A health worker shows a flyer used to explain how to prevent Dengue, Chikungunya, and Zika viruses in San Salvador, El Salvador. REUTERS/Jose Cabezas

This year might be a glimpse into the infectious world of the future.

The Zika virus outbreak "is now spreading explosively" across the Americas, with 3 to 4 million infections estimated for the region in the next year, as Mother Jones reported Thursday.

The infectious disease, which doctors suspect has caused an increase in microcephaly, is likely linked to this year’s El Niño, a global climate cycle characterized by a band of warm water in the equatorial Pacific. But the Zika virus isn’t the only public health concern El Niño brings. This year’s landscape of infectious disease might be a glimpse into a sicker future.

The Zika link to El Niño

The Zika virus is borne by Aedes aegypti, the same species of mosquito that carries yellow fever, dengue fever, and chikungunya. Virtually all mosquitoes thrive in humidity, and Aedes aegypti is no exception.

"One of the hallmarks of these mosquitoes is they like very clean water," Laurie Garrett, senior fellow for global health at the Council on Foreign Relations, told Mother Jones. "So rainfall is perfect for them. If it creates puddles, or accumulates in tires or any sort of containers, that will be a breeding site."

El Niño has helped create ideal mosquito breeding conditions in Latin America. In late December, Brazil, Paraguay, Argentina, and other countries experienced the worst flooding in half a century, driving 150,000 people from their homes and killing at least 10. The region also experienced above-average warm temperatures in 2015. Warm temperatures can promote higher rates of mosquito-biting and accelerate the development of mosquito larvae.

For the same reasons, El Niño is likely linked to the surge in dengue fever that’s also being felt across Latin America—and in general, it helps drive the spread other infectious diseases globally.

How El Niño drives other infectious diseases

“All pathogens—viruses, bacteria, fungi and parasites—are temperature-sensitive,” says Dr. Aaron Bernstein, Associate Director of the Center for Health and the Global Environment at Harvard University. “They have differences in how they reproduce and they infect people and other animals based on the temperature they’re living at.”

El Niño’s warm temperatures and heavy rainfall are strongly linked with diseases of all kinds. As detailed in a textbook chapter on internal medicine that Bernstein authored, scientists accurately predicted outbreaks of Rift Valley fever several weeks prior to epidemics in East Africa during the 2006–2007 El Niño season. El Niño can also push up malaria incidence. Research in Colombia found that a 1°C temperature rise contributed to a 20 percent increase in new cases of malaria between 1960 and 2006. Dengue fever was also found to be strongly associated with the El Niño events of 1997–1998 and 2006–2007.

Waterborne diseases can also surge during El Niño years. Bernstein says that research has shown warmer temperatures in the Bay of Bengal seems to facilitate the reproduction of cholera bacteria.

Characteristic patterns of disease outbreaks associated with El Niño events. (International Journal of Health Geographics, 2006)

If you’re feeling relieved that these outbreaks surge only during the occasional El Niño year, think again. For years, public health scientists have used El Niño events as a kind of natural experiment to investigate the occurrence of infectious disease in a world marked by warmer temperatures and more extreme weather worldwide—i.e., the conditions of climate change. Now, many scientists believe that climate change itself may be strengthening El Niño events in general.  

Does that mean that Zika virus outbreaks will worsen as climate change progresses? Too early to say. It’s hard for scientists to model the exact extent to which climate change is affecting this year’s El Niño in particular, and not every disease will necessarily become more prevalent with climate change. Still, says Bernstein, “Given what we know about how infectious diseases spread with regards to temperature and precipitation, doing nothing about climate change doesn’t make sense.”

What humans can do

Even with the double-whammy of El Niño and climate change, there are public health measures humans can take to stem the tide of warm-weather-loving infectious disease.

Brazil, where the Zika virus outbreak began in 2015, is a good example of what not to do: There, a waning economy and a government corruption scandal have enfeebled an already deficient public health system. In turn, doctors and officials have struggled to effectively deal with the virus-carrying insects, as well as with thousands of babies born with microcephaly. "We were taken by surprise," Angela Rocha, a pediatrician and infectious disease specialist at the Oswaldo Cruz University in Recife, told Reuters. Wednesday, the country’s top health official acknowledged that Brazil was “badly losing the battle” against mosquito-borne infectious diseases.

The situation highlights the need for strong public health services and proactive vaccine development. "With Zika and Ebola, we're returning to a time that's similar to the 1940s before we had polio and rubella and other vaccines,” Peter Hotez, founding dean of the National School of Tropical Medicine at the Baylor College of Medicine in Houston, told U.S. News. “It's a terrible feeling of vulnerability.”

U.S. officials have said that a vaccine for the Zika virus may not be available for years—although a Canadian vaccine developer said Friday that one could be ready for emergency use before the end of 2016.

Now, doctors in Latin America are saying that they suspect microcephaly is just the tip of the iceberg when it comes to neurological birth defects caused by the Zika virus. We’re dramatically underestimating what’s at stake for public health with this,” says Bernstein. But on the bright side, “we still have a tremendous amount of power right now to substantially reduce the risks.”

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