Although there is no absolute proof that the Zika virus is behind the surge in microcephaly in Brazil and outbreaks of Guillain-Barré syndrome in six countries, the world’s leading health authorities are close to certain that it is.
“With each passing day, the evidence that it is the cause mounts,” Dr. Thomas R. Frieden, director of the Centers for Disease Control and Prevention, said recently.
Dr. Bruce Aylward, who is leading the World Health Organization’s response, said on Friday: “At this time, the virus is considered guilty until proven innocent.”
Many rumors blaming other potential causes have arisen, and the authorities have worked hard to debunk them. Here is a look at the most prominent theories making the rounds on social media, along with responses from scientists.
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A biologist released genetically modified mosquitoes in Piracicaba, Brazil. Credit Victor Moriyama/Getty Images
Are genetically modified mosquitoes the real cause of the birth defects?
That buzzing sound you hear is a ‘no.’
A British company, Oxitec, released genetically engineered mosquitoes in Brazil in an attempt to control dengue fever. But the later microcephaly outbreaks were far away. For example, the largest mosquito release was in Piracicaba, which is 1,700 miles from Recife, where microcephaly was most common. The mosquitoes have also been released in the Cayman Islands, Malaysia and Panama without causing problems.
Mosquitoes fly less than a mile in their lifetimes. Also, only male mosquitoes were released. They do not bite humans or spread disease, and were genetically programmed to die quickly.
Entomologists have dismissed as “ridiculous” the idea that the larvicide pyriproxyfen could have caused such a large wave of birth defects. It does not attack nerve cells; it is a chemical mimic of an insect hormone that signals larvae to stop growing, and insect hormones do not endanger humans.
Pyriproxyfen was approved in the United States in 2001 and is sold as a flea treatment for dogs and cats and as a flea-killing carpet spray. Babies have crawled in the stuff for years without apparent harm. And in Brazil and Polynesia, brain damage has occurred to infants in many communities where the larvicide is not used.
Rumors have blamed both a “bad batch of rubella vaccine” and the introduction of a new pertussis vaccine in Brazil, or aluminum in that vaccine. Neither is plausible.
There was no rubella outbreak among pregnant women and no evidence of a “bad batch.” No vaccine is used only in northeast Brazil. The new pertussis vaccine has been used since the 1990s in many countries, including the United States. It was introduced because an older one caused soreness, fever and, in rare cases, seizures. Neither ever caused microcephaly.
What if another disease is causing microcephaly and Guillain-Barré?Mounting evidence points to Zika.
The outbreak of microcephaly began in northeastern Brazilian cities where doctors had already seen thousands of people with “doença misteriosa” — the mystery disease — which was later proved to be caused by the Zika virus. Although there is no rapid test for Zika, the symptoms are easily recognized — a rash, bloodshot eyes, fever and joint pain, in large numbers of patients who are almost never dangerously ill.
Although they initially misdiagnosed it, Brazilian doctors knew for months that they had a large outbreak of an unusual disease on their hands. The same thing happened on Yap Island in Micronesia in 2007 and in French Polynesia in 2013.
Within weeks after the “mystery disease” appeared, doctors began noticing an uptick in adult paralysis — Guillain-Barré syndrome, an auto-immune disease that can be triggered by viral infections. There was a similar surge in cases in French Polynesia in 2013, and cases are now surging in Colombia, El Salvador, Suriname, Venezuela and Martinique — always in step with Zika outbreaks. In some cases, Zika virus has been found in the blood or urine of the victims.
About a year after the outbreak began in northeast Brazil, cases of microcephaly began appearing among newborns there. Pathologists in Brazil, the United States and Europe have now found Zika virus in brain tissue from stillborn and aborted microcephalic fetuses and in the amniotic fluid surrounding them.
Couldn’t Brazil just have been undercounting microcephaly for years?It’s possible, but not by large enough margins to explain the current outbreak.
Some researchers believe that Brazil previously did undercount cases, but not by huge margins. Definitions of microcephaly vary, but European and North American countries report about one case per 5,000 live births to one per 10,000. Before its Zika outbreak, Brazil reported about one case per 20,000 live births — in other words, half or a quarter as many as it might really have had.
Before the outbreak, the seven states in tropical northeast Brazil where microcephaly first appeared reported about 40 cases of microcephaly a year. In October, neurologists in Recife, who normally saw microcephalic babies very rarely, found themselves treating five or more at a time. By Nov. 17, just those seven states had 400 reported cases. A month later, just one of them, Pernambuco, reported more than 600.
Eventually, the Brazilian health ministry decided that doctors were overreporting cases, so in December, it tightened its definition to include only children with heads less than 32 centimeters in circumference instead of 33. But cases continued to grow. Even previous undercounting would not explain the tremendous surge that followed Zika’s appearance.