Abstract

The Pacific spiny lumpsucker (Eumicrotremus orbis) is a common aquarium exhibit fish. Routine necropsy revealed necrosis and severe histiocytic inflammation of multiple organs including heart (epicardium and myocardium), kidney, liver, ovary, and gills associated with large numbers of intralesional organisms within the coelomic cavity and affected tissues. Although the morphology at the light microscopic level and upon initial ultrastructural examination was not specific, it was somewhat suggestive of a group of fish pathogens called Mesomycetozoa, which include Dermocystidium spp., Ichthyophonus spp., and the "rosette agent" of salmonid fish. Based on published phylogenetic analyses, Mesomycetozoa-specific polymerase chain reaction (PCR) primers were designed for conserved regions within the aligned 18S small subunit rRNA gene sequences of several species in the class Mesomycetozoa, including species from the orders Dermocystida and Ichthyophonida. PCR was performed on formalin-fixed, paraffin-embedded samples from affected fish. Rhinosporidium sp. archived case material was used as a positive control. Sequence analysis of the PCR product representing a 497bp partial sequence of the 18S SSU rRNA gene has revealed that the closest matches are Myxosporea including Myxidium rather than Mesomycetozoa, suggesting that additional investigation is necessary.

Acknowledgements

The authors wish to thank the people who work with this species and submitted specimens for necropsy.