Distribution and prevalence of the myxozoan parasite Tetracapsuloides bryosalmonae in northernmost Europe: analysis of three salmonid species

Abstract

<p>Global climate change is altering the abundance and spread of many aquatic parasites and pathogens. Proliferative kidney disease (PKD) of salmonids caused by the myxozoan <em>Tetracapsuloides bryosalmonae</em> is one such emerging disorder, and its impact is expected to increase with rising water temperature. Yet, the distribution and prevalence of <em>T. bryosalmonae</em> in Northern Europe remain poorly characterized. Here, we studied 43 locations in 27 rivers in northernmost Norway and Finland to describe <em>T. bryosalmonae</em> infection frequency and patterns in 1389 juvenile salmonids. <em>T. bryosalmonae</em> was discovered in 12 out of 27 rivers (44{\%}) and prevalence ranged from 4.2 to 55.5{\%} in Atlantic salmon and from 5.8 to 75{\%} in brown trout among infected rivers. In sympatric populations, brown trout was more frequently infected with <em>T. bryosalmonae</em> than was salmon. Age-specific parasite prevalence patterns revealed that in contrast to lower latitudes, the infection of juvenile fish predominantly occurs during the second summer or later. Temperature monitoring over 2 yr indicated that the mean water temperature in June was 2.1 to 3.2{\textdegree}C higher in rivers containing <em>T. bryosalmonae</em> compared to parasite-free rivers, confirming the important role of temperature in parasite occurrence. Temporal comparison in <em>T. bryosalmonae</em> prevalence over a 10 yr period in 11 rivers did not reveal any signs of contemporary parasite spread to previously uninfected rivers. However, the wide distribution of <em>T. bryosalmonae</em> in rivers flowing to the Barents Sea indicates that climate change and heat waves may cause new disease outbreaks in northern regions.</p>