On the farm, arthropods like flies, mosquitoes, mites, and ticks can be more than a nuisance—they can carry diseases, damage hides and wool, and even cause death. You’ve probably heard about New World Screwworm (NWS) in the news. Once nearly eradicated from North America, NWS has been recently detected in Central America and Mexico. Northward migration to the United States is a major concern, as NWS threatens both animal and human health.
What are New World Screwworms?
NWS are the larvae (maggots) of a parasitic fly that feeds on living flesh. This differs from most other maggots, which consume dead or dying tissues. Female flies lay eggs in any open wound, no matter how small—this can include:
- Small cuts or scrapes
- Umbilical area of newborn animals
- Castration or dehorning wounds
- Tick bites
Most NWS cases are in cattle, goats, and sheep, but all warm-blooded animals, even birds, and rarely humans, can be affected.
What Does a New World Screwworm Infestation Look Like?
NWS infestations are difficult to detect at first. Slight movement might be seen inside a wound, but as the larvae feed and bury deeper, the wound gradually enlarges. A blood-tinged fluid with a decay smell may seep from the wound. Hundreds of larvae can be packed deep inside.
NWS infestation is painful. Affected animals usually separate from the herd and lie down in shady areas. You may see head shaking, and animals may seem irritable. Decreased appetite and lowered milk production are common. Untreated animals can die in 7 to 14 days.
What Should I do if I Suspect New World Screwworm?
In animals, contact your veterinarian immediately. Widespread outbreaks do not currently occur in the United States, although isolated cases do pop up, especially in southern states. Suspicion of disease requires immediate attention. In humans, contact your physician.
How Can I Protect My Animals from NWS?
Clean wounds as soon as possible, no matter how small. Check them closely for NWS eggs or larvae until healed. NWS eggs are creamy and white and deposited in a shingle-like manner on or near the edges of a wound. Larvae are cylindrical with one pointed end and one blunt end, with rings of dark brown spines around the body. Female flies are slightly larger than houseflies, with dark blue to blue-green bodies and reddish-orange heads.
Even on small farms, one NWS case is too many. Stay informed, stay watchful, and don’t hesitate to contact your veterinarian if you see anything unusual. Your animals—and your neighbors’ animals—depend on it.
For More Information
- USDA: New World Screwworm
- What does a foreign animal disease look like (pdf)
- Frequently asked questions on reportable animal diseases (pdf)
Photo: Key Deer leg with NWS infestation. Mark Cunningham, USDA.