Kathryn Hanley studied Zika virus for years before anyone was concerned about it.
The New Mexico State University biologist began her Zika research a decade ago while she was studying other mosquito-borne viruses in Africa.
“It was an accident,” Hanley said. Zika was a logical “add-on” to research into viruses considered far more dangerous, including dengue, yellow fever and chikungunya, all transmitted by mosquitoes.
“I’m really a dengue person,” she said.
During her postdoctoral work at the National Institutes of Health, Hanley began studying the emergence and control of dengue virus, which remains a focus of her work. She has co-authored studies on emerging diseases around the world.
The scarcity of research about Zika has left science with more questions than answers vital to understanding the virus, she said.
Since its discovery in the Zika forest of Uganda in 1947, Zika virus has remained little studied because it was not considered a threat to public health.
That changed earlier this year when Zika was linked to a Brazilian epidemic of microcephaly – a severe birth defect marked by an underdeveloped head and brain.
“Until people appreciated that it could cause birth defects, no one was interested in Zika,” Hanley said.
She said she studies insect-borne viruses that infect both humans and animals, because they cause so many of the illnesses that afflict people today.
A key question regarding Zika: Why was it identified with birth defects only this year in Brazil, even though the virus has infected people in the Old World, probably since ancient times?
“That’s the $6 million question,” Hanley said. “I suspect that in the Old World – Africa and Asia – (Zika) has been distributed across the tropical belt for thousands of years.”
Scientists have offered some theories.
“One is that somewhere in that transition from Asia into Brazil, the virus acquired a mutation that made it more virulent,” she said.
Zika island-hopped across the Pacific Ocean from west to east, causing a 2007 outbreak in Yap island, about 800 miles east of the Philippines, where it infected 73 percent of the population in four months, according to a study Hanley co-authored.
In 2011, Zika also caused a large outbreak in French Polynesia, some 4,000 miles west of South America, followed by increased reports of babies born with microcephaly, according to studies published this year.
The virus was unknown in the Americas until 2015, although it may have turned up in Brazil as early as 2013, remaining out of sight for two years, she said.
During that voyage, the virus may have mutated in a way “that made it more likely to cross the placenta and infect the fetus, and cause brain damage,” she said.
Another possibility is that the Old World virus indeed causes birth defects, “but nobody noticed because monitoring of infant health was inadequate and because infections of humans aren’t that common,” she said.
In Africa, and possibly in Asia, Zika is transmitted by mosquitoes that feed largely on monkeys and only occasionally on humans.
“That may be driving lower rates of human infection” in Asia and Africa, she said.
In West Africa, where Hanley has done much of her research, Zika is primarily a monkey virus that occasionally spills over into humans. Yellow fever is another virus that infects both monkeys and humans.
In the Americas, Zika is transmitted primarily by the Aedes aegypti mosquito, which feeds almost exclusively on humans.
Aedes aegypti “are what we call a domestic mosquito, kind of like our dogs, only by accident,” she said. “They only really like human blood and they very rarely feed on anything else, which makes them a very good human-to-human vector.”
In the Americas, Zika has spread rapidly in dozens of Central and South American nations, and the Caribbean.
Another possibility is that Zika is largely a childhood disease in the Old World, providing children with lifetime immunity that protects women in their childbearing years.
Zika could become a childhood illness in the Americas, “but that depends on a lot of questions,” Hanley said.
Monkeys a key reservoir
Many of those questions involve monkeys.
In Africa, monkeys are a key reservoir for Zika virus. In the Americas, the virus isn’t known to infect monkeys, at least not yet.
If Zika establishes a “monkey cycle” in the Americas, it would be almost impossible to eradicate regionally when and if a vaccine is developed, Hanley said.
In the case of yellow fever, the virus infected New World monkeys soon after it arrived in the Americas in the 17th century.
Science now offers an effective vaccine for yellow fever, but the illness persists both in South America and Africa because monkeys harbor the virus.
“The same would be true for Zika in the Americas,” she said.
Another unknown is whether children who get Zika develop lifelong immunity. Pregnancy is known to suppress a woman’s immune system, “so it could be that you get lifelong immunity until you are pregnant,” she said.
‘We need more time’
A big question for U.S. residents is the potential risk here.
As of Friday, 1,132 travel-associated cases of Zika infections had been reported among U.S. residents, all acquired in other countries. No local infections have occurred within the continental U.S.
But two species of mosquitoes that transmit Zika virus – Aedes aegypti and Aedes albopictus – thrive in the southern U.S., including southern New Mexico.
Three Zika cases have been reported in New Mexico, all acquired outside the U.S.
Hanley predicts that the U.S. is at “grave risk” of local Zika virus infections.
“We know that dengue virus is transmitted along the U.S.-Mexico border region,” as well as Hawaii and Florida, “and I believe the same will happen with Zika.”
Hanley’s research will take her to Borneo later this month and, later, to South America to study the Zika transmission cycle. Asia remains a “black box” where information about the virus is scarce.
“We’ve had six months of intense research on this virus,” Hanley said. “A lot of people are really trying, but we need more time to answer some of these questions.”