Brazil is experiencing a huge outbreak of dengue fever, a sometimes deadly mosquito-borne disease, and public health experts say it is a harbinger of a coming emergence of cases in the Americas, including Puerto Rico. .

Brazil’s Health Ministry warns that it expects more than 4.2 million cases this year, surpassing the 4.1 million cases recorded by the Pan American Health Organization for all 42 countries. the region last year.

Brazil was expected to have a bad year for dengue – the number of virus cases typically rises and falls over a roughly four-year cycle – but experts say a number of factors, including El Niño and climate change, have significantly amplified the problem this year.

“Record heat in the country and above-average rainfall since last year, even before summer, have increased the number of mosquito breeding sites in Brazil, even in areas where there were few cases illness,” Brazilian Health Minister Nísia said. Trindade, he said.

The number of dengue cases has already soared in Argentina, Uruguay and Paraguay in recent months during the southern hemisphere summer, and the virus will spread across continents as the seasons change.

“When we see waves in one country, we will usually see waves in other countries, that’s how interconnected we are,” said Dr. Albert Ko, a Brazilian dengue expert and public health professor. at Yale University.

The World Health Organization you warned that dengue fever is quickly becoming an urgent global health problem, with record numbers of cases last year and outbreaks in places, like France, that have never reported the disease.

In the United States, Dr. Gabriela Paz-Bailey, chief of the dengue branch of the vector-borne diseases division of the Centers for Disease Control and Prevention, said she expected high rates of infection with the dengue. in Puerto Rico this year and that there would be more cases in the continental United States as well, including Florida, as well as Texas, Arizona and Southern California.

Dengue is spread by Aedes aegypti, a species of mosquito that establishes itself in new areas, particularly warmer, more humid parts of the United States, where we had never seen him until recent years.

In the United States, cases are still expected to be relatively few this year — hundreds, not millions — due to the prevalence of air conditioning and window screens. But Dr Paz-Bailey warned: “When you look at the trends in case numbers in the Americas, it’s scary. This is steadily increasing. »

Florida reported its highest number of locally acquired cases last year, 168, and California reported its first such cases.

Three-quarters of people infected with dengue have no symptoms, and of those who do, most cases will only resemble a mild flu. But some dengue infections are serious, causing headaches, vomiting, high fever and painful joint pain that give the disease the nickname “broken fever.” A severe case of dengue can leave a person weak for weeks.

And about 5 percent of people who get sick will progress to what’s called severe dengue, which causes plasma, the protein-rich liquid component of blood, to leak out of blood vessels. Some patients may go into shock, causing organ failure.

Severe dengue has a mortality rate of between 2 and 5 percent in people whose symptoms are treated with blood transfusions and intravenous fluids. However, when left untreated, the mortality rate is 15 percent.

In Brazil, state governments are setting up emergency centers to test for and treat dengue fever. The city of Rio de Janeiro declared a state of public health emergency over dengue on Monday, days before the start of the annual Carnival celebration, which brings tens of thousands of people to outdoor parties in day and night.

A high number of cases are being reported in Brazil’s southernmost states, said Ms. Trindade, the health minister, which are generally much colder than Rio and central and northern states. People in these areas will have little immunity to the disease from previous exposure.

Dengue comes in four different serotypes, which are like cousin viruses. Previous infection with one provides only short-term protection against infection with another, and a person who has previously had a dengue serotype is at higher risk of developing severe dengue following infection by another serotype.

“Right now, there are serotypes circulating in Brazil that have not circulated in 20 years,” said Dr. Ernesto Marques, associate professor of infectious diseases and microbiology at the University of Pittsburgh.

Brazil has launched an emergency campaign to vaccinate children in areas with the highest rates or risks of dengue transmission, using a two-dose vaccine called Qdenga, made by pharmaceutical company Takeda of Japan. Brazil has purchased 5.2 million doses to be delivered this year, plus an additional nine million to be delivered in 2025, and the company has donated another 1.3 million doses, effectively tying up most of the supply of Qdenga in the world. A company spokesperson said Takeda was working on a plan to increase supply, focusing on delivery to high-prevalence countries.

But even so, this is enough to cover less than 10 percent of the Brazilian population over two years. The only good news regarding dengue in Brazil at the moment is the publication of the results of clinical trials of a new vaccine tested by the public health research center Instituto Butantan in São Paulo. This vaccine requires only one injection and the trial found that it protected 80% of those vaccinated against developing dengue. The research center will ask the Brazilian government to approve the vaccine and has facilities to produce it, with a goal of starting providing shots in 2025.

For this epidemic, it is too late for vaccination to be truly effective, and public health authorities have few other ways to slow it.

“Insecticide resistance really limits what you can do in terms of controlling the mosquito population, and insecticide resistance is widespread,” said Dr. Paz-Bailey of the CDC. “What you can do is make sure that people have access to clinical care and that clinicians “I know what to do.”

Brazil’s medical centers are setting up extra beds for people with severe dengue fever, hoping to avoid the type of overload on health systems that has occurred during the Covid-19 pandemic and avoid deaths due to dengue.

“The old paradigm of dengue that affects children the most is not the case in Brazil: we have to think about the elderly, who are very vulnerable,” said Dr. Ko. It will be important for clinicians and the The public understands that children and the elderly should be tested for dengue fever at the first sign of symptoms, he said.

“All educated guesses were that this would be a bad year,” Dr. Marques said, “but we now know how bad it is. “It’s going to be very, very bad.”

Lily Moriconi contributed reporting from Rio de Janeiro.

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