About NWS

What is the New World Screwworm?

New World Screwworm (NWS) is a dangerous parasite that affects all warm-blooded hosts, including livestock, pets, wildlife, and humans. The screwworm fly lays eggs in open wounds, and the hatching larvae burrow into living tissue, causing severe damage and potentially death if left untreated.

New World Screwworm flies (Cochliomyia hominivorax) cause a condition called myiasis – a parasitic infestation where larvae feed on the living tissue of warm-blooded animals.

Screwworm flies lay eggs in:

Open wounds (cuts, scratches, surgical sites)
Natural body openings (eyes, nose, ears)
Soft tissue areas (naval, genital areas)

The eggs hatch into larvae that burrow or “screw” into flesh using sharp mouth hooks, creating larger wounds that attract more flies to lay additional eggs.

How is NWS Managed & Treated?

Eradicating NWS is only possible through the sterile insect technique. With this method, sterile male flies are released to mate with female NWS flies in areas of known populations, which then lay unfertilized eggs.

Animals infested with NWS should be treated according to the recommendations of their veterinarian.

When pesticides for treatment are approved by USDA, a complete list will be available here.

The best way to protect yourself and animals from NWS is surveillance, monitoring, and promptly reporting suspicions.

How New World Screwworm Spreads

The Average Life Cyle of the NWS is 10-30 Days

Adult flies mate

Females mate after three days and males mate within 24 hours. The lifespan of the adult NWS fly is about 10-30 days.

Female flies lay eggs

NWS eggs are laid on the edge of an open wound or exposed area of soft tissue. Adult female NWS flies can lay 200-300 eggs at a time. Eggs are white, oval shaped, and usually hatch within 10-12 hours.

Larvae feed on living tissue

After the eggs hatch, the larvae burrow into the wound and feed on living tissue. The larvae may be visible after the third day of infestation and may continue to burrow deeper into a wound. After about a week, the more mature larvae fall off and burrow into the ground to pupate.

Pupation occurs in the soil

Mature NWS larvae drop from their host, burrow into soil, and develop into a pupa inside darkening red-brown skin. Depending upon the weather, pupae mature for 7-54 days. In warmer temperatures (~82°F) pupae can develop in as little as seven days. In cooler temperatures (50°-59°F) pupae can take much longer to develop. When soil temperatures are constantly below 46°F it can be lethal to the pupae.

Adult flies travel to new host

Once pupae mature, they emerge from the soil as adult flies. Adult NWS flies can fly as far as 12 miles in search of a suitable host. This is how the life cycle continues and spread occurs.

The movement of infested livestock or wildlife can lead to the spread in even greater distances.

Strong winds and weather patterns can also increase the distance NWS flies travel.

Eradicated in 1966

History of NWS

1966

The parasite was eradicated from the U.S. in 1966 utilizing an innovative sterile fly program developed by USDA. This groundbreaking method involved releasing sterile male flies to break the reproductive cycle, leading to the fly’s elimination across the Southern U.S. and much of Central America. Since female NWS flies only reproduce once, hundreds of millions of larvae were sterilized with radiation at production facilities and released as adults from aircraft. Eradication efforts have continued in Central America, but the pest is considered endemic in Cuba, Haiti, the Dominican Republic, and South America. Sterile flies are the only proven solution to eliminate NWS populations.

2006

Since 2006, the United States and Panama have maintained a barrier zone in eastern Panama. This barrier zone prevents NWS from moving north from South America to screwworm-free areas in Central and North America.

2016

In 2016, there was an outbreak of NWS in the Key deer population in the Florida Keys that never reached the mainland. Cases were controlled and no new reports of NWS emerged after 2017.

2023

In 2023, APHIS confirmed an unprecedented number of NWS cases in Panama. Since then, cases have been detected in every Central American country and Mexico.

2025

In June 2025, U.S. Secretary of Agriculture Brooke Rollins announced Moore Airfield Base in South Texas will serve as the site of a new sterile fly dispersal facility. This facility is designed to provide urgent and necessary tools to help protect our livestock and wildlife.

Report Potential Cases

Regions affected by NWS are encouraged to monitor for signs and immediately report potential cases.