A parasitic fly capable of infesting open wounds in animals and humans is advancing closer to the United States boundary. This pest, known as the screwworm, lays eggs in living tissue. Upon hatching, the resulting maggots consume the host's flesh, causing severe lesions that can ultimately prove fatal.

Screwworm Presence Confirmed Near US-Mexico Border

The presence of the screwworm has now been verified across Mexico. Specifically, the fly has been confirmed in the Mexican states of Tamaulipas and Nuevo Leon. These two regions share a direct border with the state of Texas, increasing the immediate risk of entry into the US.

Impact of Surveillance Funding Changes

According to Professor Richard Wall, a biologist at the University of England, the growing spread is likely linked to several factors. He notes that surveillance and prevention programs are inherently expensive. This expense coincides with recent US federal budget reductions and decreased foreign aid, which have weakened control efforts in Central America.

The United Nations' global health security program reduced its screwworm monitoring after US funding ceased in March 2025. However, subsequent announcements have indicated that new federal funding for control measures has now been made available.

Historical Context and Current Threat Assessment

Devastating History in the US

The Animal and Plant Health Inspection Service (APHIS) at the USDA has labeled the screwworm a "devastating pest." Their report warns it causes "serious, often deadly damage to animals and people where the pest spreads." Historically, US beef producers suffered significant losses before the 1950s due to infestations.

For instance, Texas alone recorded an estimated 230,000 livestock cases and 55 human cases in 1935. Eradication efforts proved highly successful in the latter half of the 20th century, leading to the fly's removal from the US by 1966.

Eradication Method and Modern Challenges

The successful eradication relied on the sterile insect technique. This method involved mass-rearing the flies and sterilizing them using radiation. Releasing these sterilized males resulted in mating with wild females, producing no offspring and thus collapsing the population.

Professor Wall cautions that current outbreaks may be harder to manage. He states that the sterile insect technique is most effective when dealing with isolated insect populations, which may not be the case now.

Public Health Warnings and USDA Response

The US Centers for Disease Control and Prevention (CDC) advises that individuals living in or traveling to areas where screwworm flies are present face a higher risk. Symptoms in humans can include feeling or seeing maggots moving within a skin wound, sore, or even in the ears, eyes, or nose.

Infestations cause painful skin wounds that deteriorate rapidly, often accompanied by a foul odor and bleeding. Secondary infection of these wounds may also lead to systemic symptoms such as fever or chills.

The USDA confirms the screwworm is not currently established in the US. They are currently "leading an aggressive national, One Health response to keep the out of our country." The department urges the public to immediately contact a veterinarian if they observe suspicious wounds or infestations in livestock. Human health providers should be contacted by anyone suspecting they may be infested.