Identification of Chaetozone setosa (Cirratulidae) as the definitive host in the life cycle of Parvicapsula pseudobranchicola infecting Atlantic salmon (Salmo salar).

Parvicapsula pseudobranchicola is a marine myxozoan parasite of high significance to Atlantic salmon (Salmo salar) aquaculture, particularly in northern regions such as Norway and Iceland, where outbreaks have been reported with increasing frequency and severity in recent years. In this study, we aim to resolve the life cycle of P. pseudobranchicola by screening benthic invertebrate communities in areas with known infection pressure in salmon farms as well as performing a transmission trial using post-smolt Atlantic salmon and polychaetes. Sediment samples were collected using a Van Veen grab sampler, covering a 0.025 m2 area at each sample site. Polychaetes found were subjected to microscopic investigation, PCR, Sanger sequencing, histology and in situ hybridization. Fish were challenged with actinospores using two different methods. The polychaete Chaetozone setosa was identified as the definitive host of P. pseudobranchicola. Actinospores of morphotype saccimyxon, measuring 5-6 µm in diameter are described. Each spore contained three spherical polar capsules, approximately 0.7-1.0 µm in diameter. Four of the 20 challenged fish (20%) tested positive for parasite DNA in blood samples by PCR. By identifying and confirming the polychaete Chaetozone setosa as the definitive host of Parvicapsula pseudobranchicola, we resolve the sixth known marine myxozoan life cycle. These results provide immediate utility. Life cycle resolution enables targeted surveillance and site-specific risk zoning as mitigation for parvicapsulosis, that should reduce impact on fish welfare and losses from the disease.