What is New World Screwworm?
New World screwworm (NWS) has been on the minds of livestock producers as it made its way north across the southern U.S. border. NWS is a parasitic fly whose larvae feed on the tissue and flesh of warm-blooded animals. The female flies deposit their eggs into open wounds and body parts such as eyes, nose, ears, and mouths. Within 12-24 hours the eggs hatch and the larvae eat the live tissue of the animal causing pain. Wounds become infected, resulting in additional discomfort, secondary health risks, and foul smells. Once larvae have fed on the host animal and are ready to molt into adults, they will drop to the ground and molt in the soil. Adult NWS flies are similar in size to house flies but are metallic blue- green with orange eyes and three dark stripes on their backs. The NWS is primarily spread via animal movement as larvae in infested animals. Adult flies do not travel very far. NWS is not a food safety concern.
What to Look for:
Producers are encouraged to inspect their animals routinely for open wounds and closely look at body parts with openings. A few symptoms producers should look for in animals:
- Maggots inside open wounds, or inside the eyes, nose, ears, and mouths
- Wounds that take longer than expected to heal
- Foul-smelling odor coming from the wound site
- Excessive irritation around the wound including head shaking
- Reduced appetite
How to Prevent:
The greatest risk for Wisconsin producers is NWS larvae coming into the state in/on an infested animal and not being detected early.
Following a good biosecurity protocol for your farm will go a long way in keeping screwworms away. Inspect and quarantine new animals or animals returning to the farm. Remove sharp objects from animal handling and housing areas and inspect open wounds frequently. If you suspect animal disease or parasite infestations, contact your veterinarian.
WI Guidance:
Effective 7/21/26, Wisconsin has regulations on warm-blooded animals entering Wisconsin from NWS infested areas. The order can be found on the DATCP website. Contact your veterinarian for more information.
Other Resources:
Screwworm.gov | Unified Government Response To Protect the United States
DATCP Home Animal Health Waivers & Special Orders
Strengthening biosecurity practices on dairy farms – Dairy
▶ Watch: Basic Biosecurity Practices For Livestock Operations – Livestock