Flesh-Eating Screwworm Outbreak Threatens Big Game Populations

Despite having secured the border earlier this year, a new and uninvited visitor is threatening to make passage into the U.S. and stopping it isn’t quite as simple as shutting down entry points. 

A dangerous outbreak of the New World screwworm, a flesh-eating parasitic fly, is spreading northward from Mexico, raising alarms among conservationists and wildlife experts about its potential to devastate big game populations near the southern border. The outbreak has already prompted significant action, including a halt on cattle trade with Mexico to curb the parasite’s spread, as announced by USDA Secretary Brook Rollins last week.

The New World screwworm poses a severe risk not only to livestock but also to big game species such as deer, desert bighorn sheep, mule deer, and whitetail deer. The parasite burrows into open wounds, feeding on living tissue and causing extensive damage to its hosts, which can include livestock, wildlife, birds, and, in rare cases, humans. Historical outbreaks highlight the danger, including a 2016 infestation in Florida that killed 132 endangered Key deer, and significantly impacted the threatened deer’s population recovery efforts.

Conservation organizations like the National Deer Association (NDA) and the Wild Sheep Foundation (WSF) have expressed deep concern. The NDA warns that screwworm infestations could suppress deer population growth and is calling on congress to take action with a newly-introduced bill.

“The health and sustainability of North America’s deer populations are paramount to our mission,” said Nick Pinizzotto, President and CEO of the National Deer Association (NDA), in a prepared statement. “The re-emergence of the New World screwworm poses a significant threat not only to livestock but also to our nation’s wildlife, especially deer."

The WSF went on to emphasize the threat to Texas’ desert bighorn sheep, a species already under pressure from disease and competition with invasive aoudad (Barbary sheep). A screwworm outbreak could potentially wipe out the remaining bighorn population, dealing a catastrophic blow to conservation efforts. The organization warns that screwworm’s spread could also affect hunting opportunities and disrupt the ecological balance in affected regions, with ripple effects on local economies and conservation programs.

“NWS not only affects the cattle industry in Texas but will have detrimental effects on our native wildlife as well. The Desert Bighorn Sheep (DBS) population in Texas has suffered devastating die offs recently from exotic invasive aoudad. A NWS infestation would have catastrophic effects on the remaining population, potentially extirpating the species entirely.” stated Sam Cunningham, WSF Director and President of Texas Bighorn Society.

In response to the outbreak, Texas lawmakers introduced the STOP Screwworms Act, a bipartisan bill to fund a new sterile fly production facility. This approach, which has been successful in past outbreaks, involves releasing sterile male flies to suppress the screwworm’s breeding population. Both the NDA and WSF have endorsed this initiative, urging swift action to prevent the parasite from establishing a foothold in the U.S.

Beyond its impact on big game, the screwworm outbreak is considered a national security concern due to its potential to disrupt the food supply through livestock losses. The U.S. Department of Agriculture’s decision to suspend cattle trade with Mexico underscores the severity of the threat. 

As of right now, the screwworm outbreak has spread as far north as Oaxaca and Veracruz—about 700 miles from the U.S. border. As per the USDA, this "unacceptable northward advancement" is something we all ought to be paying attention to, especially as it pertains to passage of this new bill.