Protecting the Herd from New World Screwworm

By Kim Brackett

Every cattle producer knows pests are a challenge to their operations, but some pests are far more dangerous than others. 

One of the new threats to the nation’s herd comes from the New World screwworm (NWS), which is currently advancing through Central America into southern Mexico and could soon be at our border.

A nasty pest

NWS is a fly with a particularly gory way of harming our cattle. 

Female NWS flies lay their eggs in open wounds or body orifices, and when the larvae hatch, they burrow deep into the skin like a screw driving into wood. These maggots feed on cattle’s tissue, causing larger wounds as they go.

NWS flies and their burrowing larvae cause extensive damage to cattle and infestations can spread rapidly. 

In 1966, we eradicated screwworms in the U.S. through sterile insect technique. The U.S. Department of Agriculture (USDA) bred sterile male NWS flies which mated with wild female screwworms and failed to produce offspring. 

Eventually, these flies died out in the U.S., and now they only exist in a handful of South American countries.

Unfortunately, these flesh-eating flies are now on the march north.

Since 2022, USDA’s Animal and Plant Health Inspection Service (APHIS) has confirmed the presence of NWS in Costa Rica, Nicaragua, Honduras and Guatemala. 

Just last month, screwworms were discovered in southern Mexico, which raised red flags for the National Cattlemen’s Beef Association (NCBA).

Once in Mexico, it is easy for screwworms to travel north and appear on our southern border. These flies can hitch a ride on people, livestock or wild animals – not to mention the thousands of vehicles and cargo containers traveling through Mexico to the U.S.

On high alert

We know how devastating it would be if NWS returned to the U.S., which is why NCBA is raising the alarm now. 

First, we are communicating with Mexico’s cattle industry leaders so they can begin the process of screening for these flies and push for eradication efforts in their country. 

We are also supporting USDA APHIS to bolster the use of sterile flies. Recently, we have been concerned our existing sterile flies might not be getting the job done, and NCBA is pushing for more flies and better flies to help us beat back these pests.

Although we haven’t confirmed any NWS in the U.S., we need producers to be on high alert. 

NWS have orange eyes, a metallic blue or green body and three dark stripes across their backs. Individuals who see any suspicious flies should alert their local veterinarian, Extension agent or contact USDA APHIS Veterinary Services.

Please also pay close attention to cattle and watch for any open wounds or sores. If cattle exhibit irritated behavior such as head shaking, the smell of decaying flesh or if individuals spot maggots in a wound, seek treatment from a veterinarian immediately. 

Proper wound treatment and prevention is the best way to protect farms and ranches from a screwworm infestation. 

NCBA is also sharing the latest updates at ncba.org/NWS.

NCBA is always on the clock to protect the U.S. cattle industry from threats like NWS. Through producers’ vigilance and NCBA’s advocacy, we can protect the U.S. cattle herd.  

Kim Brackett is an Idaho rancher and serves as policy division chair at NCBA. She can be reached by visiting ncba.org.