First Screwworm Case in Mexico City: A Wake-Up Call Amidst Heatwave and Vulnerable Animals
Mexico City, May 8 – The detection of the first screwworm case in Mexico City has triggered an alert among specialists. On Tuesday, a dog in Tlalpan was reported with myiasis, an infection caused by screwworm larvae. The animal has been under supervision and control by the National Service for Agrifood Health, Safety, and Quality (Senasica) since April 26. While authorities maintain it’s an isolated incident, some experts are concerned about the parasite’s presence in a region not typically conducive to its development.
Adaptability of the Screwworm and Environmental Factors
Dr. Yazmín Alcalá, an academic from the Faculty of Veterinary Medicine and Zootechnics at UNAM, highlighted that while factors like altitude and typical temperatures in the Mexico Valley Metropolitan Area might slow the screwworm’s biological cycle, they don’t stop it entirely. “I resist believing this is an isolated case when it comes to an insect capable of flying kilometers. Indeed, the distribution of parasites, especially insects, is delimited by environmental and altitude conditions, but as far as we know, this fly is highly adaptable,” Alcalá told EL PAÍS.
The recent heatwave, with temperatures ranging from 32 to 45 degrees Celsius across Mexico City and much of the country, creates favorable conditions for screwworm propagation. The infection thrives in warm, humid, and wooded environments. Alcalá noted that deforestation and urban expansion have removed natural barriers, facilitating the fly’s movement into new areas. Livestock mobilization also contributes to the problem, as flies can travel hundreds of kilometers inadvertently in trailers used for animal transport.
Risk to Pets and Urban Environments
The Tlalpan case underscores the screwworm’s ability to adapt to urban settings. Although commonly associated with livestock and agricultural contexts-primarily affecting cows, horses, goats, or pigs-it can infest any living animal with open wounds, including dogs, cats, and even humans. “There are many stray dogs and cats in boroughs like Tlalpan or Iztapalapa that lack responsible owner care, making them highly susceptible to fights, wounds, and injuries. This is very concerning because, frankly, it opens the door for the fly to detect the scent of an open wound on a living animal and deposit between 100, 300, or 400 eggs,” explained Alcalá, who works in the parasitology department of the Veterinary Medicine faculty.
The public is advised to watch for potential signs of myiasis in pets, including abnormal secretions, pus, foul odor, behavioral changes (such as apathy, aggression, or loss of appetite), and excessive licking of lesions. In case of suspicion, immediate veterinary attention is recommended. “We must be vigilant and not relax surveillance measures. Senasica has done well with its limited budget, implementing good sanitary fences, treatments, cleaning, and applying healing agents. These measures must continue,” the specialist affirmed.
Screwworm’s Reappearance: A State Issue
The reappearance of the screwworm, considered eradicated in Mexico since 1991, presents a new public health challenge for the country. According to Senasica data, over 17,000 infestation cases had been recorded by March of this year.
Furthermore, amidst bilateral tensions with the United States, in July 2025, the government of President Donald Trump ordered the closure of the border to Mexican livestock following the detection of new cases. Despite this, the U.S. Department of Agriculture (USDA) has maintained its financial participation in containment and surveillance efforts, in coordination with Mexican authorities. “I am optimistic that they are working together. Regardless of political ideologies, Mexican and American scientists are uniting and proposing very viable joint projects. In this sense, I am very optimistic and see that in this case, science has overcome political differences,” Alcalá commented.
As part of the strategy to contain and eradicate the problem, Mexico plans to open a new sterile fly production plant in Chiapas in July of this year. The complex, located in Metapa de Domínguez, aims to become one of the most modern centers globally for this type of biological control, with a capacity to breed around 100 million insects. Simultaneously, early detection projects are also being developed. One such project is led by Dr. Carlos Guillermo Gutiérrez, director of the UNAM Faculty of Veterinary Medicine and Zootechnics, who has trained dogs to identify contaminated samples.
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