Taxonomy of Strongyloides in humans, dogs and cats: a comprehensive review from morphology to molecular and population genetics

Abstract The genus Strongyloides (Nematoda; Strongyloididae) comprises over 50 species of nematodes parasitic in terrestrial vertebrates, including humans ( Homo sapiens ), dogs ( Canis lupus familiaris ) and cats ( Felis catus ). Taxonomy of the genus has been shaped by over a century of morphological research, with the most widely adopted framework established in the late 1980s. Advances in molecular genetics have increasingly revealed cryptic diversity and yielded new insights into interspecific and intraspecific relationships within the genus. Despite the rapid expansion of molecular genetic data over the past decade, particularly for Strongyloides spp. infecting humans and companion animals, a synthesis of these findings remains lacking. Here, we review historical and contemporary literature on the taxonomy of Strongyloides spp. infecting humans ( Strongyloides stercoralis, Strongyloides fuelleborni ), dogs ( S. stercoralis , including host-specific lineages and cryptic taxa) and cats ( Strongyloides felis, Strongyloides planiceps, Strongyloides tumefaciens and S. stercoralis ). We provide an updated overview of taxonomic histories, host ranges and key morphological features for genus identification and species differentiation, along with a synthesis of available molecular taxonomic data informed by phylogenetic and population genetic studies. This work is intended to serve as a renewed reference for researchers, diagnosticians and clinicians working with Strongyloides spp. in medical and veterinary contexts, supporting accurate diagnosis and guiding future taxonomic research on these nematodes.