Scientists shed new light on mysterious deaths among African elephants
Scientists from Statens Serum Institut and the University of Copenhagen in Denmark have investigated the bacteria that killed 35 elephants in Zimbabwe in 2020. The study reveals that the bacteria produces a unique combination of virulence factors, including two toxins that are also found in Neisseria meningitidis – a bacterium that can cause meningitis in humans. These genetic traits may explain the bacteria’s unusual ability to cause blood poisoning and possibly cross the blood-brain barrier.
African elephants are under significant pressure from poaching and habitat loss. The population has fallen from ~500,000 in 2007 to ~350,000 in 2014, and the African elephant has now been added to the list of endangered species. In May and June 2020, hundreds of elephants died mysteriously in Botswana. The dead animals were often found at waterholes and showed signs of central nervous system damage, with many falling on their faces and walking in circles just before they died. This led authorities to point to poisoning by toxic blue-green algae (also known as cyanobacteria) as the most likely cause of death. From August to November 2020, another 35 elephants died in neighbouring Zimbabwe, but here the cause was different, namely a Pasteurella multocida-like bacterium called Bisgaard taxon 45 that has previously been isolated from tiger and lion bite wounds in humans but never from elephants. The outbreak investigation, which was led by Dr. Chris M. Foggin of the Victoria Falls Wildlife Trust, showed pathological changes and bacteria in the brain, liver and spleen. Whole genome sequencing of the bacterium from one of the elephants was also successful, making it possible to search for so-called virulence genes – i.e. small pieces of DNA that enable the bacteria to infect their host. The results were published in the renowned journal Nature Communications in October 2023, and it immediately aroused the interest of the Danish researchers.
“When I read that Bisgaard taxon 45 was the cause of the elephant deaths in Zimbabwe, I immediately thought of Magne Bisgaard, who is a former professor at the University of Copenhagen and the namesake of the bacterium,” says Jesper Larsen, who is a professor at Statens Serum Institut and the University of Copenhagen.
Jesper Larsen therefore contacted Magne Bisgaard and Henrik Christensen, who is an associate professor at the University of Copenhagen and helped characterize Bisgaard taxon 45 back in 2007. It turned out that they had already contacted Dr. Foggin and his collaborators with a view to comparing the outbreak strain with other bacteria belonging to Bisgaard taxon 45. This led to a close collaboration between Statens Serum Institut and the University of Copenhagen.
“Whole-genome sequencing and bioinformatic analyses showed that the bacterium that caused the elephant deaths in Zimbabwe contained approximately 40 unique genes, many of which encode virulence factors”, explains Astrid Rasmussen, who is a Ph.D. student at Statens Serum Institut. Two of these genes are particularly interesting as they encode a toxin that has previously only been found in Neisseria meningitidis, an important cause of life-threatening blood poisoning and meningitis in humans. The biological function of the toxin is still unclear, but it is known that N. meningitidis produces it in connection with invasive infections, so it is reasonable to believe that the toxin also plays a role in Bisgaard taxon 45.
“It is possible that this toxin is what enables N. meningitidis and Bisgaard taxon 45 to cross the blood-brain barrier, which could explain the large amounts of bacteria found in the elephant brains,” says Jesper Larsen.
Could Bisgaard taxon 45 have played a role in the deaths of elephants in Botswana?
“The cause of death among elephants in Botswana has never been verified, so we cannot rule out poisoning by toxic blue-green algae, Bisgaard taxon 45, an unfortunate combination of both, or other causes for that matter,” explains Jesper Larsen.
The researchers hope that whole-genome sequencing can help identify virulence genes involved in other cases of mass death among wild animals, including approximately 200,000 endangered saiga antelopes that died of P. multocida blood poisoning in central Kazakhstan 10 years ago.
“Such studies will not only increase our understanding of the diversity, evolution, epidemiology and pathogenesis of different Pasteurella organisms, but can also be used to design new vaccine candidates,” concludes Jesper Larsen.
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Read the scientific article in NatureCommunications: Genomic insights into the 2020 mass die-off event among African elephants