New World Screwworm: What it Means for U.S. Livestock and Wildlife
by Haley Goodwiler
There’s been some buzz about screwworm lately. But what is it and what does it mean?
Screwworms are the larvae of the Cochliomyia hominivorax fly. It’s a parasite known for infesting warm-blooded animals. What makes screwworms dangerous and buzzworthy is that they feed on the living tissue of their hosts, leading to severe wounds, infection, and even death if not treated promptly.
Here is a great resource and thorough breakdown released by the U.S. Department of Agriculture Animal and Plant Health Inspection Service.
Recent Developments
The New World screwworm is native in parts of Latin America and the Caribbean, but its recent northward spread has alarmed U.S. officials. On Sunday, Agriculture Secretary Brooke Rollins announced on X a temporary halt on live cattle, horse, and bison imports from the southern border. “The protection of our animals and safety of our nation’s food supply is a national security issue of the utmost importance,” Rollins said. “This is not about politics or punishment of Mexico, rather it is about food and animal safety.”
Brief Historical Breakdown of the Screwworm
The United States successfully eliminated screwworms in the 1960s using the sterile insect technique. The same technique will still be used today to combat this parasite.However, the parasite still exists in parts of Central and South America. This was highlighted in 2016 when screwworms reemerged in the Florida Keys, negatively impacting the endangered Key deer population and prompting a rapid, multi-agency response to control the outbreak.
I recently spoke with a local rancher in Texas who mentioned, “There wasn’t a day in my grandfather’s life that he didn’t talk about screwworms.” While researching for this article, I found a bulletin about “screw-worm” released by the Texas Agricultural Experiment Station in 1890. This indicates that producers have been dealing with this devastating parasite for over 100 years.
What Livestock Producers Can Do
In the last couple of weeks, there have been alerts to U.S. producers. Public awareness and speedy reporting to right away to your local accredited veterinarian, State Animal Health Official, or a USDA veterinarian of unusual wounds in animals is crucial. Livestock producers, veterinarians, and pet owners are urged to familiarize themselves with the signs of infestation (USDA, 2025).
Screwworms typically lay eggs in open wounds—whether caused by injury, surgical procedures like castration or dehorning, or even tick bites and navels of newborn animals. They prefer warm, moist environments, so good hygiene and prompt wound care are essential (USDA, 2018). Here are some key prevention tips:
Inspect animals regularly for any wounds or signs of distress.
Keep wounds clean and treat with appropriate antiseptics.
Apply fly repellents after procedures or when wounds are present.
Minimize exposure to environments where flies thrive—like unclean, moist, or shaded areas with poor drainage.
Report suspicious wounds that don't heal properly or appear infested, especially if paired with a foul odor.
For more information, check out these resources:
