By Paulo Prada
RIO DE JANEIRO (Reuters) - Battered by a presidential impeachment and the worst recession since the Great Depression, Brazil is getting a rare bit of relief as Rio de Janeiro prepares to host the Olympics: declining numbers of Zika infections.
Since the start of the Zika outbreak, which wreaked havoc across Brazil's northeast earlier this year, many physicians and would-be visitors have worried the Games could be a catalyst to spread the virus internationally.
Some athletes, including the world's top-ranked golfer, have said they will stay home to avoid infection because of concerns over health complications caused by Zika, notably microcephaly, a birth defect among babies of pregnant mothers infected by the virus.
Recently, however, cooler-than-normal temperatures during the southern hemisphere winter, coupled with efforts to eliminate breeding grounds for the mosquitoes that spread Zika, have cut infections by about 90 percent from a February peak, when more than 16,000 cases were reported in one week.
In Rio, an ebbing of Zika fears is reassuring authorities just over a month before the Olympics start on Aug. 5.
"Rio is not the Zika nightmare that people worried about," says Pedro Vansconelos, director of the Evandro Chagas Institute, a Brazilian research facility, and a member of the World Health Organization's emergency committee for Zika.
Hot, humid weather, which fosters mosquito reproduction, helped Zika spread rapidly from Brazil to more than 60 countries and territories.
A hot summer in Rio at the start of the year led to a spike in other mosquito-borne illnesses, such as dengue and Chikungunya. But the local outbreak of Zika was not as severe or as widespread as in the northeast, confounding scientists.
Now, with seasonal temperatures in Rio falling as low as 8 Celsius (46 Fahrenheit), infection rates for Zika, dengue and Chikungunya are waning.
According to the state government, Zika infections in Rio fell from more than 3,500 per week in February to less than 200 recently. Cases of dengue fell from 4,500 per week to under 500, while Chikungunya dropped from nearly 700 to under 50.
Statistics paint only a partial picture of Zika infections because the illness presents only minor symptoms in most people and remains difficult to diagnose.
And if temperatures rise, as they have during waves of summer-like heat in recent winters, the mosquito population could rebound and lead to more Zika transmission even before hot weather returns in earnest later in the year.
But Brazil's health ministry, local officials and Olympic organizers insist Rio will be safe for visitors. Public health workers are scouring Olympic venues and tourist sites for puddles, stagnant water and other areas mosquitoes lay eggs.
The spread of Zika depends mostly on Aedes aegypti, a mosquito that thrives in steamy urban environments. It transmits viral infections by biting an infected person and then biting someone else.
There is much unknown about Zika, including exactly how it causes complications like microcephaly.
The defect, marked by abnormally small head size that can lead to developmental problems, has been confirmed in over 1,600 infants in Brazil, mostly in the northeast. It has begun afflicting babies in other countries.
Concerns over the virus led a group of more than 200 health experts, bioethicists and lawyers to argue in a letter to the World Health Organization last month that the Olympics should be postponed or moved.
The agency responded this month by saying "there is very low risk of further international spread of Zika virus" because of the Olympics.
Epidemiologists say many more people are already traveling to countries with infections than the 500,000 visitors expected in Rio for the Games.
Using metrics for mosquito-borne illnesses in Rio during August in recent years, virologists have put the chances of Zika infection for Olympic visitors at roughly 3 in every 100,000. For a malady that often remains asymptomatic, even fewer would feel ill.
"There are people trying to blow this out of proportion. But one of the few things we do know is that the Olympics are not going to be a major contributor in epidemiological terms," said Nikos Vasilakis, a virologist at the University of Texas who has been studying the outbreak in Brazil.
HARD TO PREDICT
What he and many other scientists do not know is why the disease struck Brazil's northeast so severely compared with other corners of South America that have similar climates and the teeming, disorderly cities where Aedes aegypti thrive.
Brazil did not begin compiling Zika statistics until earlier this year. Given the patchy data and the high number of unreported cases, some researchers say microcephaly statistics may provide a better indicator of the outbreak's severity.
Since Brazil's government first noticed the link between Zika and the birth defect, 1,410 infants in the northeast have been diagnosed with it, including 366 in the hardest-hit state of Pernambuco. In Rio, a state whose 16 million people represent almost twice Pernambuco's population, just 72 cases have been confirmed.
Numerous hypotheses exist for the disparity – from the possibility that Zika is worse for people infected with other viruses beforehand to whether pesticides have made northeasterners more vulnerable.
But studies to prove any hypothesis will take years.
"Many, many things need to be looked at. How many people were asymptomatic? How many cases lead to microcephaly? What genetic factors are involved?" says Mauricio Nogueira, director of the virology laboratory at the São Jose do Rio Preto medical school in São Paulo state.
Efforts to forecast outbreaks of Zika, dengue and Chikungunya are difficult. Rates of infection for all three have worsened in recent years, aided by hotter temperatures overall, but the spread of each has been different and unpredictable.
The state of Rio during the first six months of 2016 reported nearly 60,000 dengue diagnoses, almost 50 percent higher than the same period in 2015. In nearby São Paulo, Brazil's most populous state, dengue diagnoses have more than tripled to nearly 670,000 cases.
Reports of Chikungunya, which at first appeared in small pockets in Brazil, have grown too, ballooning in both Rio and São Paulo from less than 100 in each of those states last year to more than 2,000 this year.
What is clear, scientists say, is that the viruses, once rare in Brazil and the rest of the Americas, have adapted to the region, posing serious public health challenges that go well beyond the risk posed by a big event.
"Zika is here to stay," says Vansconelos. "It has already spread rapidly and you can't blame the Olympics for that."
(Reporting by Paulo Prada; Editing by Daniel Flynn, Bernard Orr)