Global health is complicated. We knew this already, true, but a new study on cholera and river flows brings home that complexity in all its inexplicable glory. While we once assumed that warmer water meant more cholera – a grim portent of widespread cholera in a future of climate change – it seems that instead cholera has a complicated relationship with river flows.

The research, published on August 3 in the American Journal of Tropical Medicine and Hygiene looks at the connection between rivers, cholera, and ocean plankton. Phytoplankton are tiny microscopic organisms that live in the ocean. When they die, they are eaten by other organisms – zooplankton. Zooplankton and cholera go together. Warmer water temperatures mean phytoplankton death, zooplankton feasts, and cholera bacteria proliferation – exactly what we need to complement the ecological devastation of the oceans. Or so we thought. But new data has shown sea temperatures are rising and phytoplankton doesn’t die. What’s up?

Here’s what the new study tells us: large river discharges into the ocean cause phytoplankton die-off and the resulting increase in cholera bacteria. So the rising ocean temperatures are not an automatic guarantee of cholera expansion. Global warming can still cause flooding and river flows, so we’re not cholera-immune. But it’s a different process.

This also casts an interesting light on Haiti’s cholera. We know that the earthquake affected Haitian rivers. That may well be what triggered the cholera outbreak.

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