Record numbers of cases of the mosquito-borne virus have been reported in the Southern Hemisphere summer, and the surge is likely to move north.

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

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

Brazil was due for a bad dengue year — numbers of cases of the virus typically rise and fall on 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.

“The record heat in the country and the above-average rainfall since last year, even before the summer, have increased the number of mosquito breeding sites in Brazil, even in regions that had few cases of the disease,” Brazil’s health minister, Nísia Trindade, said.

Dengue case numbers have already soared in Argentina, Uruguay and Paraguay in the last few months, during the Southern Hemisphere summer, and the virus will move up through the continents with the seasons.

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