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 preparedness, animal movement controls, and fly-suppression activities should be implemented proactively at the leading edge of expansion, rather than only after human cases are detected,” the researchers write. “Coordination between human health, animal health, wildlife, and agricultural sectors will be essential to guide prevention and control measures.”

The US reported its first animal cases in Texas in June amid the reemergence. Since then, the USDA has reported 45 animal cases. So far, no human NWS myiasis cases have been reported in the US from the fly’s resurgence. But there have been travel-related cases amid the reemergence in Central America and from people returning from other locations with endemic NWS populations.