This piece was originally published in The Guardian and appears here as part of our Climate Desk collaboration.

Hundreds of elephants died in Botswana earlier this year from ingesting toxins produced by cyanobacteria, according to government officials who say they will be testing waterholes for algal blooms next rainy season to reduce the risk of another mass die-off.

The mysterious death of 350 elephants in the Okavango Delta between May and June baffled conservationists, with leading theories suggesting they were killed by a rodent virus known as EMC (encephalomyocarditis) or toxins from algal blooms.

“Our latest tests have detected cyanobacterial neurotoxins to be the cause of deaths. These are bacteria found in water,” Mmadi Reuben, principal veterinary officer at the Botswana Department of Wildlife and National Parks, said in a news conference on Monday. “However, we have many questions still to be answered such as why the elephants only and why that area only. We have a number of hypotheses we are investigating.”

Local sources suggest 70 percent of elephants died near water holes containing algal blooms, which can produce toxic microscopic organisms called cyanobacteria. Toxins were initially ruled out because no other species died, except for one horse, but scientists now think elephants could be particularly susceptible because they spend a lot of time bathing and drinking large quantities of water.

Reuben said the investigation looked at how mortality affected the elephant population and injuries on carcasses, as well as testing water samples at laboratories in Botswana, South Africa, and the U.S. He said the cause was a “combination of neurotoxins” but declined to give further details, and declined to say at which institutions tests had been carried out.

The Okavango Delta is a region of striking biodiversity that provides water to a million people.
The Okavango Delta is a region of striking biodiversity that provides water to a million people. Diego Delso / CC BY-SA 4.0