Brandon Fletcher
The alarming resurgence of the New World screwworm fly (Cochilomyia hominivorax) poses significant threats not only to livestock but also to human health.
Recent research conducted in Mexico has revealed troubling data regarding local cases of screwworm infestations over the past few years. Since 2025, hundreds of individuals have experienced infestations by the fly’s maggot larvae, with at least one confirmed death attributed to these parasites. This evidence suggests that the screwworm’s reappearance in Mexico and, more recently, the United States, constitutes a serious public health concern for both animals and humans.
“Effective collaboration among human health, animal health, wildlife, and agricultural sectors is crucial for implementing effective prevention and control strategies,” the authors stated in their study published last week in the journal Emerging Infectious Diseases.
Understanding the Threat of Screwworm Infestations
Unlike many other fly species, screwworm maggots specifically target the living tissue of warm-blooded animals for nourishment. The adult flies lay their eggs on a host through open wounds or mucous membranes, and the maggots then use their spiral-shaped bodies and sharp mouth hooks to burrow deeper into the animal’s flesh. This burrowing process can lead to severe tissue damage and significantly increases the risk of dangerous secondary infections.
For several decades, eradication initiatives in the United States and other countries throughout the Americas successfully controlled the screwworm population, maintaining a barrier that extended as far south as Panama. this barrier has weakened in recent years, allowing the parasite to gradually reestablish itself further north. Mexico was declared free from screwworm in 2003, but reports of infestations in livestock began to surface again in late 2024.
While these parasites predominantly infest livestock such as cattle, they can also affect humans (a condition known as myiasis). The research team, which includes experts from Mexico’s public health agencies, examined documented cases of screwworm infestations in humans since the parasite’s reemergence in the country.
Between April 2025 and March 2026, authorities reported 204 human cases of screwworm infestation. Most of these cases involved men, with maggots typically found on the lower extremities of the victims. Tragically, six individuals succumbed to their infestations, with one death directly linked to the screwworm.
“The resurgence of the New World screwworm in Mexico and the related human cases of myiasis are significant public and animal health issues,” the authors noted.
More recent data from Mexico’s Ministry of Health indicates that over 500 human cases of screwworm have been documented in the country since the parasite’s return.
Key Insights and Precautions
In June of this year, the screwworm made its official reentry into the United States. As of August 9, there have been 45 confirmed cases in Texas and New Mexico. Fortunately, no human cases have been reported yet, according to the Centers for Disease Control and Prevention (CDC), and the overall risk to the public remains very low.
Despite the currently low risk, there are valid concerns. Reported human cases in Mexico have been notably higher in areas where animal infestations have surged. The situation has recently deteriorated in Mexico, with a rise in human cases observed in early 2026, as highlighted in the report.
“Given the correlation between animal and human cases indicating the northward spread of the New World screwworm fly, proactive measures such as surveillance, diagnostic preparedness, animal movement restrictions, and fly-control initiatives should be implemented ahead of human cases being identified,” the authors urged.
In the United States, officials have enacted containment strategies around all identified cases. These measures include the release of sterile male flies, a critical component of the eradication efforts that previously curtailed the screwworm population. the effectiveness of these interventions in preventing a full-scale resurgence of the parasitic fly in the U.S. remains uncertain. It is evident that wherever the screwworm fly establishes a presence, both livestock and human populations face significant risks.
Daniel Mercer
