Relationship of respiratory frequency, gill histopathological lesions and Paramoeba perurans infection load in Atlantic salmon (Salmo salar L.) – a possible proxy for gill health

Gill health challenges, such as amoebic gill disease (AGD) caused by Paramoeba perurans , represent a major threat to Atlantic salmon aquaculture. Due to invasiveness and limited representativeness of most conventional diagnostic methods, early detection of gill diseases remains challenging. This study evaluated camera-based respiratory frequency monitoring as a non-invasive proxy for early gill disease detection using AGD as a model. A group of AGD-challenged Atlantic salmon was compared to an unchallenged group, and the fish were monitored for 35 days post-challenge for respiratory frequency, gill lesion severity (macroscopic and histological scoring) and pathogen load (qPCR). AGD-challenged groups showed an early increase in respiration frequency preceding the major histological changes at high amoebic load. Overall, colonization of P. perurans was confirmed by qPCR analysis in the challenged fish, in contrast to the unchallenged group. As a part of the project, a probiotic was tested and found not to have any significant protective effect against AGD under the given conditions. The appearance of sacciform-like cells exclusively in the gills of AGD-challenged groups may indicate a localized immune response. A close, positive correlation was found between respiratory frequency and infection in the AGD-challenged groups, and histopathological lesions in all groups despite only mild to modest changes, indicating monitoring of respiration as a sensitive, real-time and non-invasive indicator of early onset of gill disease under experimental conditions. • Camera-monitored respiration used as a proxy for gill disease in Atlantic salmon. • AGD challenge led to a consistent increase in respiratory frequency. • Non-invasive monitoring – a novel tool for continuous assessment of gill health.