Cases of birth defects in Brazil have doubled since the Zika virus outbreak began in the country, and data suggest that rise began in mid-2014— nearly a year before officials declared the epidemic— according to a report released Thursday by the Centers for Disease Control and Prevention (CDC).

“We observed an unprecedented and significant rise in the hospitalization rate for congenital malformations of the nervous system, Guillain-Barré syndrome, encephalitis, myelitis, and encephalomyelitis,” wrote the study authors, from Brazil's Oswaldo Cruz Foundation, whose research was released in advance of the November edition of the CDC journal Emerging Infectious Diseases. “These conditions are compatible with viral infection and inflammation-associated manifestations and may have been due to the entrance of Zika virus into Brazil.”

Researchers analyzed hospital admissions data between January 2008 and February 2016 to identify neuropathy cases that may have been associated with Zika virus infections in Brazil.

In Northeast Brazil, which officials have considered the epicenter of the Zika outbreak, congenital malformations of the nervous system (CNS) occurred in a mean value of 40 in 100,000 live births until September 2015. As of November, that number began rising and reached a mean value of 170 in 100,000 live births— a nearly fourfold increase compared to historical rates, the report noted.

In February 2016, Brazil overall saw rates of 100 such malformations in 100,000 live births, which indicated a twofold increase compared to the national baseline. A total of 1,027 children were hospitalized for congenital malformations of the nervous system between November 2015 and February 2016, researchers found.

“Our results show that the rates of hospitalization in the Northeast region for some neuropathies were subject to abrupt changes beginning in mid-2014, when it is believed that Zika virus was introduced to the country,” the authors wrote. Theories persist on whether a drought between 2010 and 2013 that led to drinking water contamination, food insecurity and diarrhea outbreaks in the region may have led that population to be disproportionately affected by Zika after its arrival, they added.

Initial introduction of Zika in Brazil may be due to sporting events from 2013 and 2014, other studies suggest.

The new report found that the onset of hospitalizations for congenital malformations of the CNS that were seen in November 2015 may stem from the first public warnings about microcephaly, which occurred in the same month. Greater public awareness of the virus could have contributed to the rise in reporting, which researchers said was one limitation of the report. However, they wrote that the rise in reported infections that were not microcephaly probably were not affected by this factor because the World Health Organization did not declare the virus a public health emergency until February 2016.

“This study can contribute to a greater understanding of the Zika epidemic in Brazil and its additional effect on certain neuropathies,” researchers wrote.

“New detection tools for outbreaks and means of communicating warnings should be pursued to identify real trends and, at the same time, minimize false alarms and panic that could be provoked in populations potentially affected by the Zika epidemic,” they continued. “The history of neuropathies caused by viruses is about to change, not only because of the effects that Zika virus has been found to have on the CNS, but also because of the effort of gathering evidence that will help construct this history.”

