Before Zika swept across most of Latin America and the Caribbean, the largest outbreak ever recorded had been in French Polynesia. Between 2013 and early 2014, researchers estimate nearly 20,000 people on the cluster of islands in the South Pacific were infected with the virus.

French Polynesia's brush with Zika underscores fears that the mosquito-borne virus could cause devastating neurological problems but it also offers insights into the disease — and hope that Zika can be contained.

Dr. Van-Mai Cao-Lormeau at the Institut Louis Malarde on the island of Tahiti says in 2013 it seemed like this nation of 270,000 people was getting hit with a dengue outbreak. But something didn't make sense.

"You know, Tahiti is a small island," Van-Mai says. "So in the lab we had relatives, family and friends who were getting sick who we know had already had dengue several times." Once you've had dengue, it's unlikely that you'll get a mild form of it again. But that's what seemed to be happening.
"We were suspicious of this being dengue because all these people already had dengue."

Van-Mai and her colleagues at the Institut Louis Malarde, the government infectious disease lab, happened to be part of a program to monitor for mosquito-borne viruses in the Pacific.

While Zika had never been detected in French Polynesia before, there had been an outbreak 5,000 miles to the west on Yap Island. In 2007, three-quarters of the roughly 7,000 residents got infected with the virus. Because of that Yap outbreak Institut Louis Malarde was already set up to test for Zika and, sure enough, Zika was running rampant in Tahiti.

"Then it spread very quickly. Very quickly we had reports of Zika virus infection occurring all across the islands of French Polynesia," says Van-Mai. But it was difficult at times to diagnose.

Doctors found that the levels of Zika virus in people's blood dropped off quickly even while the patients were still sick. "So you do not see the virus in blood and what we started to do is looking for Zika virus in saliva," says Dr. Van-Mai. "Using saliva we were able to confirm a lot of Zika virus infections." They were only testing for the presence of the virus. She says it's not clear whether the virus they were finding in saliva was capable of reproducing or being spread to other people.

In addition to low-grade fever, rash and body aches, the French Polynesian doctors noticed another symptom that could be linked to Zika and that hasn't been reported elsewhere: mouth ulcers. "A lot of people were complaining about mouth ulcers," she says.

And then about five weeks after the Zika outbreak started, doctors in Tahiti started seeing a rise in cases of Guillain-Barre syndrome — a neurological condition that can cause temporary paralysis. During the 2013-14 Zika outbreak the number of Guillain-Barre cases in French Polynesia jumped from an average of two a year to 42.

The possible link between the Zika virus and birth defects only emerged last year after health officials in Brazil reported hundreds of thousands of Zika cases. Those reports out of Brazil prompted researchers in French Polynesia to do a retrospective investigation to search for congenital abnormalities that may have been caused by the virus. Van-Mai says they found a disturbingly high number of birth defects.

"We had 17 cases of the central nervous system malformations in either fetuses [examined after a miscarriage] or babies from mothers who had been exposed to Zika during the outbreak."

She says those cases are still being investigated. The birth defects may have been caused by something else. They still don't know.

The good news is that as quickly as Zika arrived, it seemed to disappear. There hasn't been a laboratory confirmed case in French Polynesia since April of 2014.

But Zika might still be circulating, Van-Mai says. Perhaps it isn't making people so sick that they show up at the doctor, she says. And at the moment, another virus is getting the island's attention. The archipelago got slammed with a chikungunya outbreak right after the Zika outbreak, and researchers have been consumed with that ever since.

Copyright 2021 NPR. To see more, visit https://www.npr.org.