A case study of Dermotheca gasterostei (=Dermocystidium gasterostei, Elkan) isolated from three-spined stickleback (Gasterosteus aculeatus) captured in lake Vattern, Sweden.

Mesomycetozoea is a class of fungus-like protist pathogens close to the branching between the animal and fungal kingdom within the holozoan supergroup (Kirkbright et al. 2015; Mendoza, Taylor, and Ajello 2002). During the last decades the composition of the class Mesomycetozoea have gone through significant changes with an increasing number of species being added. Originally being named Ichthyosporea to reflect their occurrence in fish, these pathogens have now also been identified in invertebrates, birds, and mammals (Glockling, Marshall, and Gleason 2013). However, the parasites are predominantly found in aquatic organisms, both marine and freshwater, usually occurring under cold water conditions. These parasites, which mainly infect the skin, fins, and gills, but occasionally also internal organs, are normally placed in the genus Dermocystidium (Langenmayer et al. 2014). The genus currently contains about 20 various pathogenic organisms, but the genus is likely to contain more members in the future as the majority of pathogens have only been described to genus level. The pathogens infect a magnitude of aquatic animals (Kirkbright et al. 2015), where Dermocystidium spp. produces clearly visible cysts in host tissue, and each cyst is filled with small spherical spores. In each spore, a relatively large refractile body (vacuoplast) that pushes the cytoplasm and nucleus to the periphery of the cellular body is present (Shamsi et al. 2020). To date, all described organisms within Mesomycetozoea have been found to cause anything from mild sub-clinical infections to aggressive fatal infections. This study is a case report on the first case of Dermocystidium recorded in the lake Vattern in Sweden.