28 January 2016

The human causes of viral outbreaks like Zika

Laura H. Kahn

Only two years have passed since chikungunya, a debilitating mosquito-borne virus, appeared in the Americas. Now, a new scourge is causing havoc: the Zika virus. Historically, Zika virus outbreaks tend to follow chikungunya outbreaks, though we don’t know why. In fact, there is still much we don’t know about the virus. What we do know, though, is that human behavior and activities such as environmental destruction lead to the emergence and spread of mosquito-borne viruses. That means that with long-term political will and commitment, it might be possible to prevent the next such outbreak. 

The Zika virus was first discovered in April, 1947 when a rhesus monkey became sick during a research program on yellow fever in the Zika Forest of Uganda. For decades, the obscure virus stayed within a narrow equatorial region spanning Africa and Asia. That changed in April 2007 when it first appeared on Yap Island, one of the Federated States of Micronesia. Since then, global travel has facilitated its spread to the Americas. To introduce the virus to a new area, all that is required is for one Zika-infected individual to go to a region without the virus and get bit by an Aedes species mosquito. The newly exposed mosquito then bites another individual and the viral fire takes off, like a match dropped on kindling. Once a novel mosquito-borne virus gets in to a new region, it is very hard, if not impossible, to eradicate.

Only one in five people infected with the Zika virus get sick, and of those who do, the disease is generally mild and lasts from several days to a week. Common symptoms include fever, red eyes, a rash, and joint pain. It rarely causes death.

Unfortunately, though, the virus may lead to serious complications for some. In Brazil, which has experienced an explosive Zika virus epidemic since late 2014, disease specialists have observed an increase in rates of Guillain-Barré syndrome, a condition in which the immune system attacks the nervous system leading to complete paralysis. Though most people fully recover with intensive care and physical therapy, Guillain-Barré is potentially life-threatening.

The Zika virus also appears to be causing an epidemic of microcephaly, a type of abnormal brain development, in babies born to women who were infected during pregnancy. This devastating complication is permanent and leads to life-long disability. Over the past year, nearly 4,000 cases of microcephaly have been reported in Brazil. (The first case reported in the United States occurred in Hawaii in a woman who had spent time in Brazil while she was pregnant.) There is not yet definitive proof that the virus causes microcephaly,  as many of the mothers of the affected infants, though they reported rashes during their pregnancies, did not have laboratory-confirmed cases of Zika virus disease. It is notable, though, that before the Zika virus appeared, fewer than 200 cases of microcephaly were reported per year. 

Regardless of the lack of definitive laboratory-confirmed proof, concerns about the potential risks are so great that US health officials have issued a travel advisory against pregnant women traveling to Zika virus-infected areas.  El Salvador, which also saw an increase in microcephaly cases after the appearance of the virus, made the unprecedented decision to advise all women to avoid getting pregnant for the next two years

Global travel isn’t the only way humans help spread mosquito-borne diseases. By destroying forests and other ecosystems, we have inadvertently created incentives for mosquitoes to seek new habitats—namely densely populated urban areas. Climate change is theoretically another driving force behind the expanding geographical range of certain mosquito species, although right now, the data is too limited to make any definitive conclusions.

Currently, there is no anti-viral medication or vaccine for the Zika virus. Efforts to develop a vaccine are only in the earliest stages, which means that it will likely take years before a safe and effective one is available. What can be done in the meantime? Effective mosquito control programs are essential. Screened windows and doors, bed nets, protective clothing, insect repellent, and the elimination of larval breeding sites are all important strategies. For developing countries such as Brazil, urban slums pose serious obstacles to mosquito control because of poor sanitation and hygiene. Mosquitoes love to breed in puddles of sewage and in the water that collects in used tires.

Over the long term, though, we need strategies that limit some of the human behavior that leads to mosquito-borne outbreaks. We have no one to blame but ourselves. The question is: When will political leaders decide to be proactive and develop effective policies to mitigate these problems? Mosquitoes may thrive in slums, but they’re equal opportunity biters and don’t stop for national borders. All urban areas require clean, safe water supplies and modern sanitation systems, both for the sake of the people who live there and for the rest of the world.