Environment
Jun 07, 2026
The first case of New World screwworm in 60 years has been confirmed in a Texas calf, raising conce…
The Return of a Eradicated ParasiteThe New World screwworm, a flesh-eating parasite which infects cattle and other warm-blooded animals, has been found in a calf in Texas, the US Department of Agriculture announced on Wednesday. This marks the first confirmed case in the United States since the parasite was declared eradicated in 1966, following decades of successful containment through biological barriers and sterile fly release programs.The screwworm is believed to have travelled from Central America to Mexico before being found in the calf in LaPryor, Texas, around 50km from the Mexican border. Experts suggest that a combination of factors, including possible disruptions to sterile-fly programs during the COVID-19 pandemic, increased movement of livestock and people, and favorable weather conditions, may have contributed to its re-emergence.Understanding the Screwworm ThreatA screwworm comes from the larvae of a screwworm fly (Cochliomyia hominivorax), and humans can indeed be infected. Female screwworm flies lay their eggs in scratches and wounds of warm-blooded animals, normally livestock or wild animals. The eggs hatch into hundreds of screwworm larvae which eat the living tissue of the infected animals.The flies are attracted to the smell of open wounds on the bodies of these animals, or sometimes even of humans. Newborn calves are particularly vulnerable because the post-partum navel has yet to scar. The larvae use their sharp mouths to burrow through the living flesh of their hosts for about a week before dropping to the ground to form a pupa.Screwworm can be devastating in cattle and wildlife, which can die from infection if untreated. As adult screwworm flies are capable of travelling many kilometers in search of hosts, infestations can spread quickly across wildlife populations, livestock herds and between humans.Economic Fallout for the Beef IndustryAn outbreak in the US could heavily impact the livestock industry and cause increased beef prices. The USDA predicts that could cost the Texas economy $1.8 billion in losses. Between mid-July and mid-August 2025, Mexico reported a 53 percent rise in the number of cases in animals, indicating the rapid spread of the parasite.Washington has halted cattle imports from Mexico for the past year, citing the insect's spread further into Mexico. The US typically imports more than one million Mexican cattle annually. The import suspension has already contributed to rising beef prices by tightening the supply of beef cattle, which dwindled after a drought forced ranchers on both sides of the border to reduce herds.Mexican cattle are usually fed and fattened on US farms for five to six months before slaughter, and a diminished slaughter rate can also raise beef prices. With US cattle herds already at a multi-decade low after severe drought, high feed costs have forced ranchers to shrink their herds.Regional Vulnerability and ResponseA quarantine zone spanning 20km (12.4 miles) has been established around the affected farm in Texas with no movement of any animals including pets. The infected calf is being treated and the larvae will be killed. The primary measures being implemented include vigilance, identification and isolation of cases, treatment and elimination of larvae, and controlling movement of animals.Dr Timothy Goldsmith, a veterinary medicine professor at the University of Minnesota, noted that homeless people can be especially vulnerable to infestation because they sleep outside and have less access to hygiene products and medical care. Last year, Mexico confirmed 41 human cases, primarily in the state of Chiapas.The parasite reappeared in Panama, Costa Rica, Nicaragua, Honduras, Guatemala, Belize and El Salvador before moving on to Mexico in 2023. While the precise reasons for its resurgence are unclear, experts point to climate change, increased global travel, and disruptions to agricultural monitoring programs as potential contributing factors.Future Outlook and Prevention MeasuresThe first confirmed case in the US during the latest outbreak represents a serious challenge for ranchers and could cause beef prices to rise further. After decades of eradication, most cattle ranchers no longer have the experience or tools to diagnose and treat screwworm, experts say.Infestations can be cured, but treatment is a time-consuming, pricey and labor-intensive process. A program of sterile male release is considered the best long-term method of controlling this fly, similar to the approach that successfully eradicated screwworm from the US in the 1960s.Authorities are likely to expand surveillance efforts along the US-Mexico border and potentially implement enhanced screening protocols for livestock entering the country. The incident highlights the ongoing challenges of biosecurity in an increasingly globalized world where pests and diseases can cross borders with ease.