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San Miguel Works All Year to Keep Disease-Spreading Mosquitoes under Control

Mosquito Aedes

Mosquito Aedes

By Jesús Aguado

After coming in first place in all of México in the prevalence of dengue fever not too many years ago, the state of Guanajuato is now clean of the virus. In 2017, there were 4,100 cases registered, and that number dropped to 37 in 2018. In 2019, none has yet been reported.

However, the tropical and subtropical mosquitoes that spread the disease have adapted to survive in climates like San Miguel de Allende’s, say local health officials, and they tend to live in urban areas, according to the World Health Organization. In addition, tourist cities such as Leon, Guanajuato, and above all San Miguel have a greater potential to experience an outbreak due to the number of visitors that pass through them, including visitors from areas of the world where the disease is endemic.

Humans are the main carriers of the virus, infecting aedes aegypty mosquitos when the mosquitos bite them. The mosquitoes then go on to infect the next person they bite, turning that person into a carrier, even if they don’t exhibit symptoms.

Aedes aegypty mosquitoes are a different species than the common Culex mosquito, for example, also known as the house mosquito.

The more aedes aegypty mosquitoes there are in an area, the greater the chance that someone carrying the dengue, chikungunya, or zika virus—all of which are carried by this species of mosquito—could pass it on to others, which is why the state government works all year to minimize infestations of this particular mosquito in urban areas like San Miguel.

The state Secretaria de Salud (Health Department) handles the campaign to exterminate the aedes aegypty mosquito and its larvae. Technicians travel through the municipalities, warning residents and spraying households with insecticide or larvicide when necessary.

A female mosquito—which requires blood to produce her eggs—can lay as many as 700 eggs, and “insecticides do not kill eggs,” says Martín Millán, chief of the Juridicción Sanitaria II (Second Sanitary District) for the state of Guanajuato.

Mosquito eggs need wet conditions to hatch, Millán said. Even if the water source where the eggs are laid goes dry, eggs can survive for months or even years, waiting for the necessary wet conditions to return.

Although the city of San Miguel has not seen a single case of dengue in the last two years, according to Millán, the state campaign against the vector mosquitoes goes on all year long, with two months of especially intense spraying of outdoor areas to exterminate adult mosquitoes. This year, the next spraying period will be April 8–12. The other spraying period this year will be in August.

He did not disregard the possibility that a case could appear in the city.

“San Miguel is at risk because it receives visitors from other places,” he said. “It does not have the [normal] climatic characteristics for the reproduction of the mosquito, but the mosquito that transmits the virus has adapted.”

A campaign in three stages

The first stage of the anti-mosquito campaign involves controlling any new mosquito larvae development. Workers cover public places and visit private homes, asking people to clean their patios, rooftops, and water containers to eliminate eggs that may have attached themselves to surfaces and will develop into larvae if left unchecked. Once eggs attach themselves to the sides of a water receptacle, for example, they hatch in 48 hours.

This particular species of mosquito likes to lay its eggs in water containers. Millán points out that these water containers can be almost anything, such as bottles, cans, containers, and jars left on rooftops or patios.

“Even a bottle cap can serve as a receptacle,” Millán said. “The very rooftops that are covered become nurseries because they are a sort of pool. The mosquito doesn’t need large quantities of water; puddles and covered drainage is sufficient,” he said.

In cases where larvae may already be present, Health Department officials apply chemicals to exterminate them.

In the second stage, where a house is already infested with mosquitoes, spraying is done indoors. This breed of mosquito bites in the early morning hours and at dusk, and so it is crucial to use repellent, Millán said.

The third stage of the campaign consists of area spraying. In this stage, trucks go through the streets at night spraying nebulized insecticide designed not to exterminate anything but mosquitoes.

“During nebulization [spraying insecticide in a mist form], insecticide is applied to public spaces. The trucks come out and spray the areas where there is high infestation, and it’s done in a preventive manner … Nebulization eliminates adult mosquitoes. The activity is controlled and done with care. The machines are calibrated to spray low quantities, and the only thing they will kill are the mosquitoes. It doesn’t kill crickets, for example. The difference in weight [of the animal] is significant. It doesn’t kill dogs, cats, or people. The chemical can cause sneezing or irritation, due to the smell, but it does not represent a danger to any other species than mosquitoes,” he said.

Giant incubators?

Given that standing water is an essential element for mosquito breeding, one might think that San Miguel has some potentially gigantic mosquito incubators, namely the Presa Allende, the Presa de la Cantera, and Las Colonias and Calderón.

In reality, however, these bodies of water do not represent any risk whatsoever. Mosquito eggs need standing water to be viable, and all the abovementioned places’ water is constantly in enough movement to provide a breeding ground.

Even if larvae did manage to hatch, their numbers would be controlled by other predator species around them, according to Millán.

“The ecosystems of these places do not allow the proliferation of mosquitoes, nor do they provide a food chain,” he said.

Instilling a prevention mindset

La Biblioteca employees recently received training on avoiding the proliferation of mosquitoes and the prevention of dengue, chikungunya, and zika within their spaces. They created models for the prevention of mosquito breeding and the spread of mosquito-borne diseases.

The Biblioteca is the first organization in San Miguel that the state’s Health Department has certified free of zika, dengue, and chikungunya.

The project, called La Primera Jornada Vacaciones sin Dengue (The First Day of Vacation Without Dengue), will get its start at the library on April 9 at 8pm.


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