Human Screwworm Cases Surge as New World Screwworm Reemerges in Mexico

A new study warns that the return of New World screwworm is no longer just an animal health problem. Human cases are rising, and researchers say veterinarians will play a critical role in stopping its spread.

For decades, New World screwworm (Cochliomyia hominivorax) was considered a success story in disease eradication. After an extensive sterile insect release program, Mexico was declared free of the parasite in 2003. Now, that victory is being challenged.

A new study published in Emerging Infectious Diseases analyzed the first wave of human New World screwworm (NWS) myiasis cases following the parasite's reemergence in Mexico. Researchers identified 204 confirmed human cases between April 2025 and March 2026, with the outbreak continuing to expand northward.

From Livestock Threat to One Health Emergency

Veterinarians are already familiar with the devastation caused by New World screwworm in livestock and companion animals. Female flies lay eggs in open wounds, and the larvae hatch within 12 to 24 hours before feeding aggressively on living tissue for five to seven days. Unlike many fly larvae that consume dead tissue, screwworm larvae destroy healthy flesh, creating rapidly enlarging wounds that can become fatal if left untreated.

The same parasite also infects wildlife and people, making this a textbook One Health issue.

The researchers warn that the geographic overlap between infected animals and human cases highlights the need for coordinated surveillance among veterinarians, physicians, wildlife officials, and agricultural agencies.

Cases Are Increasing

The study found that the outbreak accelerated considerably during early 2026.

Researchers documented 111 confirmed human cases during 2025, averaging about three cases per epidemiologic week. During just the first ten epidemiologic weeks of 2026, another 91 cases were confirmed, increasing the weekly average to more than nine cases.

Most cases occurred in southern Mexico, particularly Chiapas, although by March 2026, confirmed cases had spread to 12 Mexican states, demonstrating the parasite's continued expansion.

Previous modeling studies have estimated that the reemerging screwworm population could spread northward at approximately 1.2 to 1.9 kilometers per day, emphasizing the importance of early detection before the parasite becomes established in new regions.

Who Is Most at Risk?

The average patient in the study was 61 years old, and nearly 72% of cases occurred in men. Researchers also found that more than three-quarters of patients had at least one significant underlying medical condition, including diabetes, alcoholism, hypertension, cancer, or chronic skin ulcers.

Lower limbs were the most common site of infestation, accounting for 55% of reported cases, likely because chronic wounds and ulcers on the feet and legs provide an ideal location for female flies to lay eggs.

Deaths Highlight Severity

Six patients with confirmed screwworm myiasis died during the study period. Investigators concluded that one death was directly attributable to screwworm infestation, while the remaining patients died from serious underlying illnesses, including cancer, respiratory failure, encephalitis, or septic shock. Most of those patients were elderly and medically vulnerable.

Although mortality directly caused by screwworm remains uncommon, researchers emphasize that delayed diagnosis can lead to extensive tissue destruction, secondary bacterial infections, and permanent functional damage.

What This Means for Veterinarians

For veterinary professionals, the findings reinforce the importance of recognizing screwworm quickly in both companion animals and livestock.

Because animals often become infected before humans, veterinarians serve as one of the earliest warning systems for new outbreaks. Prompt diagnosis, reporting suspicious cases, treating affected animals, and complying with movement restrictions remain essential tools for slowing the parasite's spread.

The authors conclude that surveillance efforts should move ahead of the expanding parasite rather than waiting for new human cases to appear. They recommend strengthening diagnostic preparedness, maintaining strict animal movement controls, expanding sterile fly release programs, and continuing close collaboration between veterinary medicine, public health, wildlife management, and agriculture.

As New World screwworm continues its northward march, veterinarians may once again find themselves on the front lines of one of North America's most significant parasitic disease threats.