Drought may exacerbate the spread of Zika and other mosquito-borne viruses

zika mapThis week the World Health Organization declared the spread of Zika virus the world’s fourth international public health emergency. Ever. Just one of several mosquito-borne illnesses with severe health impacts on humans, including death, the media has been quick to connect the expected spread of Zika to the global changing climate. Mosquitos thrive in wet, hot conditions and, as the world warms, researchers predict some areas will see increased rain and flooding.

Unfortunately for the global population, it’s not just warmer, wetter regions that may be susceptible to the spread of mosquito borne viruses including Zika, dengue, malaria, West Nile and chikungunya. Drought is also likely to play a major role in their infectious reach.

It’s counterintuitive: if mosquitos need water to reproduce how do abnormally dry conditions result in increased mosquito-borne viruses? The problem is that mosquitos need standing water, not the moving stuff, in order to lay eggs. While there is less active fresh water in a drought, there is an increase in water storage. Whether this is government subsidized storage facilities and reservoirs or household attempts to collect water in pots, drums and barrels, the result is the same – higher numbers of standing water sources in close proximity create the perfect breeding ground for mosquitos.


Evidence from drought stricken regions over the last few years support the theory.

Sao Paulo, Brazil is in the midst of the worst drought in over 80 years. In 2015 officials in this city of over 11 million were forced to ration water, turning it off for large portions of the day. To make up for the shortage, residents  collected water when possible in any containers available for cooking, cleaning, drinking and bathing at times when the water was turned off. During the first twelve weeks of 2015, coinciding with the city’s water rationing efforts, dengue outbreaks were nearly triple that of outbreaks during the same period in 2014 (20,764 cases vs. 7,126 cases). More than half the country’s dengue cases in 2015 came from Sao Paolo in 2015 compared to only a quarter in 2014.

Spurred by the outbreak, the city unleashed a team of 2500 disease specialists to move door-to-door and discuss dengue prevention. Millions of Sao Paulo residents, however, will never see this small cadre of public health assistants. The math, 2500 specialists to 11 million plus individuals, simply doesn’t work.

Now Zika, carried by the same mosquito species as dengue (Aedes aegypti), is exploiting its blood sucking hosts in the same way. Public health officials expect three to four million cases of Zika to appear in regions where dengue currently exists. Brazil is the epicenter of the current Zika outbreak and, while most  cases have been reported in the country’s northeast, they are rapidly appearing in Sao Paulo, following the path of dengue last year.

The Caribbean is also suffering under a severe multi-year drought. In July 2014, the spread of chikungunya was declared an epidemic. In the Dominican Republic alone, hundreds of thousands of cases of chikingunya were reported, significantly outpacing dengue infections. Chikungunya virus is of a different type than dengue, West Nile, and Zika (the former an Alphavirus, the later Flavaviruses), but is spread the same way, via mosquito (by two separate species, A. aegypti and A. albopictus). The Caribbean, reeling economically from the loss of millions of dollars in agricultural and livestock production due to extremely dry conditions may have the sustained drought to blame for being one of the hardest hit regions by the Zika virus.

Climate researchers point out that developed nations are likely to have an easier time adapting to the challenges wrought climate change but, when it comes to mosquito borne illnesses in the Americas, the United States is far from immune. In fact, only two of the thirty-five countries of North and South America, Canada and Chile, are unlikely to be susceptible to illnesses like Zika or dengue.

The four year, ongoing California drought may, in fact, be responsible for the growth of West Nile virus in 2014. 801 cases of West Nile virus were recorded in the state, 131 of which were fatal. Whereas in Brazil, water rationing has forced households to collect and store water thereby attracting mosquitos, in California the lack of water rationing for non-essential purposes in a drought stricken region is part of the problem. With fewer natural sources of fresh water, mosquitos are more attracted to artificial sources such as swimming pools, ponds and fountains.  Drawn in to urban areas, mosquitos are brought in to closer contact and, as females wait for wetter conditions to lay their eggs, the virus incubates, amplifying its possible transmission to epidemic proportions.

In developed nations, it may further be the official government response to drought conditions that exacerbates the spread of viruses like dengue and Zika. Researchers in Australia in 2009 noted that areas of the country are already primed for the a. aegypti mosquito and the viruses it carries to thrive. It is the official response to dry conditions, including the introduction of government subsidized water storage devices, that could add the last needed piece of the puzzle: an ideal breeding ground for the mosquito population to explode.

We cannot yet know how climate change will impact the world but we do know that it is likely that official interventions will have unexpected repercussions that may exacerbate risks to public health, regardless of a nation’s wealth or preparation.


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