An old pest made new headlines when a case of New World screwworm was identified in southern Mexico. Efforts to keep the pest south of Panama had been mostly effective, but the recent discovery of screwworms as far north as Mexico led U.S. Secretary of Agriculture Brooke Rollins to close the border to cattle, bison, and horses from Mexico in early May. A plan to reopen the border was announced this summer, but when a case of
New World screwworm was identified farther north in July, 370 miles south of the U.S.-Mexico border, Rollins announced the border would remain closed. New World screwworm is an economic threat to livestock and wildlife. The parasitic fly used to be common in the southern U.S. but has been eradicated in the country since 1966, with the exception of an outbreak in the deer population in the Florida Keys in 2016. Eradication was declared again in March 2017.
New World screwworm eradication was achieved by breeding and raising millions of sterilized screwworm flies and releasing them across the southern U.S. Once a female mates with a sterile male, the female will not mate again and can only lay sterile eggs, reducing the population over time. This same control strategy has been used to keep New World screwworms south of Panama. It is not easy to keep the New World screwworm population contained, though. It requires the frequent release of sterile flies, animal movement restrictions, and strict monitoring for signs of infection.
Most adult flies lay eggs in manure, decaying plant material, or decaying animal flesh. That is why one control strategy for more common flies is to regularly remove manure piles from around barns. The New World screwworm is different, though. It will lay eggs in an opening on otherwise healthy skin and the larvae, or maggots, feed on healthy tissue. According to a Kansas State Research and Extension fact sheet, wounds as small as a tick bite or a thorn prick can serve as an infestation site. The area around the naval is a common infection site in young animals. Wounds from dehorning and castration are also at risk.
Infested animals may exhibit symptoms such as pain, depression, lethargy, and reduced appetite. The maggots feed aggressively and can cause severe damage or even death in their victims. This pest can have a detrimental impact on livestock, wildlife, and pets, so USDA is working with Mexico to ensure proactive measures are taken to maintain a New World screwworm-free border.
This article appeared in the August 2025 issue of Journal of Nutrient Management on page 27.
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