NEW YORK — On May 25, conservationists were flying over Botswana’s Okavango Panhandle when they counted something disturbing: 169 dead elephants. A second flight in June revealed more carcasses, bringing the total to 356. Some of the animals appeared to have died suddenly, collapsing chest-first while walking or running. No tusks were removed, suggesting that poaching for ivory might not have been to blame.

But experts were left with few clues as to whether the cause was something sinister, like poisonings, or a naturally occurring disease from which the elephants will recover.

“As elephant populations grow, it is more likely that you will get mass die-offs, probably on a bigger scale than this,” said Mr Chris Thouless, the head of research at Save the Elephants, a conservation organisation based in Kenya. “Death is no fun, but it comes to all living things.”

But other conservationists expressed more concern.

“In Botswana, there is a huge crisis for elephants unfolding,” said Mr Mark Hiley, the director of rescue operations at National Park Rescue, a nonprofit organisation based in Britain that combats poaching in Africa. “The most important thing now is for an independent team to visit the area — sample multiple carcasses, the soil and waterways — and identify what is causing the deaths.”

Researchers from Elephants Without Borders, the conservation group in Botswana that conducted the flights documenting the problem, observed some elephants that appeared to be disoriented, including one that was walking in circles. Others were dragging their hind legs, as though paralysed, and still others appeared lethargic and emaciated. Males and females, young and old, all seem equally affected.

Botswana is home to around 130,000 savanna elephants, or about one-third of the world’s remaining population. Although there are some signs that elephant and rhino poaching may be picking up there, many conservationists still consider the country a critical safe haven for elephants.

In a report submitted to government officials, Elephants Without Borders estimated that the spate of mysterious deaths began at least as far back as March. The total number of dead elephants almost certainly exceeds 356, the authors write, because their flights did not cover the entire affected area.

Some conservationists say the country’s government is not taking the deaths seriously enough. Officials collected samples from dead elephants for testing in May, but they have not yet released results.

“This started months ago, and by now, the government should be able to tell everyone clearly what this is,” Mr Hiley said. “There are plenty of reputable laboratories that could have come up with a result by now.” The delays in testing, he added, could “literally be killing elephants.”

Dr Mmadi Reuben, the principal veterinary officer at the Botswana Department of Wildlife and National Parks, said the government was taking the deaths seriously and had responded “swiftly, adequately and responsibly.”

He said that some testing has ruled out common causes like anthrax, which is caused by bacteria that occur naturally in soil. He and his colleagues are now working with labs in Zimbabwe, South Africa and Canada to perform further testing. “It’s not going to be a one-off thing where we say, ‘We’ve sent out samples, now we’re done,’” Dr Reuben said. “It’s an ongoing dialogue with different labs.”

There is still no evidence that the deaths are foul play by humans, he added.

Cyanide, which poachers sometimes use to poison elephants, seems unlikely, because carcasses tend to be clumped together near where the poison was deployed. It also tends to kill other animals, but no other species seem to be affected in this case. However, it’s possible that other poisons could be used against elephants, and Mr Hiley says some of them can dissipate quickly.

Covid-19, he added, was unlikely, because the disease has yet to infect people in Okavango’s communities. There is no evidence yet that elephants can contract the virus.

Mr Thouless suspects that a naturally occurring disease is the most likely culprit. One leading candidate is encephalomyocarditis, a viral infection that can be transmitted by rodents and can cause neurological symptoms. It killed around 60 elephants in South Africa’s Kruger National Park in the mid-1990s. Botswana also recently emerged from a drought, which could have left some elephants stressed and more vulnerable to disease, Mr Thouless said.

At this point, he continued, the deaths do not constitute a conservation crisis, because the numbers documented so far represent a small percentage of the 15,000 to 20,000 elephants that live in the Okavango Panhandle. “This is distressing, but it’s currently trivial in population terms,” he said.

Past examples also show that when conditions are favorable, elephants can quickly rebound. For example, in 1970 and 1971, a drought in Tsavo East National Park in Kenya killed an estimated 5,900 of the park’s 35,000 elephants. By 1973, the population was back to 35,000.

“There’s a limit to how much interfering with nature is worth doing,” Mr Thouless said. “You can go to an enormous amount of effort without actually achieving anything different in conservation terms.” THE NEW YORK TIMES