A type of bacterium never seen before in wild elephants has been found in the bodies of six African Savannah elephants that died in mysterious circumstances in Zimbabwe.

Scientists think it was the cause of a septicaemia, or blood poisoning, that killed the animals in 2020.

The findings are published in the journal Nature Communications.

The study might provide more clues about the deaths of 356 elephants in neighbouring Botswana that same year.

That incident drew international headlines, when conservationists discovered the dead elephants in Botswana’s Okavango Panhandle. Some of the animals appeared to have collapsed and died suddenly while walking or running.

Researchers who made the discovery were investigating the sudden deaths of 35 elephants – mostly between August and September 2020 in North-Western Zimbabwe.

Dr Chris Foggin, a wildlife veterinarian from the Victoria Falls Wildlife Trust explained that the team had to locate the bodies using a helicopter.

Wearing protective clothing, in extreme heat, he and his team carried out post-mortem examinations of 15 elephants. “They’re such large animals,” said Dr Foggin. “So it’s quite a physical operation to get access to the organs we needed to sample.”

The researchers managed to confirm that 13 of the elephants had septicaemia when they died. Crucially they found a potential cause – a bacterium called Bisgaard taxon 45 – in six of the animals. It is a type that has been found previously in swabs taken from tiger or lion bites.

Prof Falko Steinbach from the UK’s Animal and Plant Health Agency, who was involved in the mission, told BBC Radio 4’s Inside Science that severe drought and shortage of food that year could have compromised the animals’ health, allowing the disease to overcome their immune defences.

“[This type of bacteria] wasn’t completely mysterious – it was known to exist,” explained Prof Steinbach. “But it had not been associated with septicaemia and never been found in the African elephants.”

The scientist said he suspected that the elephants were under “severe stress” because of the ongoing drought and difficulty finding food.

“This is relatively common,” he explained. “We have a lot of pathogens have that [usually] do not cause disease – and certainly not death. But if the host defences break down, that allows the bacteria to spread and in the end, it doesn’t just cause a local infection after a bite – but causes severe disease.”

Elephants are highly social animals, so the researchers are concerned that they could also transmit the bacteria to others, which would explain the large number of deaths.

Laura Rosen from US-based company Transboundary Epidemiology Analytics, said the finding was “very worrying”.

“African savannah elephants are an endangered species with only 350,000 remaining in the wild and ongoing losses estimated at 8% annually. Investigating the deaths of these elephants is crucial to sustain the future of this majestic species.”

Prof Steinbach added that more research was needed to understand “the link between this infection and the stress associated with extreme weather events such as drought, which may make outbreaks more likely”.

“Hopefully with further studies, we would be able to identify not only what leads to these outbreaks, but also maybe to come up with intervention strategies – possibly even a vaccine. But that would require substantial further investigation.”