Invasive Fly Threatens Texas Livestock: What You Need to Know About the New World Screwworm

The Texas Parks and Wildlife Department (TPWD) is urging South Texas residents to stay vigilant for signs of New World Screwworm (NWS), an invasive fly whose parasitic larvae burrow into living animals. While no confirmed cases of NWS have been identified in Texas, the pest was recently detected in a cow at a checkpoint in southern Mexico, near Guatemala, raising concerns about its potential spread.

NWS, scientifically known as Cochliomyia hominivorax, has been expanding through Central America in recent years, with more than 6,500 cases reported in Panama in 2023 alone—well above the usual yearly average of just 25 cases. This surge in cases follows the fly's northward migration through Panama, Costa Rica, Nicaragua, Honduras, and Guatemala, and has prompted significant action from both the USDA and local authorities to prevent its spread to the U.S.

What is New World Screwworm?

New World Screwworm larvae are parasitic maggots that burrow into the flesh of warm-blooded animals, including livestock, pets, wildlife, and in rare cases, humans. These larvae feed on the flesh, causing severe tissue damage. Female NWS flies can lay up to 300 eggs at a time, and in their lifetime, a single fly can lay as many as 3,000 eggs. The larvae are notorious for their ability to cause devastating wounds, with clinical signs including:

• Irritated or depressed behavior

• Loss of appetite

• Head shaking

• Smell of decaying flesh

• Presence of maggots in wounds

• Isolation from other animals or people

Efforts to Combat the Spread of NWS

NWS has not been identified in the U.S. since its eradication in 1966 through coordinated federal and state efforts. However, the rise in Central American cases has renewed concerns about its potential return to the U.S. The USDA has already invested over $109 million to work with Mexico and Central American countries on preventive measures, including the use of sterile insect techniques to control fly populations. This involves breeding sterile pupae to prevent the flies from reproducing.

The USDA has also partnered with Panama's Ministry of Agriculture Development to operate North America's only NWS pupae sterilization facility. This facility plays a critical role in halting the spread of the invasive fly.

How NWS Spreads

Once the larvae infest an animal, they eventually fall to the ground and burrow into the soil, where they pupate before emerging as adult flies. These adult flies have distinctive orange eyes and a metallic blue or green body. They are capable of flying long distances, spreading the maggots to new areas as they travel.

Livestock, wildlife, and even pets are at risk from NWS. Although rare, humans can also be affected if they come into contact with the larvae-infested wounds of an animal.

What You Can Do

Both TPWD and the USDA are calling on the public to report any signs of NWS to local and state officials. If you observe unusual wounds on animals or notice the presence of maggots, please contact the Texas Animal Health Commission or the USDA immediately. Quick reporting can help prevent a larger outbreak and protect both animal and human health.

As Texas remains on high alert, authorities continue to monitor the situation closely and work with international partners to keep the invasive fly at bay. If you live in South Texas, stay informed and report any suspected NWS cases to help safeguard the U.S. from this serious agricultural threat.