Mosquito Woes Threaten Puerto Rico as Floodwaters Recede

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By Tiffany Stecker

A tropical disease epidemic could be brewing in Puerto Rico as the island territory struggles to emerge from Hurricane Maria’s floods and wreckage.

Public health researchers are concerned that, in coming weeks or months, dengue fever transmission rates will increase in Puerto Rico as a result of rising populations of the virus-carrying Aedes aegypti mosquito. Scattered debris, stagnant pools of water, and a lack of air-conditioning due to power outages will create an ideal breeding environment for the mosquito.

And finding a way to kill the mosquitoes could be difficult as many island residents have opposed insecticide spraying in the past.

“All of these things combined may lead to another disease outbreak,” Matthew DeGennaro, an assistant professor at Florida International University who leads the university’s Laboratory of Tropical Genetics, told Bloomberg BNA.

The Zika virus, carried by Aedes aegypti, infected more than 40,000 Puerto Ricans last year. It is not expected to flare up again this year, but scientists are keeping an eye on rates of dengue, which can cause muscle pain, fever, nausea, headaches, and—in severe cases—death.

As of Aug. 25, the most recent date for which figures are available, the territory of 3.7 million people had nine confirmed cases of dengue fever, according to the World Health Organization. That number could rise quickly.

“Even in a good year, Puerto Rico has a difficult time with dengue fever,” Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, told Bloomberg BNA.

Mosquito Diseases Could Rise in Months

The Centers for Disease Control is developing a mosquito-control plan for the island with local authorities. For now, immediate rescue operations take priority, Sven Rodenbeck, chief science officer of the Centers for Disease Control’s hurricane response, told Bloomberg BNA.

The island has some time. Immediately after a hurricane, communities will see a rise in so-called nuisance mosquitoes—species that bite but don’t transmit West Nile Virus, Zika, dengue, chikungunya, or yellow fever.

Between two weeks and two months after a major storm hits, the number of mosquitoes that carry mosquito-borne diseases increases, Rodenbeck said. But he does not expect to see a substantial increase in the number of people getting sick from diseases spread by mosquitoes, neither in Puerto Rico nor in Texas and Florida, which were ravaged by Hurricane Harvey and Hurricane Irma.

But others are worried that the viruses could spread. West Nile Virus is of particular concern in Texas and Florida, where the Culex genus of mosquitoes carry the disease.

“We’re already seeing those in large numbers,” Shelly Redovan, deputy director of education and communication for Lee County, Fla.'s mosquito control district, said of the Culex mosquitoes.

Aedes mosquitoes that carry tropical diseases like dengue are increasing in the Miami area, indicating that Puerto Rico also will see an increase, DeGennaro said.

In Texas and Florida, efforts are underway to eradicate the pests. Redovan said her district, which covers about 1,000 square miles near Ft. Myers, Fla., already has exhausted its annual supply of mosquito-killing insecticides and has twice re-ordered the chemicals.

Debate Over Aerial Spraying

Puerto Ricans have a history of being skeptical of mosquito control efforts like aerial spraying of insecticides. Faced with widespread opposition from residents last year, officials on the island dropped a plan to deploy pest control aircraft to kill Zika-transmitting mosquitoes.

Aerial spraying is not the only solution, Rodenbeck said. Throwing out standing water and spraying chemicals on the ground is encouraged, and Puerto Rican officials will decide how to best control the mosquito infestations.

“It’s important to emphasize that local authorities are in charge,” he said.

DeGennaro of Florida International University thinks aerial spraying for Aedes mosquitoes would be largely ineffective in Puerto Rico because those species are primarily found in cities, making it more difficult to spray large areas.

In addition, DeGennaro thinks insecticides should be used sparingly like antibiotics: “You don’t want to use them unless you really need to.”

But Eric Wintemute, CEO of AMVAC Chemical Corp., the largest manufacturer of naled, the primary insecticide used for aerial spraying of adult mosquitoes, said the island’s viewpoint could imperil public health.

“I think there’s a fair amount of distrust Puerto Rico has with the U.S. government,” Wintemute told Bloomberg BNA. As a result, “maybe decisions are made that aren’t in the best interest of the overall population.”

Naled is an organophosphate insecticide, a class of pesticides that works by suppressing nerve function. Organophosphates can be highly toxic to humans, but the Environmental Protection Agency says the product is sprayed at very low doses—a couple of tablespoons per acre—and dissipates quickly.

Changing Views

Sonja Swiger, an extension entomologist with Texas A&M University, said she regularly encounters residents who oppose aerial spraying, but that the situation after Hurricane Harvey left some locals more open to the aerial applications.

“Some people are more understanding because of the fact that there was a disaster,” she said. “This is a rare practice in Texas.”

In a typical year, AMVAC sends out enough naled to spray 12 million to 16 million acres. This year, that figure has risen to 20 million acres, with possibly another 10 million to go, Wintemute said.

“We pulled back on some export sales just so that we could make sure we would do everything we could to meet demand,” Wintemute said. Most exports go to Mexico for use on crops.

Outbreaks Years Later

The spread of diseases after hurricanes is relatively understudied, Hotez of the Baylor College of Medicine said.

The effects can be felt several years after an event. A 2008 study found that rates of West Nile Virus doubled around New Orleans one year after Hurricane Katrina in 2005. The authors didn’t pin down a specific reason, but suggested that abandoned swimming pools and the holes left from uprooted trees may have been breeding grounds for the mosquito.

“After all three major hurricanes this season, provided there’s some funds to study this, we could learn an awful lot on disease transmission after hurricanes,” he said.

To contact the reporter on this story: Tiffany Stecker in Washington at

To contact the editor responsible for this story: Rachael Daigle at

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