Pasteurella sp. associated with fatal septicaemia in six African elephants

Abstract

The sudden mortality of African elephants (Loxodonta africana) in Botswana and Zimbabwe in 2020 provoked considerable public interest and speculation. Poaching and malicious poisoning were excluded early on in the investigation. Other potential causes included environmental intoxication, infectious diseases, and increased habitat stress due to ongoing drought. Here we show evidence of the mortalities in Zimbabwe as fatal septicaemia associated with Bisgaard taxon 45, an unnamed close relative of Pasteurella multocida. We analyse elephant carcasses and environmental samples, and fail to find evidence of cyanobacterial or other intoxication. Post-mortem and histological findings suggest a bacterial septicaemia similar to haemorrhagic septicaemia caused by P. multocida. Biochemical tests and 16S rDNA analysis of six samples and genomic analysis of one sample confirm the presence of Bisgaard taxon 45. The genome sequence contains many of the canonical P. multocida virulence factors associated with a range of human and animal diseases, including the pmHAS gene for hyaluronidase associated with bovine haemorrhagic septicaemia. Our results demonstrate that Bisgaard taxon 45 is associated with a generalised, lethal infection and that African elephants are susceptible to opportunistically pathogenic Pasteurella species. This represents an important conservation concern for elephants in the largest remaining metapopulation of this endangered species.

Fig. 1. Maps of the Kavango–Zambezi Transfrontier Conservation Area (KAZA TFCA) and elephant carcass locations.

a The Kavango–Zambezi Transfrontier Conservation Area (KAZA TFCA), shown in green, spans portions of Botswana, Namibia, Angola, Zambia, and Zimbabwe. The KAZA TFCA shapefile was used with permission from Peace Parks Foundation. b A more detailed map of the area in north-western Matabeleland, Zimbabwe where elephant carcasses were found, including the approximate location of the carcasses. Protected areas in Zimbabwe are shown in green.

Fig. 2. Gross pathology findings of the dead African elephants.

a Elephant VF120A carcass in situ, showing postmortem condition with similar presentation to anthrax; only blood smears were collected (b) Elephant VF20/112 lung tissue showing small areas of consolidation and hyperaemia. c Elephant VF20/113 stomach with haemorrhages on gastric serosa; lung in foreground and liver on bottom right shown in next panel. d Elephant VF20/113 liver in situ adjacent to stomach, with rounded border indicating gross swelling, and parenchymal hyperaemia. e Elephant VF20/113 spleen, removed, with multiple capsular haemorrhages and an enlarged splenic lymph node. f Elephant VF20/170 thoracic diaphragm and lung with haemorrhages.

Fig. 3. Histopathology of lesions in tissues from two elephants.

a–d Are photomicrographs of lesions in tissues from elephant VF20/112 demonstrating foci of necrosuppurative inflammation in the spleen (a) with intralesional bacterial colonies (arrowhead), and fibrinocellular and bacterial emboli in the lung (b) and brain (c, d). Panels e and f are photomicrographs of lesions in tissues from elephant VF20/113, demonstrating foci of necrosuppurative inflammation in the liver (e) containing gram-negative coccobacilli (arrowheads, e, f). a, b, c, and e Haematoxylin and Eosin, x40 magnification, bar 50 microns. d, f Gram, x100 magnification, bar 20 microns. Data are representative of lesions observed from these two elephants at three independent laboratories.

Fig. 4. The VF20HR isolate is a member of the Bisgaard Taxon 45 (BGT45) family.

The phylogenetic relationship of VF20HR with other members of the Pasteurellaceae family was analysed using (a) 16 S rDNA sequences (1177 nucleotides); (b) core genome multilocus sequence typing (MLST) based on 681 markers; (c) partial rpoB gene sequences (467 nucleotides) and (d) partial infB gene sequences (455 nucleotides). For the 16 S rDNA, rpoB and infB trees, the Genbank accession numbers for the sequences from Bisgaard Taxon 45 (BGT45), Bisgaard Taxon 46 (BGT46), Pastereulla multocida subspecies (Pm) and Pasteurella canis are provided in the tree. The support for monophyletic groups by bootstrap analysis is indicated as percentages. Samples are color-coded, with dark blue (VF20HR), red (Bisgaard Taxon 45), green (Bisgaard Taxon 46), light blue (P. multocida subspecies) and purple (P. canis).