Abstract
Accurate pathogen detection is essential in fish health management and disease prevention. Pathogens often target different host tissues, and monitoring alternative target organs can provide important insights into disease progression. We evaluated the spleen as an alternative to the kidney for estimating the load and prevalence of the myxozoan parasite Tetracapsuloides bryosalmonae (Tb) and its relationships with proliferative kidney disease (PKD) in juvenile brown trout. We sampled 238 brown trout across nine rivers and quantified parasite load in both tissues using qPCR. Parasite load showed a strong positive correlation between the two tissues for both raw and log10-transformed data (Pearson's r = 0.60-0.71, Spearman's ρ = 0.78), with the spleen exhibiting, on average, 4.1-fold lower parasite load compared to the kidney. Tb was found in 191 specimens, consisting of 167 spleen and 190 kidney detections. The relationships between parasite load and PKD symptoms (renal hyperplasia and anaemia) were comparable for both tissues, and segmented regression line analysis indicated that health parameters deteriorate faster after exceeding a certain parasite load threshold. In conclusion, these results suggest that the spleen may serve as a viable alternative to the kidney for Tb monitoring, providing useful insights into Tb presence, load, and PKD progression in salmonids.