Wildlife authorities say around 330 tuskers have died and that a species of cyanobacteria that produces neurotoxins was the cause.
Toxins in water produced by cyanobacteria have killed more than 300 elephants in Botswana this year, officials have said, announcing the result of an investigation into the deaths which had baffled and alarmed conservationists.
Cyril Taolo, deputy director of the Department of Wildlife and National Parks, told a news conference the number of elephant carcasses found since deaths were first reported around early May had risen to 330, from 281 in July.
"What we just know at this point is that it's a toxin caused by cyanobacteria," said Taolo, adding the specific type of neurotoxin had yet to be established.
Authorities will monitor the situation during the next rainy season, and Taolo said for now there was no evidence to suggest that Botswana's wildlife was still under threat as officials were no longer seeing deaths.
Only elephants affected
The department's principal veterinary officer Mmadi Reuben told the same news conference that questions remained as to why only elephants had been affected.
Other animals in the Okavango Panhandle region appeared unharmed.
"Our latest tests have detected cyanobacterial neurotoxins to be the cause of deaths," Reuben, principal veterinary officer at the Department of Wildlife and National Parks, said.
Cyanobacteria are microscopic organisms common in water and sometimes found in soil. Not all produce toxins but scientists say toxic ones are occurring more frequently as climate change drives up global temperatures.
Elephants face difficulty in walking
Keith Lindsay, another official, said that the animals died while walking, which was unusual.
"One elephant was observed walking in circles, unable to change direction although being encouraged by other herd members," the Elephants Without Borders, a conservation foundation, said in a report.
"Several live elephants appeared to have been weak, lethargic and emaciated, with some showing signs of disorientation, difficulty in walking or limping," it said, adding that aerial surveys revealed elephants of all ages and sex appeared to be dying.
Habitat loss and poaching
In Africa and Asia, elephant habitat is being replaced by agriculture – both by small-scale farmers and international agribusiness such as palm oil.
While Africa’s elephant population is decreasing due to poaching, Zimbabwe, South Africa, Botswana and Namibia unsuccessfully lobbied for the Convention on International Trade in Endangered Species in August for controlled sales of their ivory stocks.
They want permission to sell ivory to fund conservation as well as community development. But 32 other African nations believe all trade in elephants must end.
Africa's overall elephant population is declining due to poaching but Botswana, home to almost a third of the continent's elephants, has seen numbers grow to around 130,000.
Some cyanobacterial blooms can harm people and animals, and scientists are concerned about their potential impact as climate change leads to warmer water temperatures, which many cyanobacteria prefer.
Southern Africa's temperatures are rising at twice the global average, according to the Intergovernmental Panel on Climate Change.
"It amounts to having the right conditions, in the right time, in the right place and these species will proliferate," Patricia Glibert, a professor at the University of Maryland Center for Environmental Science, who has studied cyanobacteria, told Reuters news agency.
"These conditions are coming together more often, in more places, so we are seeing more of these toxic blooms around the world."