What’s the Deal with Screwworms?

Last week, U.S. Secretary of Agriculture Brooke Rollins announced the suspension of imports of live cattle, horses, and bison across the U.S. southern border due to the rapid northward spread of New World Screwworm (NWS) in Mexico. Producers are understandably concerned and are asking how worried they should be.

How is the Screwworm different than other fly maggots?

The NWS differs from other fly species because its larvae can burrow or “screw” themselves into the healthy tissue of any warm-blooded animal. During its roughly 20-day lifecycle, an adult female fly can lay up to 3,000 eggs into tiny, inconsequential scratches or even healthy mucus membranes, such as those of the eye or vulva. Recent surgical sites for castration or dehorning and newborn calves’ navels can also be infested. As these maggots migrate through otherwise healthy tissue, they can cause pain, production losses, and even death.

What does Screwworm infestation look like?

The most obvious sign is a wound that is not healing properly, but is rather getting larger and deeper. The wound may be accompanied by the smell of rotting flesh and a bloody discharge. Maggots may be visually evident.

How is Screwworm infestation treated?

In domesticated animals, treatment usually involves veterinary procedures to physically remove the larvae. Topical or systemic parasiticides can be useful, but may be associated with meat or milk residue issues. Treatment and labor costs in addition to production loses are reasons that it’s far preferable to eliminate Screwworm flies from the environment than to treat infested animals.

Where do Screwworms come from?

The fly species is endemic in most of South America, Cuba, Haiti, and some Caribbean islands. Until recently, through a multinational collaborative control program, Screwworms were stopped from northward migration at the narrowest piece of land between South and Central America, just a south of the Panama Canal.

What does the international control program involve?

Using science developed in the 1950’s and 1960’s, Screwworm flies are cultured in a Panamanian laboratory and then irradiated, making them sterile. When an infested area is flooded with millions of sterile flies, sterile males mate with native females. Female Screwworm flies mate only once and so that generation’s flies die out.

Using this technique, Screwworms were eradicated in 1966 in the U.S. The same technology was used to push Screwworms out of Mexico and Central American all the way down to Panama where a “biological barrier” has been successful in stopping northward spread of the pest from South America for decades.

What changed recently?

In 2023, NWS detections in Panama exploded from an average of 25 cases per year to more than 6,500 cases a year. Since then, Screwworms have been detected in Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador, and now southern Mexico.

What are we most worried about?

The greatest concern is that Screwworm flies will cross the Mexico border and the species will re-establish itself in the U.S. Current reports are that Screwworm flies have been identified some 700 miles from the U.S. border. An adult Screwworm fly is reported to be capable of traveling up to 120 miles during her lifetime, making Mexico’s current identifications important. Screwworms can also be transported by animals and people travelling from infested areas.

What’s the government doing to stop the spread?

USDA has implemented a ban on horses, cattle, and bison imports from Mexico. The ban will persist on a month-by-month basis, until significant containment in Mexico is achieved. USDA is continuing its daily communication and collaboration with Mexican animal health officials, including a review of the latest data and metrics every two weeks. In addition, a bipartisan group of U.S. lawmakers has introduced legislation aimed at stopping the spread of the New World Screwworm. The bill would authorize federal funding for USDA to build a new sterile fly production facility, increasing the number of sterile males available for release in Mexico, Central America, and the U.S. if needed.

Is there any good news at all about this?

Thus far there have been no detection of Screwworm flies in the U.S. It’s also reassuring to remember that even if Screwworm flies do cross the border, we have effective means to stop the spread. In 2016, Screwworm infestations were detected in wild deer and pets in the Florida Keys. Through the use of sterile fly release, the pest was eradicated by 2017.

What should producers be doing to protect their herds?

For now, livestock farmers’ risk is minimal. If the threat to U.S. livestock becomes more imminent, producers will want to consult with their herd veterinarian and local animal health officials about additional precautions. Within their own herds, producers should report unusual looking wounds or maggots on livestock to their herd vet or local CDFA office. Producers should not treat a maggot-infested wound without consulting with their herd vet first.

Producers may also wish to discuss their routine fly management with their veterinarian. While good fly control is important for a variety of animal health and economic reasons, there are differences between the NWS and typical dairy fly biology and certain treatments may be more effective than others. Most importantly, producers should pay attention to CDFA alerts and guidance specific for Screwworms.