The latest virus to break out of the tropics may be the most frightening.
Zika virus, which has already blazed across Brazil and pressed northward into Central America and Mexico, now is poised to jump to the United States. Infection typically causes minor or even no symptoms. But in pregnant women, it’s been linked to a birth defect called microcephaly, which leaves babies with abnormally small heads and partially developed brains (SN Online: 12/2/15).
The mysterious tropical virus is an arbovirus, one of many that are spread by insects such as mosquitoes and ticks. With the rise in international travel, the rapid emergence — and reemergence — of little known arboviruses such as Zika may be the new normal, Anthony Fauci and his colleague David Morens suggest January 13 in the New England Journal of Medicine.
“Dengue hit with a vengeance in the ’90s. Then we had West Nile in 1999, chikungunya in 2013, and lo and behold, now we have Zika in 2015 and 2016,” says Fauci, the director of the National Institute of Allergy and Infectious Diseases in Bethesda, Md. “This is a disturbing, remarkable pattern.”
Already, travelers have brought Zika home to Texas, Hawaii and Illinois, though the virus doesn’t seem to have infiltrated U.S. mosquitoes yet. But the United States, with its warm, humid regions, pockets of poverty and ready fleet of mosquitoes capable of carrying the virus, has all the right ingredients for an outbreak, says Peter Hotez, a pediatrician and microbiologist at Baylor College of Medicine in Houston.
“We’ve been wringing our hands about Ebola,” he says, but “Ebola was never a threat to the Western Hemisphere.”
A virus emerges
Scientists first collected Zika virus in 1947 from a rhesus monkey that was part of an infectious-disease study in the wetland-edged Zika forest of southern Uganda. For decades, the virus flitted between monkeys and mosquitoes, infecting humans only rarely — and until 2007, never outside of Africa and Asia. That’s when Zika escaped into the Pacific, causing an outbreak on Yap Island in the Federated States of Micronesia. The virus was spotted in French Polynesia next, in 2013. It came to Easter Island a year later, and in May 2015, the first confirmed cases cropped up in Brazil. There, Zika flourished, gaining a firm foothold in the Americas.
In less than nine months, Zika infected as many as 1.3 million people in Brazil, the European Center for Disease Prevention and Control reported on December 10, and some estimates put the number even higher. Zika virus has now spread through 18 countries and territories in Latin America and the Caribbean, the Pan American Health Organization and World Health Organization report.
“The cat’s out of the bag now,” Hotez says. “Zika virus is going to be all over.”
On January 15, the U.S. Centers for Disease Control and Prevention issued warnings (SN Online: 1/15/16) for people traveling to countries with Zika. Pregnant women, in particular, should be especially cautious, the CDC advised.
Some people consider Zika virus a mild cousin of dengue: Only about 20 percent of infected people get sick, and symptoms (typically a slight fever, rash and pinkeye, to name a few) fade quickly. But a growing body of evidence suggests that the virus could also cause a devastating birth defect.
In Brazil, the number of babies born with microcephaly is steadily ticking up. In 2015, the country recorded roughly 20 times as many cases as in previous years, and new cases — sometimes hundreds — appear every week. On January 20, Brazilian health officials reported a total of 3,893 cases, 363 more than a week earlier.
Based on the number of babies born in Brazil in 2015 and the number of microcephaly cases that year, public health researcher Ernesto Marques of the University of Pittsburgh estimates that roughly 1 in 150 babies were born with the birth defect.
“This is just a huge number,” he says. “And it’s in an outbreak that has just started.”
That Zika might wreak havoc in fetal brains isn’t all that surprising, given the virus’s effect on mice and the neurological problems sometimes observed in infected adults, says Carlos Marcondes, an entomologist who studies disease-carrying insects at the Federal University of Santa Catarina in Brazil. In lab mice, Zika virus makes a beeline for the brain. “It causes serious damage,” Marcondes says. Nerve cells break down and brain tissue softens.
In the 2013–2014 French Polynesia outbreak, at least 73 people developed neurological conditions such as Guillain-Barré syndrome, which can cause paralysis. Health officials have linked the condition to Zika virus infection in the current outbreak as well.
Early this year, scientists discovered the most concrete clues yet that Zika virus can cause microcephaly: genetic traces of Zika in the amniotic fluid of two pregnant women carrying fetuses diagnosed with the birth defect, and in four babies who were miscarried or died shortly after birth.
“The evidence is very, very strong,” says Marques, but only a few babies have been tested.
Searching for answers
Marques and collaborators in Brazil, England and the United States have begun a study to examine even more babies. The researchers aim to enroll 200 infants with microcephaly and 400 infants without the birth defect (all from hard-hit Pernambuco, Brazil) and will look for traces of Zika in maternal blood, umbilical cord blood, amniotic fluid and other tissues.
At this point, Marques isn’t expecting to figure out how the virus may cause brain damage; he just wants enough cases to tease out any link between Zika and microcephaly. “If we see signs of viral infection in the placenta or the blood or brain tissue of these babies, that would strengthen the case,” he says. The researchers have already begun enrolling people, and Marques hopes to recruit all 600 participants by June or July.
Scientists still have a lot to learn about Zika virus, Hotez says, such as how the virus passes from mother to fetus, and — if infection does cause microcephaly — what exactly Zika virus does to developing brains.
Currently, scientists have no antiviral therapy for Zika infection and no vaccine. Developing a vaccine could take years, Fauci says. “Even though we started aggressively on it a month or two ago, it’s going to take a while before we get one.”
In the United States, that could leave millions of people at risk for infection, Isaac Bogoch and colleagues reported January 14 in the Lancet. Bogoch’s team made a global map of places Zika virus could readily spread. The researchers factored in climate, flight patterns out of Brazil, and mosquito species that can carry Zika virus, Aedes aegypti and Aedes albopictus. (Both species hug the U.S. Gulf Coast, and A. albopictus fans out across the southeast and up along the coast, ranging as far north as Connecticut.)
As many as 60 percent of U.S. residents live in areas threatened by Zika virus (at least during warm seasons), the team found. For southern states, especially, “there’s a potential for ongoing transmission,” says Bogoch, a tropical infectious diseases physician at Toronto General Hospital.
Still, that doesn’t mean a Zika virus outbreak is imminent, or inevitable, he says. The virus doesn’t ordinarily pass from person to person, although at least one sexually transmitted case has been reported. And it might not move beyond a handful of confined cases. After all, chikungunya and dengue virus, which ride the same mosquitoes as Zika, haven’t hit the United States especially hard.
For all these viruses, getting control of an outbreak requires getting rid of mosquitoes, Marcondes advises in a review published December 22 in the Journal of the Brazilian Society of Tropical Medicine. “Preventing breeding is the only way,” he says.
Already, Brazil is trying out an approach to cut wild mosquito populations by genetically engineering and releasing mosquitoes that can’t reproduce.
Fauci agrees that controlling mosquito populations is key. But the recent emergence of so many tropical viruses might also require some new defensive strategies, he says. If scientists could find an antiviral drug that targeted the larger group of viruses that Zika belongs to, for instance, they could knock out several threats.
“You would automatically get chikungunya, West Nile, yellow fever, Zika and dengue with one shot,” he says.
Editor’s note: This story was updated on February 3 to add in the confirmation of a sexually transmitted case of Zika virus infection.