The arrival of New World screwworm
By the time this appears in Country Folks, the numbers will have changed.
Stories about New World screwworm (NWS) are hitting nearly every news outlet. Once endemic in the U.S., efforts to push the NWS southward began in the 1950s and resulted in eradication in the U.S. by 1966.
Now it’s back, and although there are fewer than 10 reported cases as of the second week in June, reports of NWS in the U.S. will likely ramp up.
Dr. Hailey Springer, veterinarian, Bureau of Animal Health & Diagnostic Services, discussed the issue on a producer call on June 10. NWS is in the same family as the blowflies some farmers have seen on dead livestock.
NWS is more of a pro and feeds on living tissue. Once they enter a wound, the wound expands dramatically. This is when myiasis, or infection, becomes a major problem.
The positive aspect for the Northeast is that sustained soil temperatures below 50º will kill NWS and break the cycle. However, if it travels to the Northeast, high summer temperatures will allow it to thrive and spread until winter kill-off.
An outbreak in 1976 in Texas cost about $132 million. When this issue was common in southern states, the summer range could extend to southern Canada, so the environment is right for this to be a summer pest.
“There are four life stages – eggs are laid by adult females, larvae (maggots), pupae and adult,” Springer said. “The lifecycle is about 21 days. Larvae could be visible in the wound by day three. These flies can lay eggs in a wound very quickly. The larvae grow to about 17 millimeters long and it smells bad. If untreated, they can cause death in seven to 14 days, but it’s a treatable condition.”
There is often co-infestation with other maggots, which is one aspect farmers and officials monitoring the situation must watch when sampling suspect animals. NWS burrows deep into the wound while common maggots remain on the surface of wounds.
The NWS can lay eggs in a wound as small as a tick bite, Springer said. “A lot of our day-to-day practices can cause wounds that allow the NWS in. Nearly 100% of neonates in a favorable environment can have issues with NWS if they are not prophylactically treated. Cuts, abrasions, castration or ear tag placement are also potential sites for egg laying.”
Some of the most common infection sites include navel infections, which was the case in a young calf initially identified in Texas. Mucous membranes, even a small tear in a lip or eyelid, are a prime spot for egg laying.
NWS is not a food safety issue. The human health concern is if someone is in an area where there are screwworms present and being infested with eggs followed by larvae that feed on the flesh. Some severe human cases have been in people with disabilities that render them unable to communicate a problem or people with a lack of feeling in areas of their body.
“There is zoonotic potential,” Springer said. “This fly could lay eggs on a human. All warm-blooded animals can be affected so we can see infestations in humans and pets such as the travel-associated case in Maryland in August 2025.”
NWS has been in Mexico for several years but was eradicated there in 1991. However, in 2016 there was a resurgence. The theory is that some flies were moved into the Florida Keys during a storm. The outbreak killed about 135 key deer and affected several domestic animals. Animals were treated with doramectin; the sterile insect technique (SIT) was used to halt reproduction.
Springer explained that the SIT, developed in the 1950s, hijacks an important part of the female biology.
“Females only mate once in their lifetime,” Springer said. “Male flies are irradiated to make them sterile and released in large numbers. Females mate with sterile flies and are unable to lay viable eggs. It was extremely successful and has been one of the most successful biological disease controls in history.”
Northward progression of NWS began in 2023, and by 2025, it advanced farther north into Mexico. Although officials have been closely monitoring the border area, after detecting NWS in a calf in Texas, officials expanded watch areas and issued prevention and treatment information.
“The national response is sterile fly release with increased aerial and ground release near the positive cases,” Springer said. “There are animal movement restrictions and surveillance near known cases. Southern ports of entry remain closed to livestock.”
The FDA has issued Emergency Use Authorization (EUA) for several products for NWS treatment and control, including an injectable doramectin product, Dectomax®-CA1, for prevention and treatment of NWS. Sterile fly production continues, and officials plan to increase production.
For additional current information on NWS management, visit aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm.
Information on EUA drugs for preventing and treating NWS is available at tinyurl.com/NWStreatment.
by Sally Colby
