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The growing threat of the screwworm—Cochliomyia hominivorax, the larval stage of a fly that affects livestock, wildlife, and even humans—is advancing toward Mexico. Recent studies conducted by specialists from the National Autonomous University of Mexico (UNAM) indicate that cattle smuggling may be accelerating the spread of the pest and increasing the country’s vulnerability.
“The results of the study, whose full version will soon be published, show that the parasite has been expanding across Central America at an average rate of 1.6 kilometers per day,” said Alejandro Zaldivar Gómez, specialist from the Laboratory of Disease Ecology and One Health at the Faculty of Veterinary Medicine and Animal Science, UNAM. “This rate of advance allows us to project its geographical reach in the short and medium term, providing critical information to design surveillance and control strategies in vulnerable areas. However, in recent months, the spread has accelerated significantly, with outbreaks reported as far as 300 kilometers from previously affected areas.”
Zaldivar Gómez notes that these long-distance movements are further evidence that illegal cattle movement is currently driving the spread of the fly in Central America—an assertion supported by regional health authorities. Official inspection reports from Honduras and Nicaragua, published by the World Animal Health Information System (WAHIS), describe how the parasite entered these countries through smuggled livestock.
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Map 1. Modeling of possible introduction routes of the screwworm into Mexican territory. The analysis evaluates potential entry points at three strategic sites identified as cattle collection and fattening centers, using least-cost pathways based on biogeographical conditions that favor the parasite’s dispersal and survival. Credit: Alejandro Zaldivar Gómez and Óscar Rico Chávez.
Map 2. Cattle smuggling routes across Central America. Credit: InSight Crime, Las entrañas del contrabando de ganado de Centroamérica hacia México, 2022.
“These long propagation distances can only be explained by cross-border cattle trafficking,” said Kurt Duchez, Regional Coordinator for Mesoamerica on Wildlife and Natural Resource Crimes at Wildlife Conservation Society (WCS). Duchez highlights this connection by comparing the map of projected routes for the spread of the screwworm, developed by the UNAM team, with the clandestine cattle movement routes documented by InSight Crime. “If the spread continues unchecked, it is highly likely that the parasite will follow the same cattle trafficking route that crosses the Maya Biosphere Reserve in Guatemala and extends into Mexico.”
The recent report of the first confirmed case by Guatemalan authorities adds further urgency to this likely route. If the screwworm reaches Mexico, areas with high cattle density could face severe economic and public health consequences. The following map shows the most vulnerable zones in the country’s south, at high risk of infestation due to their proximity to the border and concentration of livestock. These factors are compounded by illegal cattle trafficking, which facilitates the spread of the fly and increases the threat of infection.
Map 3. The map illustrates cattle population density in the region. Areas with higher livestock concentrations appear in darker shades, indicating zones of high vulnerability due to the availability of susceptible hosts. Credit: Alejandro Zaldivar Gómez and Óscar Rico Chávez.
Map 4. The map illustrates the temporal trend of the fly’s dispersal across Central America, modeled through linear regression. It highlights the earliest outbreaks detected in each country, as well as those reported in the WAHIS system of the World Organisation for Animal Health (WOAH). The analysis estimates a spread rate of 1.6 km per day, along with isolated outbreaks indicating rapid, localized dispersal events of up to 300 kilometers. This modeling suggests the parasite’s imminent arrival in Mexico if the current trajectory continues without control measures. Credit: Alejandro Zaldivar Gómez and Óscar Rico Chávez.
Once the border is crossed, containing the screwworm’s spread in Mexican territory would be extremely difficult. Past experiences have shown how complex and costly it is to eradicate this pest; its elimination from Mexico and the United States in 1991 cost more than USD 955 million in today’s currency.
Duchez emphasizes the urgent need to implement surveillance and control measures along the border to halt the entry and spread of the screwworm in Mexico. Without such measures, the country risks facing a health crisis and losing billions of dollars in public and livestock sector resources. The WCS expert argues that the containment strategy recently published by Mexican authorities will not be sufficient unless the root cause of the problem—cross-border cattle trafficking—is addressed. “Collaboration and coordinated action between regional governments and cattle producers are essential to contain this threat before it’s too late.”
[i] Zaldivar-Gómez, Alejandro; Rico-Chávez, Óscar. 2024. Modeling the Spread of Screwworm (Cochliomyia hominivorax) in Central America: Insights from Biogeographic Factors and Control Measures. In preparation.
[ii] Illegal cattle ranching includes livestock rearing in protected areas and Indigenous territories, as well as cross-border smuggling. This activity has severe environmental, economic, and health consequences in Central America and Mexico. It is linked to deforestation, illegal land appropriation, and community displacement, and it facilitates money laundering and the exploitation of natural resources.