RIO DE JANEIRO/BOGOTA/MIAMI – Alarmed by the growing global scare about the spread of the Zika virus, Brazilian officials and Olympic organizers are telling would-be visitors to the Games to fear not.
The month of August, when Rio de Janeiro will host the Olympics, is mid-winter in the Southern Hemisphere so the weather will be drier and cooler than usual in the tropical city, providing a less hospitable climate for the mosquito that spreads the virus.
“There’s not a history of much activity for the mosquito at that time,” Rio’s mayor, Eduardo Paes, recently told reporters.
But it’s not that simple, scientists say.
True, rainfall and temperatures for the month are generally below the annual average.
But, even if less active than in warmer months, the mosquito, Aedes aegypti, never actually disappears.
Its eggs, which can lay idle for more than a year, can hatch in a matter of minutes with any quick surge in humidity or heat, which have been common in recent years, even in the tropical winter.
A Reuters review of municipal health records shows that mosquito-borne infections in August of some years can be as bad or worse than in the usual peak months for infections in others.
“Weather is relative,” said Nancy Bellei, director of clinical virology at the Brazilian Society of Infectology. “You can’t just hope for cool temperatures and hope that the virus won’t spread.”
In addition to climate, contagion depends on other factors, like whether a virus is actually circulating near a given population, whether those people have had previous exposure to it, and how prevalent the virus may be at any particular time.
Aedes aegypti, which transmits the virus by biting an infected person and then moving on to bite another, wasn’t even known to carry the Zika virus in the Americas until last year.
If they fall ill, patients generally show signs of Zika or the related viruses of dengue and Chikungunya, which are carried by the same mosquito, within a week of the mosquito bite.
Municipal health data for dengue in Rio, a city of more than 6 million people, show how variable the rate of infection can be. Of all the cases recorded each year since 2011, infections in August ranged from less than 1 percent of the annual total, in 2012 and 2013, to nearly 6 percent in 2014.
But August of some years can be worse than the typical peak months of others. Consider last August, when Rio recorded 794 cases of dengue. The figure is greater than the 773 cases reported in 2014 during the combined months of March, April and May, normally three of the worst months for infection.
This year, the El Niño weather phenomenon, which is causing higher temperatures in southern Brazil but is expected to fade by midyear, could be contributing to another surge in local dengue infections. In January, municipal health records reported 1,122 cases, compared with only 165 a year ago.
Much is still unknown about the Zika virus, including believed links to suspected brain deformations in as many as 4,000 developing babies in Brazil. Researchers are also studying whether the virus can be transmitted through sex, blood transfusions or other contact with bodily fluids, like saliva and urine, where scientists recently found traces of the virus.
But scientists agree the mosquito remains far and away the primary means of transmission.
Any big gathering provides opportunities for viral infections, from the common cold to sexually transmitted diseases.
Brazil’s government expects as many as 500,000 foreign visitors during the Rio Olympics, which start Aug. 5 and end Aug. 21. City and Olympic officials said they will inspect venues and tourist attractions daily to ensure they are clear of puddles and other possible breeding sites for mosquitoes.
But scientists say authorities are being short-sighted by thinking only about local weather and insect conditions. Some visitors, they note, will take advantage of their Olympic travel to see other parts of Brazil and Latin America, where the virus is also present and where the climate and rate of mosquito reproduction will be wholly different.
Though the risk of infection or illness could indeed remain low for individual travelers, especially for a virus that doesn’t even cause symptoms in 4 out of 5 people infected, the Games could still enable Zika to travel farther than it already has.
“The biggest risk is a collective one,” said Chris Barker, an epidemiologist who studies Zika, dengue and related viruses at the University of California, in Davis. “The probability that at least some travelers to Rio will get Zika is significant.”
Some scientists worried that foreign visitors to Brazil for the 2014 World Cup soccer tournament would fall sick with dengue. But a 2015 study published in The Lancet, the British medical journal, reported that very few actually did.
Still, that event was held in 12 different cities, some of which were suffering from the worst drought in Brazil in decades and a consequent drop in the mosquito population.
Rio recorded a total of 2,649 dengue infections that year, compared with 18,059 in 2015. It recorded more than 130,000 in 2012, the worst year for dengue so far this decade.
Like many urban landscapes across Latin America, Rio is rife with unplanned development, poor construction and inadequate water, garbage and sewer services. That offers many nooks and crannies, many of them indoors, where spilled or poorly stored water, regardless of rainfall, can sit idle and provide beds for mosquitoes larvae.
“Heat can be even more of a problem than rain,” said Denise Valle, an entomologist who studies Aedes aegypti at the Oswaldo Cruz Foundation, a public health and medical institute that is among those leading the research for Zika, dengue and other tropical diseases. “The eggs are everywhere, in neighborhoods, in houses. If it warms up, the mosquito comes back.”
Nearly 100 Colombians suffering from the Guillain-Barre syndrome, a rare nerve disorder, also have symptoms of the mosquito-borne Zika virus, Colombia’s National Health Institute has said.
Colombian health authorities recently announced that three people who had been infected with the Zika virus had died after developing the Guillain-Barre syndrome, the first time health officials had said the Zika virus could cause deaths.
“We have confirmed and attributed three deaths to Zika,” Martha Lucia Ospina, head of Colombia’s National Health Institute, told reporters during a news conference on Feb. 5.
“In this case, the three deaths were preceded by Guillain-Barre syndrome,” said Ospina, an epidemiologist, adding that six further deaths were under investigation for a possible link to Zika.
Colombian Health Minister Alejandro Gaviria has said there was a “causal connection” between Zika, the Guillain-Barre disorder and the three deaths.
Guillain-Barre is a rare disorder in which the body’s immune system attacks part of the nervous system and damages the nerve cells, causing muscle weakness and sometimes paralysis.
The symptoms can last a few weeks or several months and while most people recover fully from Guillain-Barre, some have permanent damage, and in rare cases people have died from the syndrome, according to the United States Centers for Disease Control and Prevention (CDC).
Reported cases of Guillain-Barre have increased as the Zika outbreak spreads across Central and South America. So far, the Zika virus has spread to more than 25 countries and territories in the region, according to the World Health Organization (WHO).
Brazil, Colombia, El Salvador, Suriname and Venezuela have all reported an increase in the number of cases of Guillain-Barre syndrome, just as French Polynesia did during large outbreaks of Zika there in 2013 and 2014, the WHO says.
Colombia reports an average of 242 cases of Guillain-Barre syndrome a year, but reported 86 cases in the five weeks to Jan. 30. People of all ages can be affected, but it is more common in adults and in males, the WHO says.
Much remains unknown about Zika, for which there is no vaccine.
Scientists are studying a potential — but unproven — link between the surge in cases of Guillain-Barre and the Zika virus, and are trying to determine if a Zika patient is more likely to develop the Guillain-Barre syndrome.
“We do not know if Zika virus infection causes GBS (Guillain-Barre syndrome). It is difficult to determine if any particular germ “causes” GBS,” the CDC says on its website.
The cause of Guillain-Barre cannot always be determined, but it is often triggered by an infection, such as HIV, the mosquito-borne dengue virus, or influenza, according to the WHO.
A team of CDC experts is expected to arrive in Colombia this week for a three-week visit to investigate the possible links between the Guillain-Barre syndrome and Zika.
After Brazil, Colombia is the country most affected by Zika, with more than 25,000 cases so far, more than 3,000 of them involving pregnant women.
Although Zika usually causes mild, flu-like symptoms often lasting for up to one week, the virus has also been linked to thousands of suspected birth defects.
The WHO declared the Zika outbreak an international health emergency on Feb. 1, after a spike in Guillain-Barre syndrome cases and microcephaly, a neurological disorder.
The health agency cited a “strongly suspected” relationship between Zika infection in pregnancy and microcephaly, a condition marked by abnormally small head size in newborn babies that can result in developmental problems.
Brazil is investigating the potential link between Zika infections and more than 4,000 suspected cases of microcephaly. Researchers have identified evidence of Zika infection in 17 of these cases, in either the baby or the mother, but have not confirmed that Zika can cause microcephaly.
Barbados on Tuesday confirmed three cases of Zika in pregnant women, bringing to seven the number of people on the Caribbean island with the virus, which is believed to be linked to birth defects.
The women will be given specialized obstetric care, the Ministry of Health said in a statement. The new cases were announced on the Barbados government information services Facebook page.
Zika, a primarily mosquito-borne illness, has spread rapidly through Latin America and the Caribbean. It generally causes mild symptoms but has been blamed for a rapid rise in the number of children born with microcephaly — abnormally small heads and brains.
Barbados said that link has not been confirmed.
“The situation is still evolving and information is being updated regularly,” the Ministry of Health said.
The World Health Organization has declared a global medical emergency to combat Zika and individual countries and regions are beginning to mobilize. With no cure or vaccine for the virus, some countries have taken the extraordinary step of urging women to delay getting pregnant.
According to the Pan-American Health Organization, 26 countries have confirmed cases, spanning 7,000 km (4,400 miles) from Mexico to Paraguay.
The hardest-hit country is Brazil, which hosts the Summer Olympics starting in August.
Brazil has warned pregnant women not to travel there but Games organizers have said by the time the Olympics start, the main mosquito season will be over and they don’t expect the illness to affect the sporting extravaganza.