The growing threat of the screwworm—the larva of the Cochliomyia hominivorax fly that affects livestock, wildlife, and even humans—is advancing towards Mexico. Recent studies conducted by specialists at the National Autonomous University of Mexico (UNAM) suggest that cattle smuggling may be accelerating the parasite's spread, heightening the country’s vulnerability.
“Our findings, soon to be publishedi, indicate that the parasite has been spreading across Central America at an average rate of 1.6 kilometers per day,” explained Alejandro Zaldivar Gómez, a specialist at the Laboratory of Ecology of Diseases and One Health within UNAM’s Faculty of Veterinary Medicine and Zootechnics. “This spread rate allows us to project its geographic reach over the short and medium term, offering critical data for designing surveillance and control strategies in high-risk areas. Yet in recent months, the spread has accelerated significantly, with new outbreaks reported up to 300 kilometers away from previous infection points.”
According to Zaldivar Gómez, these long-distance movements provide further evidence that illegal cattle traffickingii is likely fueling the fly’s expansion through Central America—a claim backed by regional health authorities. Official inspection documents from Honduras and Nicaragua, published by the World Animal Health Information System (WAHIS), detail how the parasite has entered these countries via smuggled cattle.
Map 1. Modeling of potential routes of screwworm introduction into Mexican territory. The analysis assesses potential fly entry points at three strategic Mexican sites, identified as cattle aggregation and feeding centers, via lower-cost routes shaped by biogeographic conditions that favor the parasite’s dispersal and survival. Credit: Alejandro Zaldivar Gómez and Oscar Rico Chávez.
Map 2. Cattle trafficking routes through Central America. Credit: InSight Crime, Cash Cows – The Inner Workings of Cattle Trafficking from Central America to Mexico, 2022.
“These long-distance spread events can only be attributed to cross-border cattle trafficking,” explains Kurt Duchez, Regional Coordinator for Mesoamerica on Wildlife and Natural Resources Crimes at the Wildlife Conservation Society (WCS). Duchez emphasizes this link by comparing the map of projected screwworm spread routes, developed by the UNAM team, with the clandestine cattle smuggling pathways documented by InSight Crime across the region. “If the spread is allowed to advance unchecked, it is highly likely to follow the cattle trafficking route through Guatemala’s Maya Biosphere Reserve and into Mexico.”
The recent report of the first confirmed case by authorities in Guatemala adds seriousness and urgency to this probable route. Should the screwworm reach Mexico, areas with dense cattle populations could face severe economic and public health impacts. The following map highlights the most vulnerable regions in the southern part of the country, at high risk of infestation due to their proximity to the border and high cattle density. Illegal cattle trafficking exacerbates these risks by further facilitating the fly's spread and heightening the threat of contagion.
Map 3. This map shows cattle population density across southern Mexico, with darker shades indicating areas of higher concentration and increased vulnerability due to the abundance of susceptible hosts. Credit: Alejandro Zaldivar Gómez and Oscar Rico Chávez.
Map 4. This map illustrates the screwworm’s spread across Central America over time, developed with a linear regression model. It highlights the first detected outbreaks in each country, along with those reported in the World Organization for Animal Health’s (WOAH) WAHIS system. The analysis estimates a spread rate of 1.6 km per day, with isolated outbreaks suggesting rapid, localized dispersal events of up to 300 kms. This model points to an imminent arrival of the parasite in Mexico if its current trajectory continues unchecked. Credit: Alejandro Zaldivar Gómez and Oscar Rico Chávez
Once the screwworm crosses the border, controlling its spread within Mexican territory will be challenging. Past experiences have demonstrated the complexity and cost of eradicating this pest; efforts in Mexico and the U.S. in 1991 exceeded $955 million in today’s currency.
Duchez emphasizes the urgent need for implementing surveillance and control measures at the border to prevent the entry and spread of the screwworm in Mexico. Without these measures, the risk of a health crisis and losses amounting to billions of dollars for the public sector and livestock industry remains alarmingly high. He asserts that the containment strategy proposed by Mexican authorities will be inadequate unless it addresses the underlying issue: cross-border cattle trafficking. “Coordinated collaboration and action among regional governments and livestock producers are vital to contain this threat before it’s too late.”
[i]Zaldivar-Gómez Alejandro, Rico-Chávez Oscar. 2024. Modeling the Spread of Screwworm (Cochliomyia hominivorax) in Central America: Insights from Biogeographic Factors and Control Measures. In preparation.
[ii] Illegal cattle ranching encompasses cattle ranching in invaded protected areas and indigenous territories, as well as cross-border cattle smuggling. This activity generates serious environmental, economic and health consequences in Central America and Mexico. It is linked to deforestation, illegal land grabbing and displacement of communities, as well as facilitating money laundering and the exploitation of natural resources.