 million annually.⁵ Another incursion into the United States could cost millions of dollars from livestock losses, trade embargoes and eradication work.³ Pets, livestock, wildlife and even humans may suffer and die from screwworm myiasis.³

Climate impact on New World Screwworm

NWS is not tolerant of cold weather. Adults do not survive air temperatures below 20° Fahrenheit and larvae dropping from the host between 15° and 20° Fahrenheit usually die. Pupation, when larvae transform to an adult fly, does not occur in dry soil when the mean average daily temperature is below 46° Fahrenheit.⁴

Historically, year-round populations were restricted to the lower quarter of Texas, southern New Mexico, Arizona, California and parts of Florida. In areas where temperatures were too cold to support fly development over winter, annual expansions occurred every spring and summer from areas where NWS presence remained yearround.⁴

If there is an infestation in your area

In areas where NWS is found, measures should be implemented to prevent animal wounds and avoid NWS myiasis.⁶ For example, to the extent possible, eliminate or delay performing wounding procedures such as dehorning, branding, shearing, ear notching, tail docking and castration.⁶

Untreated umbilical cords of newborn animals and foot lesions are commonly infested sites. Immediately treat all wounds with approved insecticides; it may also be prudent to follow up with precautionary spraying of animals with insecticide before transport.⁶

Information for veterinarians

Currently, there are no animal drugs approved by the U.S. Food and Drug Administration (FDA) for the treatment or prevention of NWS myiasis in animals.⁷ Veterinarians may prescribe the use of FDA-approved products for uses that are not on the label, in accordance with the existing regulations (21 CFR Part 530) with a valid veterinary/client/patient relationship. The FDA’s Center for Veterinary Medicine provides information to support veterinarians with the identification of FDA-approved animal drugs labeled for indications other than NWS myiasis that scientific literature indicates may be effective to prevent or treat NWS myiasis.⁷ Visit the FDA website for a list of options.

Immediately report signs to your State Animal Health Official (www.usaha.org/saho/) or USDA Area Veterinarian in Charge (www.aphis.usda.gov/contact/animal-health).

For more information, visit BQA.org/Screwworm

Sources:

1. https://www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm
2. https://www.aphis.usda.gov/sites/default/files/pest-alert-new-world-screwworm.pdf
3. https://www.aphis.usda.gov/sites/default/files/bro-new-world-screwworm.pdf
4. https://entomology.k-state.edu/extension/human-and-animalhealth/New%20World%20Screwworms_June2025.pdf
5. Novy, J.E. 1991. Screwworm control and eradication in the Southern United States of America. Special Issue of World Animal Review FAO, pp. 18–27. https://www.fao.org/4/u4220t/u4220T0a.htm. Accessed 4/22/25.
6. https://www.aphis.usda.gov/sites/default/files/nws_myiasis_disease_strategy.pdf
7. https://www.fda.gov/animal-veterinary/safety-health/new-world-screwworm-information-veterinarians