Flesh-eating screwworms feast on humans in Mexico; human cases top 500 - Ars Technica

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Since the flesh-eating screwworm fly reinvaded Mexico in 2024, human cases of the voracious parasite have been mounting. According to a July surveillance report from Mexico’s Ministry of Health, there have been over 500 human cases in 2025 and 2026. A study this week in the journal Emerging Infectious Diseases analyzed the information on over 200 confirmed cases reported between April 2025 and March 2026 and found that the infestation pace is increasing.

Among the 200 confirmed cases in the study, six deaths were reported. At least one of the deaths was directly caused by the fly’s maggots, researchers in Mexico reported in the study.

The New World screwworm (NWS) fly was once endemic in Mexico—as well as the US. The flies lay hundreds of eggs in the wounds, inflamed tissue, mucous membranes, and orifices of warm-blooded creatures. The resulting maggots hatch within a day and begin ravenously feasting on the living flesh of their victims. While the fly’s maggots generate ghastly, life-threatening wounds in a variety of warm-blooded species, including humans, they’re particularly devastating to livestock.

Amid a decades-long eradication effort, the fly’s populations were wiped out from the US, Mexico, and Central America. This was largely done with massive aerial releases of sterilized male flies, a technique that works because female NWS flies only mate once in their lifespans. Mexico declared NWS eradicated in 2003. Panama declared eradication in 2006 and kept an active, biological barrier along the Darién Gap at the border with Colombia. But in 2022, the barrier was breached, and Mexico began seeing cases in animals in 2024.

Mexico’s cases

Since the breach, there have been more than 36,000 animal cases of NWS in Mexico, with nearly 2,000 active as of August 5, according to the US Department of Agriculture. While human NWS infections—called myiasis—are a small fraction of the overall cases, the infestations can be severe and horrifying. The maggots can cause painful, rapidly enlarging deep wounds, which can destroy muscle, cartilage, and bone—even the skull—if not caught quickly. The wounds pose the risk of irreparable damage, secondary infection, sepsis, and, in extreme cases, death.

To better understand the human health impact of NWS’s reemergence in Mexico, researchers surveyed public health, epidemiological, and laboratory reports of confirmed NWS myiasis cases between April 2025 and March 2026. In 2025, the cases were being reported at a rate of about three per week. But in 2026, the pace increased to just over nine a week.

The study identified 204 cases in all, but two did not have sufficient data to be included in the analysis. Of the 202 remaining cases, researchers found 145 (72 percent) were in male patients. The age of fly victims ranged from 12 to 93 years, but skewed toward older adults, with a mean of 61. Most of the myiasis cases (77 percent) were in patients with one or more underlying health conditions, most commonly diabetes, alcoholism, cancers, and skin conditions.

When the flies infect humans, they seem to have the most success infesting the legs and feet, the study found; More than half (55 percent) of cases were in the lower limbs. But the flies are not picky. The next most common sites were reported as the head/neck, including the nose, mouth, and eyes. Fewer cases were seen in the upper limbs, trunk, ear, genitalia, and abdominal cavity.

Six deaths were reported among patients with confirmed NWS myiasis, but it’s unclear in most of the cases how much the infestation contributed to the deaths. Only one had NWS myiasis listed as the cause of the death. The other deaths were in patients with cancers, encephalitis, respiratory failure, and septic shock. The average age of patients with NWS-linked deaths was 79.5 years.

One Health approach

The geographic distribution of the human cases unsurprisingly mirrored the distribution of reported animal cases. But it remains unclear why men seem to be at higher risk of the flesh-eating maggots. They speculated that occupational exposure, outdoor activities, contact with animals, unattended wounds, or delayed healthcare seeking could be factors in the imbalance.

Overall, the researchers conclude that the cases highlight an important link between human and animal health.

“Because animal and human cases reflect northward spread of the NWS fly and that spread is expected to continue, surveillance, diagnostic...