Mexico reports more than 5,000 cases of screwworm, primarily in cattle and other animals, according to gov’t data.
Published On 28 Aug 2025
Mexico has reported a 53 percent increase in cases of flesh-eating screwworm in animals over the past month, as the parasite continues its northward trajectory towards the United States border, according to government data.
Mexican government data indicate that there were 5,086 cases as of August 17, including 649 active ones, the Reuters news agency reported on Thursday, citing updated data not yet made public.
The parasite has been found primarily in cattle, as well as in horses, sheep, and dogs, the news outlet said.
There have also been dozens of human infections.
Mexico has confirmed 41 human cases over the past year, primarily in the state of Chiapas, according to Mexican media. The first human case was confirmed in the US state of Maryland on August 4 in a patient who had travelled to El Salvador.
Mexico and the US eradicated screwworm decades ago, but the parasite returned around 2023 after moving northward from Central America.
The infestation has disrupted Mexican cattle exports, with losses valued at $1.3bn over the past year, according to the Mexican National Agricultural Council.
The US Department of Agriculture (USDA) said that left unchecked, screwworm threatens $100bn in economic activity linked to the cattle and livestock industries.
Screwworm infestations begin with parasitic flies that lay their eggs in warm-blooded animals. The eggs hatch into larvae, which burrow into the flesh of their host, killing them if left untreated.
Infections are challenging to treat, and so is preventing future outbreaks.
The Mexican and US governments are building “sterile fly” production facilities in the states of Chiapas and Texas to reduce the population of screw flies.
A sterile fly facility produces a large number of male flies and sterilises them. These are then released to mate with females in the wild, which reduces the wild population over time. This method eradicated screwworm from the US in the 1960s.
The two facilities and another in Panama will need to produce about 500 million sterile flies a week to push screw flies back to South America, where the parasite is endemic, according to the USDA.
The US Department of Health and Human Services (HHS) has also approved emergency veterinary drugs to prevent the parasite from spreading.
They include “animal drug products that may be approved for other purposes, or available in other countries, but not formally approved” for the screwworm, HHS said.
“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,” it said.
