There isn’t enough evidence to claim that Zika virus causes abnormally small heads in babies of infected mothers, according to a UW-Madison study that challenges government findings.
A government official said evidence of the link is “sufficient and growing.”
The prevalence of microcephaly in northeast Brazil, where a Zika outbreak drew worldwide attention in 2015, was similar or higher in previous years, according to a new study by Dr. Leonelo Bautista in the December edition of the journal Annals of Epidemiology.
An increase in reported Zika infections during pregnancy was not followed by an increase in cases of microcephaly, said Bautista, a UW-Madison associate professor of population health sciences.
In addition, he found that babies of Zika-infected mothers didn’t have significantly more microcephaly, which can cause developmental problems and even death.
One factor that made it seem like there was a spike in microcephaly cases in 2015 is that criteria for the condition changed that year to include children with larger heads than before, he said. Studies suggesting a link between Zika and microcephaly were too small to draw conclusions and didn’t account for genetic causes of the birth defect, he said.
The Centers for Disease Control and Prevention and the World Health Organization, which said pregnant women should avoid traveling to Brazil and other places affected by Zika, declared in April 2016 that the virus caused microcephaly.
The CDC published its findings in the New England Journal of Medicine, saying its scientists had conducted a “rigorous weighing of evidence.”
Other researchers have questioned the link between Zika, which is mostly spread by mosquitoes, and microcephaly. But Bautista said his study is the first one published to analyze the evidence and call it into question.
The CDC and other government agencies “jumped to the conclusion that these two events were associated,” he said Wednesday in an interview.
In a public talk about his research earlier this year, Bautista said the agencies “created a false crisis.”
“It points to a major failure of the peer-review system in health sciences,” he said. “It sets a dangerous precedent by establishing causal links mostly by official decree.”
Margaret Honein, acting director of congenital and developmental disorders at the CDC’s National Center on Birth Defects and Developmental Disabilities, said in a statement that “there is sufficient and growing evidence about the causal link between Zika virus infection during pregnancy and brain abnormalities, microcephaly and other birth defects.”
Since April 2016, studies have provided “strong evidence” that Zika virus infection during the first trimester of pregnancy poses the most risk for the fetus, Honein said. The virus can persist and replicate in brain and placental tissues, and imaging results have provided an estimate of the time between infection and birth defects, she said.
Birth defects potentially related to Zika occur about 20 times more often in pregnancies with lab evidence of possible Zika infection in the United States, Honein said.
In his study, Bautista said heightened media attention to Zika infections in 2015 may have led more cases to be reported. The definition of microcephaly was changed in late 2015 to include a head circumference of up to 33 centimeters, compared to less than 31 centimeters before, he said.
Combined with an underreporting of microcephaly in a widely used government database in previous years, the 2015 statistics seemed to show an uptick, he said. But upon further analysis, other information showed similar or higher rates of microcephaly before 2015, he found.
Bautista, a native of the Dominican Republic, said Latin American countries have experienced significant economic and social ramifications from policies based on the assumption that Zika causes microcephaly.
Women have avoided pregnancy or had abortions, tourism dropped and resources were shifted to Zika from diseases known to be more problematic, like cholera, he said.
It’s possible Zika causes microcephaly, but the evidence available doesn’t prove it, Bautista said. More research is needed, he said.