New World Screwworm Returns to Humans as Cases Cluster Across Southern Mexico
Human myiasis has reappeared in Mexico with the return of Cochliomyia hominivorax, the New World screwworm fly that had been eliminated from the country in 2003. A retrospective review of national surveillance data from April 13, 2025, through March 14, 2026, identified 204 confirmed human cases, with 202 cases available for case-level clinical analysis. The pattern is not random. It is concentrated in the south, centered on Chiapas, and already extending into neighboring states that have long been considered vulnerable to northward spread.
The clinical problem is simple in mechanism and serious in consequence. Adult female flies seek out wounds, inflamed tissue, mucous membranes, and body orifices, then deposit eggs that hatch within 12 to 24 hours; the larvae feed on living tissue for 5 to 7 days. In practical bedside terms, the infestation can escalate quickly from a neglected wound or mucosal lesion into destructive tissue invasion, bacterial superinfection, pain, and disability. In settings where clinicians may not immediately suspect a parasitic cause, recognition is often delayed until larvae are seen.
Geographic Spread Tracks the Southern Border and Coastal States
The first confirmed human cases in the surveillance period appeared in Chiapas during epidemiologic week 16 of 2025. The pathogen then surfaced in Campeche, Yucatán, Tabasco, and Oaxaca over the following months. By epidemiologic week 10 of 2026, confirmed human cases had been reported in 12 of Mexico’s 32 states, a marker of widening circulation rather than isolated importation.
State-level burden was heavily skewed. Chiapas accounted for 119 of 204 confirmed cases, or 58.3%. Yucatán followed with 23 cases, then Oaxaca with 16, Quintana Roo with 12, Veracruz with 8, Guerrero and Tabasco with 7 each, Campeche with 6, Puebla and San Luis Potosí with 2 each, and the State of Mexico and Michoacán with 1 each. At the municipal level, Tapachula, Huixtla, and Mérida carried the heaviest loads, together representing nearly one-third of all analyzed cases. For frontline clinicians, those numbers mean the risk is no longer theoretical in endemic-border regions; the fly is established enough to leave a recognizable geographic signature.
Older Adults, Men, and Lower-Limb Infestations Dominate the Case Series
Among the 202 cases with sufficient individual-level information, 145 occurred in men and 57 in women. Mean patient age was 61.1 years, with men averaging 59.2 years and women 65.9 years. The age distribution, spanning 12 to 93 years, places much of the burden in older adults, a group more likely to have chronic wounds, mobility limitations, or impaired self-care. Those factors matter biologically because screwworm larvae exploit disrupted skin barriers and prolonged exposure of tissue.
Lower limbs were the most frequent anatomic site, reported in 111 cases, or 55.0%. Neck involvement followed, then nose, mouth, eyes, head, upper limbs, trunk, ear, genitalia, and abdominal cavity. Several patients had more than one anatomic site recorded, although the surveillance reports did not allow separation of distinct infestations from contiguous spread or multiple descriptive terms for the same lesion. Even with that limitation, the anatomic pattern points to a disease that often follows chronic lower-extremity wounds, ulcers, and areas difficult to keep protected in hot, humid environments.
Concurrent Illnesses Suggest the Patients Most at Risk
A large share of patients had additional medical problems. One concurrent condition was recorded in 53.0% of cases, two in 15.8%, three in 4.5%, and four in 3.5%. Overall, 76.7% had at least one concurrent condition. Diabetes mellitus was the most frequent, appearing in 41 cases, followed by alcoholism, hypertension, neoplasia, and conditions associated with cutaneous ulcers.
Those comorbidities fit the biology of infestation. Diabetes impairs wound healing and local immune function, alcoholism often tracks with poor nutrition and reduced wound care, and ulcerative skin disease creates the open tissue that gravid flies seek. Neoplasia and other chronic illnesses can further reduce mobility, hygiene, and the ability to detect an infestation early. In this context, screwworm myiasis behaves less like a rare zoological event and more like an opportunistic infection of damaged tissue in medically vulnerable patients.
Deaths Were Rare but Clinical Severity Was Real
Six deaths occurred among patients with confirmed human New World screwworm myiasis during the study period. At the time of reporting, one death was attributed directly to myiasis. The other recorded causes of death included basal cell carcinoma, squamous cell carcinoma, encephalitis, respiratory failure, and septic shock. Those data do not support a simplistic reading that screwworm was the sole driver of mortality in every fatal case, but they do reinforce a more important point: this infestation appears in patients who are often already medically fragile, and it can compound existing disease in dangerous ways.
Management remains straightforward in principle and labor-intensive in execution. Diagnosis depends on identifying larvae. Treatment includes removal of larvae, wound or lesion care, management of bacterial complications, and analgesia; ivermectin may be used in selected cases. In resource-constrained settings, the main barrier is not the absence of a treatment algorithm but late recognition, especially when wounds are chronic, foul-smelling, or hidden in areas that are not inspected carefully.
One Health Surveillance Is the Only Plausible Containment Strategy
The reemergence of C. hominivorax in humans cannot be separated from animal health. The same obligate parasite infests livestock, wildlife, companion animals, and people, so animal cases can function as an early warning system for local circulation. That is why the surveillance response has to be integrated across veterinary and human sectors, with coordinated case finding, movement controls, diagnosis, reporting, and fly suppression. Sterile insect technique previously drove eradication across North America and much of Central America, and continued sterile-fly release has served as a biological barrier against reinvasion from endemic areas farther south.
Environmental conditions also shape risk. Moderate to heavy rainfall and temperatures around 35°C favor fly development, and a northward-moving outbreak has been modeled at 1.2 to 1.9 km per day. Against that backdrop, the Mexican case series is more than a retrospective count. It is an early clinical warning that human disease is following animal circulation, and that clinicians in areas along the southern coast need to inspect wounds with a higher index of suspicion, especially in older adults with diabetes, ulcers, malignancy, or other chronic illness.
What happens next will depend on whether surveillance outpaces fly movement, whether clinicians recognize the infestation early, and whether animal and human health systems coordinate tightly enough to interrupt transmission before the parasite advances farther north.