In seven countries that recently experienced Zika outbreaks, there were also sharp increases in the numbers of people suffering from a form of temporary paralysis, researchers reported Wednesday.
The analysis, published online in the New England Journal of Medicine, adds to substantial evidence that Zika infections — even asymptomatic ones — may bring on a paralysis called Guillain-Barré syndrome.
The syndrome can be caused by a number of other factors, including infection with other viruses. Researchers studying the Zika epidemic in French Polynesia had estimated that roughly 1 in 4,000 people infected with the virus could develop the syndrome.
The Centers for Disease Control and Prevention has said that the Zika virus is "strongly associated" with Guillain-Barré, but has stopped short of declaring it a cause of the condition.
The new data suggest a telling pattern: Each country in the study saw unusual increases in Guillain-Barré that coincided with peaks in Zika infections, the researchers concluded.
"It's pretty obvious that in all seven sites there is a clear relationship," said Dr. Marcos A. Espinal, the study's lead author and the director of communicable diseases at the Pan American Health Organization, which collected data on confirmed and suspected cases of Zika infection and on the incidence of Guillain-Barré. "Something is going on."
In Venezuela, officials expected roughly 70 cases of Guillain-Barré from December 2015 to the end of March 2016, as mosquitoes were spreading the virus. Instead, there were 684 cases.
Similarly, during five months in which the Zika virus was circulating in Colombia, officials recorded 320 cases of Guillain-Barré when there should have been about 100. From September 2015 to March 2016, while Zika infections peaked in El Salvador, cases of Guillain-Barré doubled to 184 from 92.
The researchers included patients with both suspected and confirmed Zika infections, as reported by national health officials.
Dr. Kenneth C. Gorson, professor of neurology at Tufts University School of Medicine, who was not involved with the new analysis, called it compelling.
"This is a substantial public health burden for countries that may not have well-developed health systems in place," he said. "They have to have enough ventilators and ICU beds." About one-third of patients with Guillain-Barré require breathing assistance, he said.
Over all, Espinal and his authors found increases in Guillain-Barré that were two to 10 times what would normally be expected. Roughly 500 million people in Latin America and the Caribbean are at risk for Zika virus infection, so even modest increases in the incidence of Guillain-Barré are worrisome.
"Countries need to have health services prepared because this data suggests Guillain-Barré syndrome is closely associated with Zika virus," Espinal said. "And Zika virus is not going away tomorrow."
The nations in the study included the Dominican Republic, El Salvador, Honduras, Suriname, Venezuela and Colombia, along with the state of Bahia in Brazil. (National data from Brazil was not available until February 2016.)
Collectively, they reported a total of nearly 1,500 cases of temporary paralysis. The reported incidence was 28 percent higher for men and increased with age for both sexes, in line with previous research.
Temporary paralysis is a potential neurological complication of dengue infection, too. But Espinal and his colleagues looked for a similar link to dengue and found none.
Gorson noted that the continental United States has no formal monitoring system for Guillain-Barré. As the number of Zika cases in Florida and elsewhere increases, he said, "you won't know if a Guillain-Barré case is related to Zika infection."
"We can do it," he said of such surveillance. But "there's no funding from Congress to do it."