Baylisascaris spp.

Sapp, Sarah G. H., Gupta, Pooja, Martin, Melissa K., Murray, Maureen H., Niedringhaus, Kevin D., Pfaff, Madeleine A. & Yabsley, Michael J., 2017, Beyond the raccoon roundworm: The natural history of non-raccoon Baylisascaris species in the New World, International Journal for Parasitology: Parasites and Wildlife 6 (2), pp. 85-99 : 95-96

3.1. Molecular epidemiology of Baylisascaris spp.

Microscopy has been traditionally used to identify Baylisascaris spp. based on adult morphological characters although some species can be difficult to distinguish, especially if only immature worms are found. However, because of the similarity among the sizes of eggs in feces or larvae in tissues, molecular markers have been developed to facilitate identification. For example, multiple single nucleotide polymorphisms (SNPs) in mitochondrial and nuclear gene sequences of B. columnaris , B. procyonis and B. transfuga , have been used to develop species-specific diagnostic molecular markers for rapid identification of different Baylisascaris species ( Blizzard et al., 2010; Testini et al., 2011; Franssen et al., 2013). Studies have also characterized the genetic diversity, investigated the population structure and the phylogenetic relationships among Baylisascaris species, which is important for understanding the zoonotic potential and host specificity of these parasites. To discuss the phylogenetic relationships among the Baylisascaris spp. that occur in the New World in a broader context, this section includes data published on Baylisascaris spp. from Asia and Europe and Baylisascaris procyonis from raccoons.

3.1.1. Phylogenetic relationships

Based on analysis of numerous genetic targets ( Table 7), Baylisascaris is most closely related to the genus Ascaris . The entire mitochondrial genome has been sequenced for four species— B. transfuga , B. ailuri , B. schroederi and B. procyonis (all samples from China) ( Xie et al., 2011a, 2011b; Li et al., 2012). Phylogenetic analyses of these mitochondrial genomes and concatenated partial mitochondrial and nuclear genes (12S rDNA, 18S rDNA and 28S rDNA) provide the strongest support for the relatedness of the genus Baylisascaris with Ascaris and other members of the order Ascaridida ( Xie et al., 2011a, 2011b; Li et al., 2012).

Within the Baylisascaris genus, phylogenetic relationships have been investigated using several molecular targets (i.e., numerous mitochondrial genes, nuclear 5.8S, and second internal transcribed spacer (ITS-2) rDNA sequences). The three ursid-specific Baylisascaris species ( B. transfuga , B. ailuri and B. schroederi ) are more closely related to each other than to B. procyonis and B. ailuri is more similar to B. transfuga than to B. schroederi ( Xie et al., 2011a, 2011b; Li et al., 2012). Evidence from nuclear 5.8S and ITS-2 rDNA sequences also showed higher genetic similarity between B. transfuga and B. schroederi compared to B. procyonis ( Zhao et al., 2012) .

Although no molecular data are available for B. laevis , B. melis or B.devosi ; mitochondrial and nuclear genes of B. columnaris and B. potosis have recently been characterized and their phylogenetic relationship with other Baylisascaris species has been examined. The first phylogenetic analysis of B. columnaris from pet skunks in Europe showed closer affinity to B. procyonis compared to B. transfuga ( Franssen et al., 2013) based on mitochondrial cytochrome c oxidase 1 and 2 (CO1 and CO2), ribosomal ITS1-5.8S-ITS2 and ribosomal 28S genes. This result was expected because B. columnaris was previously shown to be very similar to B. procyonis based on partial mitochondrial CO2 gene sequences (Danguodobiyam et al., 2009). Phylogenetic analyses of the mitochondrial CO1 and ITS2 rDNA gene sequences showed that B. potosis had high genetic similarity to B. procyonis and B. columnaris ( Tokiwa et al., 2014) .

A comprehensive phylogenetic assessment of different Baylisascaris species at common gene targets would facilitate a better understanding of genetic similarities/differences between the different species. Given that many species of Baylisascaris span a very wide geographic scale, genetic differences likely exist among these populations suggesting the existence of cryptic species. For example, B. melis is endemic to both Eurasia and the Americas and B. transfuga similarly has a large geographic and host range. The morphologic variability in B. transfuga also should be examined using molecular tools to assess if these indicate the presence of multiple species. Genetic studies are critically needed to evaluate species validity and both fine-scale and broad-scale geographic variability for all Baylisascaris species.

3.1.2. Population structure

Genetic markers are widely used to assess population structure and provide important insights into host-parasite transmission dynamics. For example, giant panda ( Ailuropoda melanoleuca ) populations were genetically distinct across the three mountain ranges in China but B. schroederi were not, suggesting little coevolution between hosts and parasites and high levels of parasite gene flow ( Zhou et al., 2013; Xie et al., 2015). High genetic variation within the parasite populations on each mountain range was observed, but the lack ofpopulation diversity across the mountain ranges suggested a homogenous parasite population, based on the complete mitochondrial cytb, atp6 and cox1 gene targets. In contrast, use of microsatellite markers revealed two genetic clusters in B. procyonis across the Grand River in Western Michigan, USA ( Sarkissian et al., 2015). Lack of population structure in B. schroederi parasites across the mountain ranges in China indicates the fast evolving rate of parasites compared to their hosts and microsatellite markers may be able to further confirm if there is any recent genetic divergence between the parasite populations. Thus, choice of appropriate genetic marker is important while assessing parasite population structure and understanding host-parasite evolutionary dynamics.

Low genetic diversity was found within B. columnaris in the Netherlands. Multi-locus genetic analysis revealed four distinct genotypes, possibly owing to the differences in the host or geographic origin of these parasites ( Franssen et al., 2013). However, a wider sampling of infected hosts from other geographical regions is warranted to reveal the true genetic diversity of these parasites.