So, it ended up happening precisely as predicted… the New World Screwworm crossed casually over our southern border in the first week of June and attacked a U.S. calf. That incident was followed by dozens of additional cases in sheep, goats, and pet dogs, with reports trickling in steadily over the next two months. Previously confined to South America and a few of the Caribbean islands, New World Screwworm is now making itself at home in our own back yard.
You may already know that screwworm is the voracious, carnivorous offspring of a very boring-looking fly. Other fly species may settle into a wound and consume byproducts of the healing process, but screwworm larvae have a very aggressive appetite for healthy, living flesh. They can and will dive deeper until they kill their host in a very painful way. That’s bad news for the animal, of course, but the bigger picture predicts damaging effects on the entire livestock industry and the U.S. economy.
In 2025, as this foreign threat was creeping closer and closer to the lower 48, travel and trade restrictions were hastily enacted to prevent its entry into the U.S. Since it’s now present in both Texas and New Mexico, however, that target has been adjusted to zero in on its possible spread to the other 46 contiguous states. In a typically well-organized fashion, this has resulted in a confusing array of regulations for movement of any warm-blooded animal from one state to the next. If you are traveling with your pet, you will now need to obtain and carry an extra certificate stating that your companion isn’t harboring any screwworm larvae. Requirements vary from state to state and the regulations may change without warning, but you will not be allowed on a plane without the necessary documents—end of story.
What if you’re driving, though? Many families with pets prefer this mode of travel because bringing an animal onto an airplane is complicated and expensive. So much paperwork. Everyone knows, however, that you don’t get stopped for “Pet Documents” when crossing state lines in an automobile. I’m not pointing out this giant loophole to get you to cheat the system. I’m pointing out that, if you drive from state to state during this foreign fly crisis, you must police your own behavior, acting in the best interest of both the family pet and the greater community. When you move through a state where screwworm has been observed, you risk carrying them back to your home state. Flies are attracted to the nose, ears, and any small scratch on the skin, so examine your pet meticulously every day for any pain, ear infections, or tiny open wounds. In addition, certain flea and tick drugs (a list is available at FDA.gov) are active against screwworm larvae. One of these should be used to protect your dog or cat during the trip. While most of these require veterinary approval, nitenpyram can be obtained over the counter. Importantly, if you suspect your pet may be suffering from this parasite, seek immediate veterinary care.
Dr. M.S. Regan