The New World screwworm, a parasitic fly that feeds on living tissue, has been confirmed spreading dangerously close to the United States, prompting federal health officials to declare a public health emergency.
While the source material describes a case in Maryland involving a patient who contracted the infection while traveling to El Salvador, the broader concern centers on the parasite's northward movement through Central America and the Caribbean. The fly has crept within a few hundred miles of the US border, primarily threatening livestock in states like Texas.
The screwworm was once endemic throughout much of the southern United States before scientists developed an innovative solution in the 1950s. Researchers discovered that exposing fly eggs to low doses of radiation rendered them sterile. Since female flies typically mate only once, encounters with sterile males prevented egg fertilization and populations declined dramatically. This technique pushed the parasite south over decades, eventually establishing a containment barrier at the Darién Gap separating Panama from South America.
However, this progress has eroded in recent years. About 89,000 screwworm cases in animals have been detected over the past three years in Central America, according to FDA officials. The parasite likely crossed northward through livestock trade, and wildlife may also carry it across the border.
Female screwworm flies lay eggs in wounds on warm-blooded animals. Once hatched, hundreds of larvae use sharp mouths to burrow through living flesh, eventually killing their host if untreated. The economic threat to US livestock production is substantial, particularly for cattle-heavy regions like Texas.
The US Department of Health and Human Services declared a public health emergency on August 18, opening pathways to test whether existing anti-parasitic medications like ivermectin could control screwworm infestations in animals. However, experts emphasize that human risk remains low. The Maryland case was diagnosed before the insect matured, eliminating transmission risk. Screwworm transmits slowly to humans compared to bacterial or viral infections, and endemic areas in Central America report only a few hundred human cases combined annually.
"This is really about protecting livestock," said Marty Makary, FDA chief, noting the parasite's primary threat lies in agricultural damage rather than human health.
In response, the USDA reopened a Texas breeding facility for sterile flies in June and announced an 850 million dollar plan in August to combat the parasite. Federal agencies are expanding monitoring programs, trapping flies at borders, and continuing inspections of livestock destined for slaughter.
International cooperation remains critical to containing the screwworm, experts stress, as fighting the parasite requires coordinated efforts across multiple countries. Recent budget cuts to scientific research and foreign aid programs have complicated these prevention efforts at a vulnerable moment.