SÃO PAULO, Brazil — 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.  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 life.  Brazilian researchers say that Zika, an obscure mosquito-borne virus that made its way to the country only recently, 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 to 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 virus strains.  The Zika virus has already reached several countries in Latin America, including Mexico, and the Centers for Disease Control and Prevention warns that it could spread in parts of the United States as well. There have already been cases diagnosed in the United States, in travelers who visited affected countries, and the C.D.C. expects these instances to increase.  “I cried for a month when I learned how God is testing us,” said Gleyse Kelly da Silva, 27, a toll road attendant in Recife, a city in northeast Brazil, describing how an ultrasound exam had detected microcephaly in the seventh month of her pregnancy with her daughter, Maria Giovanna, born in October.  Just a few months earlier, Ms. da Silva sought medical attention after experiencing some of Zika’s symptoms: fever, joint pain and a red rash.  “I had never heard of Zika or microcephaly,” said Ms. da Silva, the mother of three other children. “Now I just pray that my daughter can endure life with this misfortune.”  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 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 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, including 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 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 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. The authorities even declared victory against the pest in 1955.  But the mosquito re-emerged in Brazil in the late 1960s, outpacing eradication campaigns. Now, when President Dilma Rousseff’s 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 that the political upheaval in Brazil — in which Ms. Rousseff is fighting impeachment proceedings — weakened efforts to respond to Zika. Ms. Rousseff overhauled her cabinet in October, dismissing various ministers from her own Workers Party, including Mr. Chioro. In doing so, she ceded more power to the centrist Brazilian Democratic Movement Party, or P.M.D.B., 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.  “The health minister, a politician dedicated to the business of ranching, has a profile that’s the opposite of what’s required to lead the effort to deal with microcephaly,” Ligia Bahia, a specialist on Brazil’s public health system at the Federal University of Rio de Janeiro, said in a column in the newspaper O Globo.  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. The federal authorities do not yet have a precise estimate of the number of cases, as reporting such figures is not compulsory.  Some researchers emphasize the role that climate change may play in Zika’s spread. As temperatures increase in some areas, they argue, mosquitoes can multiply more quickly, potentially enhancing their collective ability to transmit diseases.  Additionally, increased precipitation in some areas creates places where mosquitoes can breed. And droughts, like those that recently afflicted parts of Brazil, can cause people to hoard water in containers, providing additional mosquito habitats.  “The mosquito is exquisitely adapted to human hosts, living in close proximity to humans and feeding repeatedly,” said Maria Diuk-Wasser, a scholar with expertise in entomology, zoology and parasitology at Columbia.  Paula Moura contributed reporting from São Paulo, Vinod Sreeharsha from Rio de Janeiro, and Donald G. McNeil Jr. from New York  A version of this article appears in print on December 31, 2015, on page A1 of the New York edition with the headline: Mosquito-Borne Peril to Babies Unsettles Brazil. Order Reprints| Today's Paper|Subscribe
The mosquito-borne Zika virus, suspected of fueling a spike in microcephaly, is at the core of one of the most alarming Brazilian health crises in decades.