35 elephants suddenly dropped dead in 2020. Their case has finally been solved.

• In 2020, 35 elephants dropped dead in Zimbabwe over a month timespan.
• There were no signs of external trauma or intentional poisoning.

It took a team of scientists nearly three years to figure out what caused a mass die-off of elephants in Zimbabwe, but they finally closed the case.

Between August and September 2020, 35 elephant carcasses were retrieved from various safaris and protected lands in Zimbabwe — with eleven of these animals dying within a 24-hour period.

The team responsible for figuring out what happened initially ruled out intentional poisoning, poaching, and drought conditions, per the study published in Nature Communications. There didn't seem to be any signs of external trauma on the elephants.

They then decided to look inside and sample the brain, liver, and spleen tissue from 15 elephants and confirmed that six elephants had bacteria known as Bisgaard taxon 45 in their system.

They concluded that most, if not all, the elephants died as a result of septicemia, or blood poisoning, caused by the bacteria.

"Although there was not culture or molecular evidence to confirm Bisgaard taxon 45 in more than six mortalities in Zimbabwe," the study said, "the elephants examined were in good body condition and unlikely to have died of drought-related starvation or severe dehydration alone."

The bacteria is not known to kill African elephants, scientists said, but they are beginning to see a potential pattern of cases like this in herds across Africa. There have also been cases of blood infections killing off elephant populations in Asia.

The study authors said they are still stumped as to how the bacteria was transmitted to the elephant populations.

However, the authors write that climate change-induced factors like heat, drought, and population density could make this type of bacteria transmission more common.

"It's premature to say that climate change has influenced (this) but it may do so in future if we get more and prolonged droughts, or the rainfall patterns (change) and we have a much harsher dry season," Dr. Chris Foggin, one of the study's co-authors and a veterinarian at Victoria Falls Wildlife Trust, told CNN. "I do think that if that is the case, then we are more likely to see this sort of mortality event occurring again."

The 35 deaths in Zimbabwe also coincided with a mass die-off of elephants in Botswana in 2020. At least 350 elephants died within a three-month timespan, and researchers at the time concluded that blue-green algae contaminated local water sources and poisoned the elephants.

Foggin told CNN that the scientists did not believe the deaths in Botswana and Zimbabwe were connected.