Oklahoma Aims to Protect Livestock From Possible Screwworm Outbreak

By Seth Millstein

On May 11, Secretary of Agriculture Brooke Rollins announced that the U.S. is suspending all livestock imports from Mexico due to a resurgence of New World screwworm. Mexican authorities detected the parasitic fly, which was ostensibly eradicated decades ago, in southern Mexico earlier in the month after an outbreak in Panama years ago. Now, officials are increasingly concerned that the screwworm could reach the U.S., and wreak havoc on American farm animals.

Rollins says that the suspension will apply to live cattle, bison and horses, and will be renewed "on a month-by-month basis, until a significant window of containment is achieved." In the meantime, U.S. officials and cattle farmers are on edge.

"It is very, very bad, and it's not just cattle," Dr. Rod Hall, the state veterinarian for Oklahoma, tells Sentient. "The screwworms can affect any warm-blooded animal, so it would be devastating to any of our livestock species."

What is a New World Screwworm?

The New World screwworm isn't a worm at all, but a parasitic fly whose larvae infest and burrow into the tissue of warm-blooded animals. Female screwworms are attracted to open wounds, and bodily orifices in general, and that's often where they lay their eggs. Once the larvae hatch, they burrow into the host creatures' tissue with their powerful mouth hooks.

While several fly species are attracted to open wounds, screwworms are unique in that they infest healthy, living tissue, as opposed to the flesh of dead creatures. What's more, screwworms can lay up to 400 eggs at once, so even a single pregnant fly is bad news for any warm-blooded creature unfortunate enough to encounter one.

"When they get to a certain point, they fall out, burrow into the ground for a week or so, and then they turn into more flies," Hall explains. "So oftentimes, by the time a human realizes that an animal is infected, the damage has already been done, and the next generation [of screwworms] is in the soil, waiting to turn into adult flies so they can lay more eggs."