In 2015-16, the world got a small taste of the coronavirus pandemic with the Zika virus outbreak. Although the method of transmission of Zika is very different, and it's seldom fatal, there was a similar scrambling to understand the disease and work out how to contain it. A particular puzzle was why the effects were so much more severe in Northeast Brazil than elsewhere. New evidence points the finger at water contamination.
Zika virus was first detected in Africa in the 1940s. Although related to other mosquito-borne viruses that cause lethal conditions such as dengue fever, it was little studied because effects were usually mild. That was until its arrival in the Americas, where infections triggered a wave of birth defects.
The outbreak was widespread, but its most severe effects were much more concentrated. The World Health Organization lists “increased risk of preterm birth, fetal death and stillbirth” as consequences of infection during pregnancy, but the most severe effect was children born with significantly smaller brains (microcephaly). Children that were born with Zika-induced microcephaly are too young for us to fully know the long-term consequences, but severe brain damage is likely.
From the beginning, scientists wondered if microcephaly was more common in Northeast Brazil because of a more serious strain of the virus or because it was interacting with a local environmental factor. Professor Stevens Rehen of the Federal University of Rio de Janeiro argues for the environmental theory, and blames cyanobacteria in the water supply.
In PLOS Neglected Tropical Diseases, Rehen reports that water collected in Northeast Brazil at the height of the crisis averaged 20,000 bacteria per milliliter. Some cyanobacteria produce saxitoxin, and half the drinking water sources in the area were significantly contaminated. As its name suggests, saxitoxin is poisonous, but Rehen argues in combination with Zika infection, the effects become much worse.
Cell cultures and mouse studies both confirm Zika and saxitoxin have a synergistic effect, where the combination causes three times the brain cell death of Zika on its own. Pregnant mice infected with Zika and given saxitoxin in their water gave birth to offspring with microcephaly-like symptoms. The theory also explains reports that milder microcephaly cases were common in the area prior to Zika's arrival.
Like Covid-19, Zika was a magnet for conspiracy theories, along with serious scientific explanations that turned out to be wrong. However, Rehen has credibility to draw on with a distinguished career and leader of a team that first showed a biological basis for the Zika-microencephaly association.
The high cyanobacteria concentrations in this region were a product both of inadequate infrastructure and the outbreak coinciding with the worst drought on record from 2012 to 2017, probably exacerbated both by global climate change and regional deforestation.