PIERRE — South Dakota has not reported any cases of New World screwworm, the flesh-eating fly larvae that attack open wounds on livestock, wildlife and pets, state officials said. But animal industry leaders and the state veterinarian cautioned that vigilance is still needed as outbreaks continue in parts of the southern United States.
Current outbreak and state response
The U.S. Department of Agriculture confirmed that cases surfaced this summer in Texas and parts of southeastern New Mexico, with a combined total of 35 reported infections, primarily in cattle. South Dakota State Veterinarian Mendel Miller said the state has no immediate concerns because federal and Texas containment strategies appear to be working and because South Dakota requires veterinary inspections and health records for animals entering the state.
"The system's in place to prevent it from getting here, provided everybody follows the rules and follows the system," Miller said.
Those measures include documenting animal movements and enforcing health checks at points of origin or entry, tools state officials say are critical to keeping screwworm from hitching a ride on livestock shipments.
Why South Dakota has so far been spared
Historically, screwworms reached as far north as South Dakota between 1947 and 1950 through infested livestock shipments. State and federal officials point to colder winters as a natural barrier that has limited the insects’ permanent establishment in northern states. Miller said milder winters linked to broader climate changes could allow the traditional southern boundary for screwworm to drift northward, but for now "we still have winter."
He added that the ground freezing in South Dakota remains a significant deterrent to the larvae becoming established.
What producers should know and do
Veterinarians and animal health officials emphasize biosecurity and the importance of following existing movement and inspection rules. Practical steps for producers include:
- Maintain thorough health records for animals brought into the state.
- Report suspicious lesions or infestations on livestock to a veterinarian promptly.
- Follow transport and inspection requirements for animals moving across state lines.
- Work with county and state animal health officials when buying animals from areas with known outbreaks.
USDA descriptions of the pest note that the larvae burrow into wounds, feeding "like a screw being driven into wood," and that while the flies themselves are about the size of a common housefly, their orange eyes and blue- or green-toned bodies make them distinct. While rare, human cases can occur; most reports involve livestock and wildlife.
| Location | Reported cases |
|---|---|
| Texas | Majority of recent cases |
| Southeastern New Mexico | Included in 35 total cases |
| South Dakota | 0 reported cases |
State officials said they are monitoring the situation and coordinating with federal partners. For now, they urged producers to follow the rules that have kept screwworm out of South Dakota for decades and to contact the state veterinarian's office or local extension if they suspect an infestation.
