For almost 60 years the Zika virus languished as little more than a medical curiosity. Then, just about two years ago, it began to wreak havoc across the Americas. Since then researchers have tried to learn what they can about the virus. They’ve learned that it can cause microcephaly in babies when pregnant women are infected, and that the virus can be sexually transmitted. But one thing is startlingly clear when Frank J. Bia, M.D., M.P.H., FW ’79, emeritus professor of medicine, talks about Zika virus: there is still so much that physicians and researchers don’t know, like how long the virus remains in an infected person or how it manages to cause so much damage to the developing brain of a fetus.
In his talk at internal medicine grand rounds on Thursday, Bia discussed "Zika Virus Infections in the Americas and their Consequences,” and traced its history since it was discovered in 1947 in Uganda.
Unraveling Zika’s secrets has proven difficult. It’s tricky to diagnose, often asymptomatic, and when a patient does exhibit symptoms, it can present like such other viruses as the closely related dengue fever. But the need to learn about this virus is dramatic. By December of 2015, 1.3 million suspected cases of the Zika virus had occurred. Early this year, the World Health Organization declared the Zika virus outbreak an international public health emergency, for only the fourth time in its history. With a dramatic increase in the number of babies being born with brain abnormalities like microcephaly, and because of Zika and the virus’s clear link to Guillain-Barre syndrome, researchers are scrambling to understand this mysterious virus.
It was first isolated in Uganda in the Zika Forest, a field site for a British colonial research institute studying yellow fever. The researchers had placed primates out in the forest and waited for them to be infected by mosquitoes in the hopes of studying their viruses and symptoms. An Asian species of Rhesus monkey that was not native to Zika Forest came down with symptoms that looked similar to yellow fever. But when researchers isolated the virus, they found it was something else all together. They called it “Zika Virus 776”, named after the monkey from which the virus was isolated, No. 776.
The word “Zika” means overgrown in the local Luganda language, a tribute to the densely forested area where the virus was first discovered. In 1948, the virus was isolated from the Aedes africanus mosquito from that same forest. One of the first human cases of Zika occurred in Nigeria, but in 57 years there were only thirteen more cases. “From 1957 to 2015, this virus was a curiosity,” said Bia, “until it began to spread across the Pacific to the Americas.”
Several types of Aedes mosquitos transmit the virus, but by 1966 it was clear that the major vector was Aedes aegypti, which also transmits dengue fever and yellow fever. It is not your average mosquito that transmits malaria—it bites during the day and has a tendency to hide in houses. Bed-netting, which is generally effective in preventing the transmission of malaria, doesn’t have the same effect at preventing Aedes aegypti from spreading Zika.
The United States is home to two types of mosquitoes that can transmit the Zika virus: Aedes aegypti and Aedes albopictus. Both are prevalent in the South, while only the latter exists in areas like New England.
Tracing the spread of the virus takes a fair amount of detective work. The outbreak began in West Africa, moved to Southeast Asia, then to Micronesia and French Polynesia and finally to Brazil in 2014. Researchers aren’t entirely clear how the virus got to Brazil from Polynesia. “There’s a lot of speculation about what happened between June and August of 2014,” Bia said. Whether it was a tourist coming to see the World Cup or perhaps team members from French Polynesia that competed in the World Sprint Championship in canoeing, researchers can’t be sure.
In 2016, a Physician Note published in the journal Ultrasound in Obstetrics and Gynecology detailed a potential link between Zika virus and fetal brain abnormalities. “Indeed, this was just the tip of the iceberg,” said Bia. “But it was only in retrospect, looking at ultrasounds, that people began to realize what was going on in Brazil.”
Bia calls the increase in recorded microcephaly cases between 2010 and 2015 “astronomical.” By 2015, Brazil had recorded about 7,000 cases, a 20-fold increase compared to previous years.
Fetal autopsies have shown that the Zika virus causes a highly destructive form of microcephaly, leading to some miscarriages and hypoplasia in certain parts of the brain. But researchers aren’t sure exactly how these babies will fare as they grow or how long the virus remains in their system.
The Zika virus can be sexually transmitted from men to women and it can be passed through contaminated blood at a blood bank. Doctors aren’t sure how long to tell women to wait before trying to have children if they’ve been infected with the Zika virus. The problem is complicated in places like Puerto Rico, where access to contraception is limited. Puerto Rico has a population of 3.5 million people and it’s estimated that a quarter of them will be infected over the next year. As public leaders suggest that women use contraception to keep safe, there is concern that cultural, religious, and logistical barriers to contraception will put more people at risk. “We’re dealing with a public health problem that involves more than just Zika virus,” said Bia.
There is a lot of concern and debate about the summer Olympics in Rio de Janeiro, Brazil. Several athletes and one TV anchor have declined to attend the Olympics over concerns about the Zika virus. Bia presented recently published data that showed a mathematical modeling of the likelihood of infection. It estimates the risk of Zika virus infection for tourists at about 3.2 cases per 100,000 visitors. Assuming the Olympics draw 500,000 visitors, that means 16 cases of infection. Like a lot of things involving the Zika virus, this is all speculation. “We don’t know a lot,” said Bia. “But we’re going to find out.”