Flesh-eating screwworm cases surge 53% in Mexico, inching towards US border

Mexico has reported a sharp rise in cases of flesh-eating screwworm in animals, with infections jumping 53 percent in the past month as the parasite moves closer to the US border.

Government data show 5,086 cases as of August 17, including 649 active infections, Reuters reported on Thursday. The parasite has been detected mostly in cattle, but also in horses, sheep, dogs — and even humans.

Mexico has confirmed 41 human cases over the past year, mostly in the southern state of Chiapas. On August 4, US health officials reported the first American case in Maryland, in a patient who had recently travelled to El Salvador.

The parasite had been eradicated decades ago in both Mexico and the US, but made a comeback around 2023, spreading north from Central America.

The economic toll is already high. Mexico’s cattle industry lost an estimated $1.3bn over the past year, according to the National Agricultural Council. The US Department of Agriculture (USDA) warns the pest could threaten $100bn in livestock-related economic activity if left unchecked.

Screwworms begin with flies laying eggs in warm-blooded animals. The larvae burrow into flesh, and untreated infections can be fatal. Preventing outbreaks is notoriously difficult.

In response, Mexico and the US are building new “sterile fly” factories in Chiapas and Texas. A similar facility in Panama is already operational. The goal: churn out about 500 million sterile male flies per week. Released into the wild, they mate with females without producing offspring, cutting the population over time. The same method wiped out screwworm in the US in the 1960s.

The US Department of Health and Human Services has also approved emergency veterinary drugs to fight the parasite. Some of these are not yet formally approved in the US but are being fast-tracked.

“The risk to human health in the United States remains very low, but the potential future threat to animal populations and the food supply chain requires proactive action,” HHS said.