At the beginning of summer 2020, over a period of a few weeks, elephants of all ages were struck by the same illness.
These pachyderms walked in circles for hours before collapsing, at different locations in the Okavango Delta in northern Botswana.
Nearly 350 elephant carcasses were found.
In an email exchange with The Associated Press at the time, Mark Hiley, director of the conservation group National Park Rescue in Botswana, called the event “one of the greatest disasters of the century” for elephants.
Several theories were circulating, including COVID-19, anthrax or the encephalomyocarditis virus.
In the fall of 2020, the government of Botswana concluded that toxins produced by cyanobacteria contained in water points were responsible for the deaths of the elephants.
But authorities were unable to provide solid data.
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No sample
As the pachyderm carcasses were found in an isolated location, difficult to access, during a pandemic during which restrictions prevented certain movements, it had been very difficult to obtain samples in the field, recalled the authors of a study published a few days ago in the journal Science of the Total Environment.
“No complete in situ water and tissue samples contemporaneous with mortality had been collected,” the researchers wrote.
In the absence of solid data, scientists therefore decided to explore an “alternative strategy” to understand and elucidate what had happened.
They used satellite data to analyze the distribution of carcasses in relation to water points that contained cyanobacteria, also called blue-green algae.
Thanks to aerial photos from the time, scientists knew exactly where the carcasses were located. They cross-referenced this information with an analysis of the water bodies.
Using various data from satellite images, including the NDVI or Normalized Difference Vegetative Index, researchers managed to estimate the quantity of cyanobacteria that bodies of water could contain in the summer of 2020.
Nearly 3,000 waterbodies were analyzed and the study concluded that high concentrations of elephant carcasses were found near waterholes where significant cyanobacteria blooms were observed in 2020.
About twenty of these water points showed an “unprecedented” proliferation of cyanobacteria during “the period of mass mortality”.
Climate-induced diseases
Researchers believe that the shift from the 2019 drought in Botswana, one of the worst droughts on record in that region, to a period of particularly heavy rain in 2020, may have triggered “extreme growth” of cyanobacteria in swaths of water where the elephants drank.
On average, elephants would have walked 16.5 kilometers after drinking the toxin-containing water and died 33 hours after being contaminated.
The paper’s authors warn that climate change could cause more such disasters and highlight “the alarming trend of sudden climate-induced diseases affecting large ungulates.”
Ungulates are herbivorous mammals that have hooves.
The study also highlights that it is possible that other, smaller species experienced the same fate as elephants during this period in this region, but went unnoticed.
A new framework for mass mortality studies
The researchers believe that the techniques used during the study, such as spatial analysis and remote sensing, provide a new framework for studying mammal die-offs.
“Integrating spatial analysis, remote sensing and ecohydrological assessment not only circumvents logistical challenges, but also provides a scalable model for studying wildlife mortality events in equally inaccessible regions, where traditional field-based methods may be impractical or impossible,” the study concludes.
Researchers from the University of Botswana, the Natural History Museum London, Queen’s University Belfast and the Plymouth Marine Laboratory took part in the study.
