The World Health Organization (WHO) announced on January 28th that the Zika virus is “spreading explosively.” The virus was unknown to most people until recently, and is one of the scary new viruses that emerge abruptly in the modern world. The causes for that are complex and have developed, unnoticed, over years or decades.
This virus was first isolated in Uganda in 1947 and had never been seen before, but it had probably lurked chronically in African monkeys, or some other native reservoir, for millennia. The same virus later turned up, in the same forest, within mosquitoes of the Aedes genus, and those mosquitoes are now identified as vectors of Zika, transmitting the virus from host to host when they bite. Later it was confirmed to infect people too, not only in Africa but also in Asia, in the range where Aedes mosquitoes reside. The symptoms, such as headache, fever, a rash, bloodshot eyes, were generally mild. Then, in 2007, Zika virus caused more than a hundred cases on Yap, an island in the southwestern Pacific, having somehow gotten there from the Asian mainland. Six years later, Zika emerged more dramatically in French Polynesia, sending an estimated 28000 people into medical care. Seventy-two patients had severe neurological symptoms, and 40 of them contracted Guillain-Barré syndrome, a dangerous autoimmune dysfunction, sometimes triggered by infection. That was the first signal that Zika virus infection could be dire.
In April of last year Zika virus infections were diagnosed in northeastern Brazil—a new location for the virus, where a huge population of susceptible humans is now within reach. In October, a drastic increase in cases of microcephaly among infants born to mothers in the northeast was observed. Amniotic fluid from at least two of those mothers contained evidence for the presence of Zika virus, suggesting (but not proving) the link between Zika and microcephaly. As the numbers rose further, the WHO issued warnings and called a meeting for February 1, to consider proclaiming Zika a public health emergency. However, as we saw during the Ebola events of 2014, such a declaration doesn’t necessarily trigger a coordinated and efficacious international response.
This is a story of biogeography as well as medicine and public health, and of the consequences of human travel and transport.
First, how did Zika virus get to Brazil? Possibly it traveled in the blood of athletes—when competitors from French Polynesia and other Pacific islands came to Rio de Janeiro, in August 2014, for the Va’a World Sprint Championship in outrigger canoeing.
Second, vectors were necessary, for transmitting it from human to human. One competent mosquito (Aedes aegypti, commonly called the yellow fever mosquito) is an African creature that probably reached the Americas on sailing ships at the time of the slave trade. Another, known as the Asian tiger mosquito (Aedes albopictus), arrived more recently, reaching South, Central America and the southern United States, probably by way of egg-laden water amid shipments of used tires from Asia. If those mosquitoes hadn’t been transplanted by human activity, decades and centuries before Zika virus, then the virus itself couldn’t have taken hold between the Pacific and Atlantic oceans.
The WHO has cautioned that Zika is likely to spread throughout the Americas, except for Canada and Chile. Several South and Central American countries, including El Salvador, Jamaica, and Colombia, have already advised women to delay pregnancy.
The question about such cases is: Will infected people infect Aedes mosquitoes, who will infect other people? It is a big question, as the Asian tiger mosquito is now also present across southern Europe, and both the tiger mosquito and the yellow fever mosquito inhabit most Asian cities. By one account, more than half the humans on Earth live within areas infested by Aedes mosquitoes. Public health officials will need to be vigilant in reducing standing water near human habitations, to prevent those mosquitoes from laying eggs that mature to adulthood. They will also face social realities: how to tell poor, disempowered women without access to birth control means not to get pregnant.
The danger of Zika and other newly emerged threats is the result of things people do as a modern society, such as traveling and transporting people and things speedily around the globe. In 2012, MERS coronavirus emerged from Saudi Arabia, stirring our concerns. In 2014 it was Ebola. This year it is Zika.