Out of Africa

Nearly fifty years ago, I was living in Entebbe, then capital of Uganda, studying mosquito-borne viruses at the East African Virus Research Institute. One of those, a dengue-like virus, was named after the Zika forest, a little residual pocket of trees down the hill from the institute, where it was first isolated. I was coauthor on a 1964 paper describing 12 strains of Zika virus from mosquitoes collected there (Bull World Health Organ, 31:57-69, 1964), and we deduced that it cycled between mosquitoes and monkeys in the Zika treetops.

Since then I hadn't thought much about Zika. Its life has been largely sporadic, then silent - the medical literature has cataloged fewer than 40 cases in more than 50 years, and none from the last 25. The few cases we saw were in single outbreaks in Nigeria, Indonesia, and Malaysia. But all that has changed: The virus has reappeared, spreading and sickening hundreds.

This spring, Zika burst out on Yap, an island group in the Federated States of Micronesia (FSM), in the Western Pacific. The virus causes skin rash, conjunctivitis, and pain in the fingers and toes. The outbreak was relatively mild: A few patients had low-grade fever, but nobody had to be hospitalized.

So how did the virus make its way from Africa to the Western Pacific, where is it going, and why did it appear in Micronesia? And why as an epidemic, its first?

In a new location, once it gets into the ubiquitous dengue vectors Aedes aegypti or Ae. albopictus, Zika can take off and cause an epidemic. (One or both of those vectors is almost certainly behind the Yap epidemic.) The FSM are closer to Asia than Africa, but an infected human or mosquito can travel anywhere by plane within hours. Mosquitoes infected with malaria have disembarked from planes at various European international airports and caused local cases of what is known as "airport malaria"; the same could have happened with Zika.

For an infected traveler to cause an outbreak, a concatenation of circumstances is required. Travelers must arrive during the mosquito season, and during those few days they have sufficient virus in their bloodstreams to infect mosquitoes that bite them. The mosquitoes must be able to incubate and then transmit the virus to the local population. So, out of many infected passengers, only a handful will ever give rise to a local case. Zika epidemics may have happened before in Asia, since the virus is easily confused with dengue.

If molecular studies reveal that this epidemic originated from Asia, sequencing strains identified as Zika from Malaysia and Indonesia may show that they are the same virus, or as yet unnamed, closely related viruses of the same group. Will the epidemic spread all around the Pacific Rim, to mainland countries? This happens: An unrelated virus, chikungunya, has infiltrated the islands of the Indian Ocean and the countries around its rim, causing thousands of cases and many more severe neonatal infections and even fatalities (The Scientist Daily News, "Hunting a chikungunya vaccine").

I am betting on Zika virus spreading widely, because the Western Pacific is virgin soil, with a population with no preexisting immunity and therefore 100% susceptible. I'd also put money on it turning more virulent; with hundreds of cases, you get many more mutations. When it first appeared, dengue, too, was relatively harmless but then became deadly over time.

Prevention and control are the same for both Zika and dengue: eliminating standing water, covering skin, and using mosquito nets. Of course, officials have recommended these prevention strategies for many years now, and dengue remains a major cause of hospitalization and death among children in Asia. In 2004, more than 160,000 people living in the Western Pacific region, the site of the current Zika epidemic, developed dengue and dengue hemorrhagic fever. Hundreds have already died. Indeed, on July 23, the World Health Organization issued a warning that the Western Pacific Region may be on the verge of a major dengue outbreak.

If people aren't adequately protecting themselves against a pathogen that's been wreaking havoc for decades, will they step up their efforts against a new, still relatively harmless virus? Not likely.

Jack Woodall is former director of the Nucleus for the Investigation of Emerging Infectious Diseases in the Institute of Medical Biochemistry at Brazil's Federal University of Rio de Janeiro.