For four years starting in 2012, dry weather crackled across northeastern Brazil. Temperatures rose, vegetation died, and fresh water started to evaporate more quickly from the region’s reservoirs. Though the conditions were devastating for local human populations, they were ripe for bacterial blooms to flourish in the water. Among the bacterial species known to establish themselves in reservoirs and wells during such droughts is the cyanobacterium Raphidiopsis raciborskii.
More often than not, the strain of R. raciborskii that colonizes reservoirs and wells in the region is one that secretes saxitoxin, a chemical that lets the bacterium thrive in the salty, mineral-rich water associated with dry spells. Although saxitoxin aids the bacterium’s survival, it is—as its name indicates—toxic to humans, who often ingest it when eating contaminated freshwater shellfish. In large amounts, the neurotoxin can be deadly, causing respiratory failure; in lesser amounts, it leads to numbness and partial paralysis.
Because of this toxicity, Brazilian water quality guidelines state that levels of saxitoxin must be lower than three micrograms per liter, which would keep people safe if they consume the contaminated water only infrequently. During droughts, however, that water is likely to be more contaminated than normal, putting people at risk of higher exposure to saxitoxin.
Northeast Brazil was the epicenter for cases of microcephaly. But the incidence of Zika was higher in other regions of Brazil.—Stevens Rehen, Federal University of Rio de Janeiro
To explore the effects on the brain of regularly drinking contaminated water, Katie O’Neill of the University of Adelaide and colleagues set up a lab experiment with cultured nerve cells in 2016. Continuous, low-level exposure to saxitoxin impaired the cells’ growth, the team found, making it hard for them to form the spiny protrusions that are essential for cell-to-cell communication. The neuro-toxin also disrupted expression of proteins involved in mitochondrial function and in programmed cell death, the team found. It was a hint that, even in low doses, saxitoxin may pose a risk of neurological damage.
As the dry spell lingered into the 2010s, northeast Brazil was hit with a second health crisis: an outbreak of illness spread by mosquitoes infected with the Zika virus. Zika swept across South America and other regions of the world in 2015. As it did so, doctors treating patients sickened by the disease began to note that pregnant mothers who gave birth after being infected sometimes had babies with microcephaly, a condition in which the baby’s head, and often also its brain, are much smaller than normal.
“Northeast Brazil was the epicenter for cases of microcephaly,” says Stevens Rehen, a neuroscientist at the Federal University of Rio de Janeiro. “But the incidence of Zika was higher in other regions of Brazil.” That discrepancy led Rehen and his colleagues to wonder if some environmental factor was compounding the effects of viral infection in pregnant women in the northeast region, leading to more-severe brain damage in their babies. When his team saw that northeast Brazil suffered its worst drought on record at the same time as the Zika outbreak, the researchers decided to test whether saxitoxin and Zika together spurred serious changes to brain tissue.
The team started tests in human brain organoids, growing clumps of nerve cells from the reprogrammed skin cells of donors. After cultivating the cells for 50 days, Rehen and his colleagues infected the brain organoids with Zika virus and then treated them daily with low doses of saxitoxin. After 13 days of treatment, the team looked at the organoids under the microscope and found that the clumps of brain cells exposed to both Zika and saxitoxin had 2.5 times more dead cells than organoids that were only infected with Zika. Organoids exposed to both saxitoxin and Zika also had levels of the virus that were three times higher than what was found in Zika-only cultures, suggesting that the toxin promoted viral replication. Organoids treated only with saxitoxin had a level of cell death similar to what was observed in untreated organoids.
Wanting to check the results in animals, Rehen and his colleagues set up an experiment in which female mice were given either clean or saxitoxin-contaminated drinking water for a few days before and a few days after mating. After the mice became pregnant, the researchers infected them with Zika. Baby mice born to Zika-only mothers had fairly normal brains, the team found, but those born to Zika-infected mice drinking saxitoxin-laden water had unusually small brains, with around a 30 percent reduction in the thickness of their cortex, a layer of the brain known to be essential for cognition. Those mice also had twice the number of dead nerve cells in their brains as the pups whose moms were only infected with Zika or were not subjected to either treatment, the researchers reported earlier this year.
“This paper demonstrates that a long-standing problem with cyanobacteria toxins in the water resources of the region has played a role in making the impact of the Zika outbreak in the region much worse,” says Alexandre Anesio, a biogeochemist at Aarhus University in Denmark who was not involved in the work.
Rehen notes that the research not only shows a connection between saxitoxin and Zika, but also exposes a potential reason for the observed economic disparity in severity of illness. Initially, “we were surprised by the fact that many babies with microcephaly were born at Brazilian cities with very low gross domestic product,” he writes in an email to The Scientist. In light of his team’s findings, it seems that “unfortunately, these malformations were probably exacerbated by avoidable environmental cofactors associated with poverty and lack of basic needs.”
The importance of improving access to clean water is clearly shown in this new work, notes Fabiano Thompson, a micro-biologist at the Federal University of Rio de Janeiro who was not involved in the study. “Even in countries like Brazil, which has 10 percent of the fresh water of the planet, water is a pricey resource,” he says. “Governments need to be very careful with this.”