The emergence of sarcoptic mange in Australian wildlife: an unresolved debate

Abstract

Due to its suspected increase in host range and subsequent global diversification, Sarcoptes scabiei has important implications at a global scale for wildlife conservation and animal and human health. The introduction of this pathogen into new locations and hosts has been shown to produce high morbidity and mortality, a situation observed recently in Australian and North American wildlife.Of the seven native animal species in Australia known to be infested by S. scabiei, the bare-nosed wombat (Vombatus ursinus) suffers the greatest with significant population declines having been observed in New South Wales and Tasmania. The origins of sarcoptic mange in Australian native animals are poorly understood, with the most consistent conclusion being that mange was introduced by settlers and their dogs and subsequently becoming a major burden to native wildlife. Four studies exist addressing the origins of mange in Australia, but all Australian S. scabiei samples derive from only two of these studies. This review highlights this paucity of phylogenetic knowledge of S. scabiei within Australia, and suggests further research is needed to confidently determine the origin, or multiple origins, of this parasite.At the global scale, numerous genetic studies have attempted to reveal how the host species and host geographic location influence S. scabiei phylogenetics. This review includes an analysis of the global literature, revealing that inconsistent use of gene loci across studies significantly influences phylogenetic inference. Furthermore, by performing a contemporary analytical approach on existing data, it is apparent that (i) new S. scabiei samples, (ii) appropriate gene loci targets, and (iii) advanced phylogenetic approaches are necessary to more confidently comprehend the origins of mange in Australia. Advancing this field of research will aid in understanding the mechanisms of spillover for mange and other parasites globally.

Keywords: Conservation Medicine; Network; One Health; Phylogeny; Sarcoptes scabiei; Wombat.

Fig. 1

Neighbour-net analysis using SplitsTree of publicly available 16S rRNA gene sequences retrieved from GenBank (July 2015). Bootstrap values greater than 80 are included. Human and animal S. scabiei mites from Europe cluster away from other global S. scabiei mites. The Australian derived mites are shown to cluster closely with Asian and African mites, which conflicts with the assumption that Australian mites are consequential to European origins. There is limited network support for internal subclades in both the European clade and the Asian, Australian, African and North American clade

Fig. 2

Neighbour-net analysis using SplitsTree of publically available COX1 sequences retrieved from GenBank. Bootstrap values greater than 80 are included. COX1 sequences analysis supports host-separation rather than geographic location is the biggest influence on S. scabiei diversity. Dog, wallaby and wombat sequences are shown to be clustering closely. The majority of sequences are branching away independently within both clades as unique sequences

Fig. 3

Representation of four different scenarios of how mites were introduced into Australian wildlife. Each line represents a different host, introduction period and are genetically unique. (i) S. scabiei was already present in Australian wildlife via the dingo prior to European settlement (ii) a single manifestation from European settlers and their domestic dogs (iii) after initial European settlement, a second and new introduction of S. scabiei was introduced from other regions across the world and (iv) combinations of all three situations (illustrating all three at once). Colours indicate species as follows: red - dingo, yellow - European domestic dogs, blue - European settlers, and green - second introduction of mites from other ethnic countries. Each of these possible scenarios would produce different clade structures on a phylogenetic tree, respectively as follows: (i) a single Australian S. scabiei subclade within the larger S. scabiei phylogeny with divergence time pre-dating European arrival, (ii) single or two Australian S. scabiei subclades within the larger phylogeny with divergence time associated to European arrival, (iii) a further subclade within (ii) associated to reintroduction times, and (iv) a single S. scabiei subclade within the larger S. scabiei phylogeny distinctly separate from the smaller subclades associated to European settlement and other more recent introductions