Baylisascaris transfuga (Nematoda: Ascarididae) in a Zoo Brown Bear (Ursus arctos) in Bulgaria: Morphometric Characterization, Molecular Identification, and Phylogenetic Analysis

Introduction: The artificial environment of zoological gardens poses risks for the transmission of parasitic diseases, among which ascarid infections in bears are exceptionally common. Due to the zoonotic potential of Baylisascaris transfuga and the morphological overlap of its eggs with other roundworms, accurate diagnosis is critical within the “One Health” framework. Materials and Methods: Following anthelmintic treatment with ivermectin and praziquantel, adult nematodes were collected from a captive brown bear (Ursus arctos) at the Sofia Zoo, Bulgaria. Detailed morphometric examination was performed for species identification, complemented by molecular-genetic and phylogenetic analyses targeting the nuclear 28S rDNA and mitochondrial COI genes. Results: Morphometric analysis revealed a specific “miniaturised” morphotype of B. transfuga, characterized by body lengths and egg dimensions that fell within or below the minimum ranges reported for wild populations. Crucially, despite the observed changes in general morphometric parameters, the male reproductive structures—specifically the spicules—exhibited high metric conservativeness (533–570 µm), confirming their reliable diagnostic value. Nuclear 28S rDNA sequences provided definitive taxonomic validation, showing a match with Palearctic lineages and distinguishing the isolate from the Nearctic cluster. At the same time, the mitochondrial COI marker captured intraspecific diversity, identifying three distinct new haplotypes co-existing within the single host. Conclusions: The observed general reduction in morphological measurements combined with reproductive metric stability demonstrates environmentally induced phenotypic variation. Furthermore, the simultaneous presence of multiple COI haplotypes highlights the cumulative nature of the infection through successive exposure over time (trickle infection), although simultaneous infection with these genetic variants cannot be entirely discarded. The global macrogeographical differentiation captured by both markers suggests noticeable genetic divergence among geographic lineages within the distribution range of the taxon, while future broad-scale investigations are required to fully clarify underlying microevolutionary patterns. These findings underscore the high adaptive potential of B. transfuga and emphasize the necessity of integrating morphometrics with molecular tools for accurate wildlife monitoring in captivity.