On the Southern border of the U.S., swarths of livestock face off against a tiny, silent, but vicious killer: the screwworm. After its reemergence in recent years, the small fly is currently posing a big threat to Texas agriculture.
Cochliomyia hominivorax, the New World Screwworm (NWS), is a species of parasitic fly. After it lays eggs inside of a mammal, its hundreds of offspring maggots erupt and consume the flesh of the host, pupating into full adults to begin the process again. In a state so dependent on ranching, Texas farmers fear what an outbreak of these insects could mean for their cattle herds. A few well-placed eggs could reintroduce the parasite, resulting in millions or billions of dollars in damages and rising beef and milk costs for consumers, per the Texas Animal Health Commission. Monitors in Mexico have reported NWS increasingly close to the Texas border, prompting concerns that it may reach the United States.
This isn’t the first time Texas has had to deal with these petite invaders. NWS was present in the U.S. for the first half of the 20th century until the government stepped in to exterminate it in the 1950s. Rick Machen was the executive director of the King Ranch Institute for Ranch Management before retiring this February, and he has been working with cattle for decades, since before the screwworm was eliminated from Texas. He explained a key strategy the U.S. could turn to as it tries to prevent the bugs’ potential return.
“The current technology that we have points to sterilization of the male flies,” Machen said, “Because of the unique phenomenon that the female will only breed once in her lifetime, if that mating is with a sterile fly, then, in fact, the eggs that she produces will be sterile. The process that we used to eradicate the screwworm about 60, 70 years ago, that technology is still viable.”
One sterile male can eliminate thousands of unborn screwworms. This strategy has been used for decades, with United States Department of Agriculture officials in Panama managing to keep the screwworm below the Darien Gap, the jungle landbridge between Colombia and Panama, since 2006. Recent events briefly threw this procedure out of order, allowing some number of flies to pass the isthmus and enter North America.
“What happened was, during the COVID pandemic, all federal government employees were furloughed and couldn’t go to work,” Machen said. “So the facility, in effect, was closed for six weeks … and during that time, obviously there were no sterile flies produced. There were no sterile flies being released.”
Flies present in Colombia were able to cross the Darien Gap, taking advantage of the warm spring and summer weather, and have been progressing north over the last several years. Sightings of the screwworm have occurred only a few dozen miles from the U.S.-Mexico border. Dr. Bud Dinges, Texas State Veterinarian and executive director of the Texas Animal Health Commission, is in charge of handling the potential response to a possible appearance of the insect in the state.
“Production levels of meat and milk; veterinary, medication, and labor costs in livestock production; and the health status of wildlife populations are all impacted by a NWS outbreak,” Dinges said. “During the 20th century, the presence of NWS cost the U.S. livestock industry more than $100 million annually. Reintroduction of NWS into the U.S. could cause tremendous financial burden due to livestock losses, trade embargoes, and eradication effort costs.”
There are few readily-available treatments for screwworm-affected animals, with only two commercial products available for livestock, and three for pets, Dinges explained. Screwworms are often lethal, and are impossible or very expensive to remove, meaning preventative efforts are the preferred way of mitigating the cost.
“Both internationally and domestically, animal health officials, including the Texas Animal Health Commission, are conducting extensive outreach campaigns to educate on NWS and how to prevent infestations,” Dinges said. “U.S. border states, including Texas, are maintaining surveillance traps on the U.S.-Mexico border to promote early detection of NWS.”
Still, the fast progress of the screwworm thus far has ranchers worried. Machen, for one, believes the idea of an outbreak to be a case not of if, but of when.
“The facility in Panama is running at what I understand to be maximum capacity. I think that their capacity is to produce 100 to 110 million flies per week,” Machen said. “As I understand it, it’s running at capacity … I try to be optimistic and take the position that the flies won’t make it to Texas. But I think the reality is that they probably will, given their progress.”
An incursion of screwworms pose a threat to both ranchers and non-ranchers alike, including humans in cities and towns throughout Texas. Mammals, including pets, that live in urban areas are particularly ideal breeding grounds for screwworms.
“If the screwworm gets here, it is no respecter of warm-blooded mammals,” Machen said. “It will infect white-tailed deer, it will infect dogs, calves, and so the white-tailed deer is gonna be the vector that’s gonna potentially bring the screwworm very, very close to us and very close to our pets. We as urban dwelling pet owners are gonna have to be diligent and watch because any opening, a wound as small as a tick bite is enough to give a female screwworm fly the avenue by which to infect that animal with her larvae … it will be a concern for urban-dwelling pet owners, as well as farmers and ranchers alike.”
The screwworm may be one of the most notable and dangerous parasites, but it’s not the only one that could cause damage to Texas wildlife and people. Lamba Omar Sangare studies parasites at Texas A&M University, focusing on a widespread but clandestine creature called Toxoplasma gondii.
“It’s estimated one third of the human population is infected by Toxoplasma,” Sangare said. “And people are infected, but they are not aware of the infection until they are immunocompromised … this parasite can reactivate in the brain and then kill the individual.”
When treatments exist for Texan parasites, they are often expensive, difficult to obtain, or have major side effects. Larger, corporate ranches like the King Ranch may be able to obtain these treatments, but they prefer to prevent infection rather than attempt to cure it after it happens.
“I would say parasites are what we call a time bomb,” Sangare said. “And we have a tendency to underestimate this time bomb.”
