There was a time in the not-so-distant past when some Texas cattle hands riding the range had just one job: detecting and treating “wormies.”
The wormies were calves infected with New World screwworm (NWS), according to San Angelo, Texas, veterinarian and rancher Dr. Chris Womack. He said calves are the perfect prey for NWS, because, unlike most other myiasis parasites that feed on dead and necrotic tissue, NWS feed on living tissue.
On a recent episode of the Have You Herd? podcast from the American Association of Bovine Practitioners, Womack noted one of the ideal feeding sites for the burrowing NWS myiasis (maggots): the fresh navel of a newborn calf, along with any other open wounds like disbudding sites. But cattle and calves certainly are not the only hosts of NWS. Infestations can occur in all mammals, including humans.
That fact leaves Womack concerned about the possible spread of the pest. If it does cross the U.S. border, he believes the “sentinel animal” in which it is first detected will be either a companion animal or a wildlife species.
The current rate of spread also alarms him. NWS was basically eradicated from the U.S. in 1966, thanks to collaborative biological efforts that resulted from public-private partnerships between donors and the U.S. and Mexican governments. “At the time more than $2 million in private funds were raised,” he Womack shared. “It was that big of deal to producers that they were willing to put their own dollars up to fight it.”
As an example, the state of Texas lost an estimated 180,000 head of cattle to NWS in 1935 alone, according to T.R. Lansford III, DVM, deputy executive director and assistant state veterinarian with the Texas Animal Health Commission.
“Through medicine or miracle, they figured out they could irradiate the male fly and make him sterile,” Womack recalled. Because NWS females mate only once in their lifetimes, mating with a sterile male results in sterile larvae, breaking the fly’s production cycle.
“They bred and raised hundreds of millions of sterile male screwworm flies and blanketed the southern U.S. border with them,” said Womack. After they had pushed them out of the United States, those same efforts continued in Mexico for 25-30 years, until the parasite had been driven all the way across the Panama Canal and south of the Darien Gap.
But that progress shifted in the second half of 2024. Womack said a rancher in Panama told him NWS had been detected in Panama in early August 2024. “From that time until Thanksgiving, it migrated clear across Central America and into Chiapas, Mexico,” noted Womack. “That’s really terrifying.”
In addition to closing – then re-opening and closing again – the U.S./Mexican border to imports of cattle, bison, and horses, USDA also is ramping up eradication efforts in Mexico and Central America. State departments of agriculture in the border states of New Mexico, Texas, and Arizona also are involved. But in the meantime, the deadly parasite has continued its march north through Mexico, and has been detected within 370 miles of the U.S./Mexico border.
Womack also is concerned that range cattle and wildlife are not as closely observed as they used to be. And while there may be fewer cattle on open pastures in Texas compared to 60 years ago, Texas is now home to tens of thousands of dairy calves at calf ranches, many of whom travel to various outposts around the country as they mature.
USDA resources indicate that NWS flies are slightly larger than the common housefly. They have orange eyes, a metallic blue or green body, and three dark stripes along their backs. Symptoms of effected animals include irritated behavior, head shaking, the smell of decay, and the presence of maggots in open wounds.
Producers who suspect their animals may be infected with NWS should contact their veterinarians and are encouraged to submit maggot samples to a diagnostic lab.
Womack lamented that because the problem was so well-controlled for so long, there is a generational gap in knowledge about NWS. “Because it was so fantastically eradicated, it was a non-issue for decades. But now it’s an issue again. We need to be aware of and vigilant about this problem,” he stated.
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