Cryptic lineage diversity in the zoonotic pathogen Angiostrongylus cantonensis

Delimitation of species is still a necessity among parasitic pathogens especially where morphological characters provide limited discernibility. Identification of cryptic lineages (independently evolving lineages that are morphologically similar) is critical as there could be lineage-specific traits that are of epidemiological importance. Angiostrongylus cantonensis is a zoonotic pathogen that can cause eosinophilic meningitis in humans. Recent reports of single marker sequence divergence hint at the potential for cryptic diversity in this lungworm. However, to definitively address if single marker divergence corresponds to independent evolving lineages, a multilocus approach is necessary. Using multilocus data, our goal was to determine if there were cryptic lineages within Thailand, a country plagued by several outbreaks and isolated cases of A. cantonensis infection. We analyzed the genetic structure of A. cantonensis samples collected from snails, Achatina fulica, across provinces in Thailand. Multilocus data (mitochondrial sequence data and 12 nuclear microsatellites) and individual based analyses were used to test for cryptic lineages. We found strong linkage disequilibrium patterns between mitochondrial haplotypes and nuclear-identified genetic clusters. There were clearly two divergent and independent clades. Moreover, within each clade, the data suggested additional substructure where individual provinces were likely to harbor unique genetic clusters. The results indicate there are at minimum two and possibly up to eight cryptic lineages within the assumed single species of A. cantonensis. Importantly, the two main clades do not show geographic affiliation and can be found in sympatry. With recent studies highlighting A. cantonensis strain diversity in pathogenicity and infectivity, it will be important to determine if these critical epidemiological traits are associated with specific lineages.