Frequently Asked Questions, Courtesy of the Beef Checkoff
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 can affect all mammals, including humans and household pets, and rarely birds.3
NWS is most common in livestock.3
This pest can infest a wide variety of wounds, from tick bites to cuts and dehorning or branding wounds.3
Infestations are very common in the navels of newborn animals and the genital regions of their mothers.3
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
All wounds in animals within a control area should be treated with insecticide every 2 to 3 days until healed to prevent female NWS flies from laying eggs and to kill any eggs or larvae already present.3
All detected myiases must be treated. The recommended protocol for treatment is as follows:3
Clean the wound and surrounding tissue with warm water and apply a mild antiseptic.
If myiasis is found, physically remove as many larvae as possible.
Apply an effective topical treatment to kill any remaining larvae.
Regularly treat affected animals with longer-lasting systemic insecticide.
In areas where NWS is found, measures should be implemented to prevent animal wounds and avoid NWS myiasis.8
For example, to the extent possible, eliminate or delay performing wounding procedures such as dehorning, branding, shearing, ear notching, tail docking and castration.8
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.8
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
An eradication program to remove NWS from the United States began in 1957.4
It used a biological control technique (sterilized insects) developed by USDA’s Agricultural Research Service.4
This method is an ecologically safe and proven way to eradicate NWS fly populations by taking advantage of the fly’s own biology.4
The sterile-insect approach eradicated NWS from the United States in 1966.4
In a cooperative program, the Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG) maintains a permanent sterile fly barrier along the border of Panama and Colombia to prevent the re-establishment of screwworms.4
NWS is endemic in Cuba, Haiti, the Dominican Republic and countries in South America, with cases spreading north to Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador and Mexico.1
Although 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
Since 2006, the United States and Panama have maintained a barrier zone in eastern Panama.3
This barrier zone prevents NWS from moving north from South America to screwworm-free areas in Central and North America.3
In 2023, APHIS confirmed an unprecedented number of NWS cases in Panama.3
Since then, cases have been detected in every Central American country and Mexico.3
NWS can threaten the livelihood of livestock producers.3
It can cause millions of dollars’ worth of production losses and economic damage.3
Screwworm also pose a threat to humans in infested areas.3
New World Screwworm, Cochliomyia hominivorax, is found in the Western Hemisphere, primarily in tropical areas of South America and some Caribbean Islands.6
Old World screwworm, Chrysomya bezziana, is found in warmer areas in the Eastern Hemisphere (e.g. Southeast Asia, South Asia, Africa, Persian Gulf and Middle East).6
According to the USDA NWS Disease Response Strategy, movement controls will be a primary strategy of NWS response efforts.8
USDA may restrict interstate commerce from the infested states, asking the states (or adjoining countries) to provide resources to maintain and enforce movement controls.8
State movement controls may be placed on individual infested animals or premises with infested animals.8
Of most importance is the control of livestock movement within and out of an infested area, using a system that requires inspection for wounds and myiasis. This may include permit requests for permitted movement.8
All decisions in regard to movement control will be based on science-based assessments of the current extent of NWS infestation, risk of spread and the interaction of other factors, such as seasonal climate and weather conditions.8
NWS myiasis is not a food safety concern.9
Animals used for human food production must pass inspection before and after slaughter 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 is held for testing to ensure there are no medication residues present.10
USDA FSIS will remove any animal from food production if residues are found.10
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.