What You Need To Know About The Screwworm Now That It’s Reached The U.S.

For more than a year, federal officials in the United States have been monitoring and warning the agriculture sector about the New World Screwworm, a fly parasite with serious consequences for the farming and equestrian industries.

In 2024, the screwworm was reported in livestock in Mexico. Despite increased surveillance, the screwworm was found in the United States this month, according to the United States Department of Agriculture. On June 3, USDA officials confirmed that the screwworm was found in the umbilical area of a calf in South Texas.

The New World Screwworm is considered a serious threat due to how fast its larvae can infest in living tissue and rapidly progress, causing fatal maggot infestation in warmblooded animals, including horses and humans, according to the American Association of Equine Practitioners.

The screwworm has the potential to spread to every state. It is most commonly known to exist in the Caribbean, Central and South America, according to the AAEP.

The risk to horses comes from their susceptibility of myiasis, or infestation, by fly larvae that feed on living tissue. Eggs are laid in wounds or areas like the nose and sheath of horses, where larvae can feed and grow. Screwworm larvae can emerge from eggs in 12 to 24 hours. After about seven days, the larvae drop and burrow into the ground. Adults emerge after another week as flies.

Untreated wounds can develop multiple infestations, which raises the fatality risk in horses and other animals. Horses can be carriers and spread the disease if they have open wounds or mucous membrane injuries, according to the Equine Disease Communication Center. Prognosis for horses is usually good if treated and managed early.

A widespread outbreak of the screwworm could have serious consequences, disrupting livestock trade, contribute to increased veterinary and surveillance costs and place a greater burden on producers, animal health officials and event organizers. Longterm spread could have an impact on food production and the national economy.

Early detection and response is critical to stop the spread, health officials have stressed.

“USDA invested heavily in the tools needed to eliminate NWS ever since cases started increasing in Central America and Mexico. The United States has defeated this pest before, and we will do it again,” said Dudley Hoskins, Under Secretary for Marketing and Regulatory Programs, in a statement.

Federal officials are working to deploy sterile screwworm flies in infested zones, up to 4 million flies per week. The U.S. successfully eradicated the screwworm with this method during previous outbreaks.

Horse owners can protect their pets by monitoring any wounds for drainage, foul odor or uncommon irritation, according to a press release from the United States Equestrian Federation. Cleaning and covering even minor cuts as soon as they are found will help reduce risk or screwworm intrusion.

Consistent fly control, good stable hygiene and careful monitoring after injuries or surgery are good tips to help reduce risk. If a horse develops a wound that worsens quickly, has a bad smell or has visible maggots, owners should contact their veterinarian immediately.

The USDA released a list of pesticides that are appropriate to use against the screwworm. You can see that list here. The USDA also launched a map of active screwworm activity in the U.S. See that map here. And the agency released a fact sheet with more information here.

For horse owners interested in learning more, the USEF is hosting a free webinar on June 18 at 4 p.m. EST. Register here.