Screwworm parasite spreads across North America; over 500 human cases in Mexico
The New World screwworm (Cochliomyia hominivorax), a fly whose larvae feed on living tissue, is spreading through Mexico and parts of Central America, raising concerns for both human and animal health. Mexican authorities have reported more than 500 human infections since the outbreak began, with a noticeable rise in cases during 2026.
Female screwworm flies lay eggs in open wounds or exposed mucous membranes, such as the nose, mouth, eyes and ears. The eggs hatch within about a day, and the larvae burrow into the wound, feeding on living flesh. This condition, known as myiasis, can cause severe tissue damage if left untreated, affecting muscles, cartilage and even bone.
A study published in the journal Emerging Infectious Diseases, issued by the US Centers for Disease Control and Prevention, examined 202 confirmed human cases in Mexico between April 2025 and March 2026. Researchers found that reported infections rose from roughly three cases a week in 2025 to more than nine a week in 2026, indicating a sharp upward trend.
More than half of the infections involved the legs or feet, while others affected the head and neck, including the nose, mouth and eyes. Six patients died during the study period, although only one death was directly attributed to the screwworm infestation. The affected individuals were predominantly older adults, with an average age of about 61 years. Around 72 per cent were men, and 77 per cent had at least one underlying health condition.
The New World screwworm poses a significant risk to livestock. Cattle and other animals are especially vulnerable because the larvae feed on living tissue, and a serious outbreak could lead to major economic losses for the livestock industry if the parasite becomes established in new areas. In humans, the condition is rarely fatal but requires prompt medical treatment to remove the larvae and prevent secondary infections.
The United States had successfully eradicated the screwworm in 1966 through decades of control efforts, including the sterile insect technique. Mexico was declared free of the parasite in 2003. However, the screwworm reappeared in Mexico in 2024 and has since spread through parts of the country and into Central America. In June 2026, the US Department of Agriculture confirmed a screwworm infection in a calf in Zavala County, Texas. Additional cases have since been detected in Texas and New Mexico.
The authors of the study emphasize that human and animal cases reflect the northward spread of the screwworm fly, and that spread is expected to continue. They recommend proactive surveillance, diagnostic preparedness, animal movement controls and fly-suppression activities at the leading edge of expansion, rather than waiting until human cases are detected. Coordination between human health, animal health, wildlife and agricultural sectors is essential to guide prevention and control measures.
US authorities are now working to prevent the parasite from becoming established in American livestock. The situation underscores the importance of cross-border cooperation in managing infectious disease threats that affect both humans and animals. Public health and agricultural agencies are monitoring the spread and implementing measures to contain it.
