Assessment of the environmental distribution of the protozoan parasite Perkinsus olseni by next-generation sequencing, qPCR and histopathology allows the identification of alternative bivalve hosts

Perkinsus olseni is a protozoan parasite that infects a wide variety of molluscs worldwide, causing economic losses in the aquaculture sector. Consequently, Perkinsosis has been catalogued by the World Organization for Animal Health (O.I.E.) as a notifiable disease, and international measures have been established to control it (O.I.E., 2019). In the present study, we analysed the spatial and temporal distribution of transmissible stages of Perkinsus olseni in an endemic area of the parasite from 2016 to 2018. The pathogen was detected using high-throughput sequencing of the 18S rRNA gene and a specific real-time PCR assay (qPCR) in samples of water, sediment and several bivalve species. Histopathological assays were also conducted on bivalve samples. P. olseni was not detected in environmental samples by qPCR; however, eDNA sequencing revealed its presence in both the water and sediment at all sampled points, showing a seasonal pathogen prevalence. As expected, the parasite was detected in clams, but a few cases were also found in mussels and cockles. The presence of the parasite was confirmed in Cerastoderma edule by histology and qPCR using RNA to evaluate the presence of proliferative life stages of the parasite. Therefore, this is the first time that P. olseni has been found in C. edule but with low abundance and infection intensity levels.