Researchers, alert to the rapid increase in cases, say that Zika’s spread to Brazil reflects how easily viruses are jumping from one part of the planet to another.

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SÃO PAULO — A little-known virus spread by mosquitoes is causing one of the most alarming health crises to hit Brazil in decades, officials here warn: thousands of cases of brain damage, in which babies are born with unusually small heads.

Many pregnant women across Brazil are in a panic. The government, under withering criticism for not acting sooner, is urging them to take every precaution to avoid mosquito bites. One official even suggested that women living in areas where mosquitoes are especially prevalent postpone having children.

“If she can wait, then she should,” said Claudio Maierovitch, director of the department of surveillance of communicable diseases at Brazil’s health ministry.

The alarm stems from a huge surge in babies with microcephaly (my-kroh-SEF-uh-lee), a rare, incurable condition in which their heads are abnormally small. Brazilian officials have registered at least 2,782 cases this year, compared with just 147 in 2014 and 167 the year before.

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At least 40 of the infants have recently died, and some Brazilian researchers warn that cases could multiply in the months ahead. Those babies who survive may face impaired intellectual development for the rest of their lives.

Brazilian researchers say that an obscure mosquito-borne virus that made its way to the country only recently — Zika — is to blame for the sudden increase in brain damage among infants.

But other virologists caution that more testing is needed to prove the dangerous link between the virus and brain damage, leaving the full extent of the threat to the country, and the hemisphere, unclear.

“Why this may have happened in Brazil and not elsewhere is, at this stage, difficult to answer,” said Alain Kohl, a virologist at the University of Glasgow who studies Zika.

“Perhaps it was never properly registered in other areas, or the situation in Brazil is, indeed, different,” he added, citing the possibility that the link between Zika and microcephaly could be related to particular strains of the virus.

The Zika virus has already reached several countries in Latin America, including Mexico, and the Centers for Disease Control and Prevention (CDC) warns that it could spread to parts of the United States as well. There already have been cases diagnosed in the United States, in travelers who visited affected countries, and the CDC expects these instances to increase.

Gleyse Kelly da Silva, 27, a toll road attendant in the city of Recife in northeast Brazil, learned of the problem when an ultrasound exam detected microcephaly in the seventh month of her pregnancy with her daughter, Maria Giovanna, born in October. Just a few months earlier, da Silva had sought medical attention after experiencing some of Zika’s symptoms: fever, joint pain and a red rash.

No one knows precisely when the Zika virus made the leap to Brazil from its place of origin in Africa. Some researchers say it could have arrived during the 2014 World Cup, when Brazil welcomed travelers from around the globe. Others think the virus may have come during a canoe race weeks later, when paddlers from French Polynesia, the site of a recent Zika outbreak, arrived in Rio de Janeiro.

Researchers, alert to the rapid increase in cases, say that Zika’s spread to Brazil reflects how easily viruses are jumping from one part of the planet to another.

They are particularly worried that the disease is wreaking havoc in a region where the population has not encountered it before, and that climate change may be allowing viruses like Zika to thrive in new domains.

The Brazilian government has stopped short of officially advising women not to get pregnant, but confusion and fear are spreading along with the virus.

“The situation is incredibly frightening,” said Andreza Mireli Silva, 22, a worker in a shoe factory in Sergipe State in northeast Brazil who is seven months pregnant. She said she was trying to avoid mosquito bites by wearing long pants despite the heat of the Southern Hemisphere summer and applying insect repellent every three hours.

Zika, named for the forest in Uganda where scientists discovered it in the 1940s, often goes unnoticed in the people it infects and was not considered especially life-threatening before spreading to Brazil. But the advance of the virus here is focusing scrutiny on the resilience of a worrisome pest: Aedes aegypti, the mosquito that carries Zika and other diseases, among them yellow fever and chikungunya.

“Brazil offers the ideal conditions for Zika to spread so quickly,” said Ana Maria Bispo de Filippis, a leader of the research team that has linked Zika to microcephaly. The country, she added, has “a susceptible population in which the majority of people never had contact with the disease.”

Before Zika’s arrival, Brazil was already grappling with a much deadlier epidemic of dengue, another virus transmitted by Aedes mosquitoes. Brazil had nearly 1.6 million cases of dengue in 2015, according to estimates from the health ministry, up from 569,000 in 2014. At least 839 people have died from dengue in Brazil this year, an 80 percent increase from the previous year. Some health officials say that changes in weather and rainfall may be behind the surge.

Brazil waged war on the Aedes aegypti mosquito for decades during the 20th century before a vaccine was developed for yellow fever. Health agents deployed across the country to destroy habitats like water barrels and other open water sources where the mosquitoes thrive. Authorities even declared victory against the pest in 1955.

But the mosquito re-emerged in Brazil in the late 1960s, outpacing eradication campaigns. Now, at a time when President Dilma Rousseff’s beleaguered government is under fire over corruption, an economic crisis and its handling of the surge in dengue cases, the spread of Zika is unleashing even more criticism.

After virologists identified a Zika outbreak in May in northeast Brazil, the health minister at the time, Arthur Chioro, played down the discovery.

“Zika virus doesn’t worry us,” he told reporters, calling it a “benign disease.”

After that dismissive response, public-health experts say the political upheaval in Brazil — in which Rousseff is fighting impeachment proceedings — weakened efforts to respond to Zika. Rousseff overhauled her cabinet in October, dismissing various ministers, including Chioro.

In doing so, she ceded more power to the centrist Brazilian Democratic Movement Party, or PMDB, which controls both houses of Congress. While Zika was raging, she named as her health minister, Marcelo Castro, a psychiatrist from that party who stopped practicing medicine years ago to focus on his own business interests and politics.

Researchers warn that they are only just starting to understand Zika’s impact on Brazil and the potential for it to spread to other countries in the Americas.

Some researchers emphasize the role climate change may play in Zika’s spread. As temperatures increase in some areas, they argue, mosquitoes can multiply more quickly, enhancing their collective ability to transmit diseases.

Increased precipitation in some areas also creates new places where mosquitoes can breed. And droughts, like those that recently afflicted parts of Brazil, can cause people to hoard water in containers, providing mosquito habitats.

Neither the Zika outbreak in Latin America nor its possible link to microcephaly in infants has led to changes in travel advice from the CDC or the Pan-American Health Organization, the regional office of the World Health Organization.

Because Zika is spread by the same mosquito species linked to dengue and chikungunya, both agencies are sticking with the advice they gave for those outbreaks: that all travelers, including pregnant women, do everything they can to avoid mosquito bites, like using insect repellent day and night, wearing long pants and long sleeves, and staying in places that are screened and air-conditioned.