New World Screwworm

What is New World Screwworm?

• New World Screwworm (NWS) is a devastating pest of livestock and other mammals. Screwworms are a fly larvae, or maggots, that burrow into the flesh of living animals causing serious, and even deadly, damage to the animal.¹
• An infestation occurs when screwworm flies are attracted to and lay eggs in an open wound.²

• NWS myiasis is not a food safety concern. ⁹
• Animals used for human food production must pass inspection before and after harvest to ensure food safety requirements are met.¹⁰
• Detection of any illness that results in meat products being unsafe for consumers will prevent the animal from entering the food supply.¹⁰
• Any animal pharmaceutically treated for NWS will be held for testing to ensure there are no medication residues present.¹⁰
• The U.S. Department of Agriculture (USDA) Food Safety Inspection Service will remove any animal from food production if residues are found.¹⁰ 

• Screwworm infestations begin when a female fly lays eggs on a wound or orifice of a live warm-blooded animal.⁴
• Eggs hatch into larvae that burrow into the wound to feed on the living flesh.⁴
• After about 7 days of feeding, larvae drop to the ground, burrow into the soil and pupate.⁴
• The pupa is a transitional stage from larva to adult fly.⁴

• The adult screwworm fly emerges from the soil after 7–54 days depending on temperature and humidity.⁴
• Female flies mate after 3 days, and males can mate within 24 hours of maturation.⁴

• One female can lay up to 3,000 eggs in her lifespan.⁴
• Female flies are attracted to the odor of a wound or opening such as the nasal or eye openings, umbilicus of a newborn, or genitalia.⁴
• Wounds as small as a tick bite may attract a female to feed.⁴

• Look for the following signs in warm-blooded animals (including pets and birds):³
• Irritated behavior
• Head shaking
• The smell of decay
• Presence of maggots in a wound
• Egg masses may be around or in the wound; larvae may be visible by the third day of infestation.⁴
• Because they feed on live flesh, NWS maggots may burrow deep into wounds or openings, while other species of maggots may appear around the outer surface of the wound.⁴
• Screwworm infestations are very painful. Animals may become depressed, stop eating and separate themselves from other animals or people.⁴

• Immediately report any suspicious wounds, maggots or infestations to a local accredited veterinarian, your State Animal Health Official or USDA (www.aphis.usda.gov/contact/animal-health).^3,4

• Treatment includes cleaning of the wound, removal of the larvae, and application of larvicides and measures to prevent secondary infections. Veterinary care is essential to avoid complications.⁵
• In areas where NWS is found, measures should be implemented to prevent animal wounds and avoid NWS myiasis.⁷
• For example, to the extent possible, eliminate or delay performing wounding procedures such as dehorning, branding, shearing, ear notching, tail docking and castration.⁷
• 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.⁷

• An adult screwworm fly can travel up to 125 miles before laying eggs in a wound.⁶
• Screwworms can also be transported by animals and people travelling from infested areas.⁶

• NWS can threaten the livelihood of livestock producers. It can cause millions of dollars’ worth of production losses and economic damage.³
• During the 20th century, the presence of NWS cost the U.S. livestock industry more than $100 million annually.⁴
• Another incursion into the United States could cost millions of dollars from livestock losses, trade embargoes and eradication work.⁴
• Pets, livestock, wildlife and even humans may suffer and die from screwworm myiasis.⁴