First Report of the Malacosporean Parasite Tetracapsuloides bryosalmonae in the Faroe Islands Reveals Widespread Distribution and High Prevalence in Brown Trout.

The myxozoan endoparasite Tetracapsuloides bryosalmonae (Tb), the causative agent of proliferative kidney disease (PKD) in salmonids, has predominantly been reported in temperate regions of continental Europe and North America. Here, we present the first record of Tb in the Faroe Islands, extending the known northern distribution of the parasite. We sampled 161 brown trout (Salmo trutta) and three Atlantic salmon (Salmo salar) from 16 streams across the Faroes. Quantitative PCR (qPCR) analysis of kidney tissue detected Tb in 14 streams, with mean prevalence of 65.6%, reaching 100% in six streams. Parasite load, quantified as Tb 18S rRNA gene copies per qPCR reaction, ranged from 109.5 to 281,219 (mean 16,127) among infected fish. Despite high Tb prevalence and load, no clinical signs of renal hyperplasia, indicative of PKD, were observed. Sanger sequencing of the Tb 18S rRNA gene showed 99.8% to 100% identity with European isolates. These results indicate Tb is well established in Faroese freshwater and is genetically similar to European isolates. Given the known temperature dependence of PKD, climate warming is likely to increase disease risk in northern regions. Continued monitoring is essential to assess the potential long-term ecological impacts of Tb on wild salmonid populations under changing environmental conditions.