BUENOS AIRES — Argentina is grappling with its worst outbreak of dengue in seven years as the population of Aedes aegypti mosquitoes, which also transmit the Zika virus, expands in the subtropical northeast of the country.
The outbreak, which some officials are describing as an epidemic, has the authorities in Misiones and Formosa Provinces scrambling to arrest the spread of the virus. There were nearly 4,900 reported cases of dengue in Argentina in the first five weeks of the year, according to data compiled by the Pan American Health Organization. Experts say the figure could be 10 times higher.
Officials have attributed only one death to the virus, but with weeks of the Southern Hemisphere’s summer remaining — providing ideal breeding conditions for the mosquitoes — the outbreak has not yet peaked, experts say.
In 2009, the year of Argentina’s last severe dengue outbreak, described as perhaps its worst ever, there were nearly 27,000 reported cases and five deaths.
“We’re expecting something even worse than in 2009,” said Hernán G. Solari, a physics professor at the University of Buenos Aires who researches dengue and Aedes aegypti mosquitoes. “The outbreak will keep growing until March or April.”
In Argentina, there have been only a handful of cases of the Zika virus that has hit neighboring Brazil. And those infections were transmitted abroad, according to local news media reports.
Zika, which some researchers have linked to microcephaly, a disorder that causes babies to be born with small heads, has been concentrated in the northeast of Brazil, more than 2,000 miles from Misiones. That makes it difficult for the virus to reach Argentina.
But the high density of Aedes aegypti mosquitoes, benefiting from a muggy climate and ideal breeding grounds in poor urban areas, like discarded buckets, plastic bottles and old tires partly filled with rainwater, means the provinces could be at risk, experts say.
“I think the conditions are there for Zika outbreaks,” said Jorge Osorio, a professor of pathobiological science at the University of Wisconsin who arrived this week in Misiones to advise the provincial government and investigate dengue prevention methods. “We have a mosquito population and we have people traveling from Argentina to Brazil.” Misiones is in northeast Argentina, bordering three Brazilian states and Paraguay.
Nicolás Schweigmann, a biologist at the University of Buenos Aires who tracks mosquito populations here, said this city — which has large numbers of mosquitoes in summer — would also be under threat. “Wherever you have the vector, there is easily the possibility of transmission,” he said.
The national Health Ministry did not respond to a request for comment. But Dr. Jorge D. Lemus, the health minister, told the Clarín newspaper that he was concerned there could be an outbreak when Argentines return from their summer vacations, often taken in infected areas of the continent.
Dengue is far more common in countries like Brazil and Colombia, where there are hundreds of thousands of cases a year, but the outbreak in Argentina is causing alarm among experts, who say mosquitoes are widening their range across the Americas.
Argentina had rid itself of the Aedes aegypti mosquito in the 1960s during a regionwide campaign to extinguish the insect. But it re-emerged in 1986 after Brazil suffered a reinfestation, and there have been periodical outbreaks of dengue in northern provinces since the mid-1990s.
Beyond Misiones and nearby Formosa, there have been secondary outbreaks this year in other provinces, including Buenos Aires, Argentina’s largest and most populous. Scientists have even been mapping the risk of dengue nationwide to help officials combat the virus.
Dr. Jorge Derna, a physician in Posadas — one of the worst-hit cities — who works at both public and private clinics, said he was seeing about 10 patients a week with symptoms of dengue. Some have been in such excruciating pain, he said, that rather than sit for hours in waiting rooms, they have chosen to lie on the floor.
Because provincial hospitals and clinics might be overwhelmed, or because symptoms are sometimes mild, many cases go unreported.
There is no specific treatment for dengue, also known as breakbone fever, which causes nausea, headaches and extreme aches in muscles and joints. Doctors normally recommend painkillers and rest to help the fever pass.
On rare occasions, the virus can become fatal. There are four types of dengue, which gives the virus the opportunity to re-establish itself in people who have developed immunity against one of the strains.
It is unclear why mosquito populations have proliferated, although many experts point to the humidity and rainfall caused by El Niño, a weather pattern that recently led to flooding across swaths of northeastern Argentina and Paraguay. Still, they say, the 2009 outbreak occurred after a drought.
Urbanization, which has favored the rise of dengue worldwide in recent years by amplifying breeding conditions, is another likely factor. A surge in border crossings as Argentines shopped for goods that were cheaper in neighboring countries like Brazil may have also helped transmit the virus.
The authorities in Misiones are now moving furiously to fumigate. But Dr. Lemus, the health minister, has warned that the mosquitoes may have become resistant to some pesticides. The authorities are also clearing away debris where water can collect and ordering residents to do the same.
Some critics say those efforts are too reactionary. “We have a cultural problem,” said Dr. Solari, the physics professor. “There’s a lack of awareness: We act when the threat is already upon us.”
In Buenos Aires, the municipal authorities always move to tackle dengue at this time of year. But this summer they have put special emphasis on the efforts, officials said, emptying flower vases in cemeteries, a common mosquito breeding ground, and putting up posters warning people to prevent their backyards from turning into a haven for the insects. With inflation accelerating, the Argentine government has also temporarily reduced the price of a popular repellent.
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