Complex interaction between proliferative kidney disease, water temperature and concurrent nematode infection in brown trout

Proliferative kidney disease (PKD) is a temperature-dependent disease caused by
the myxozoan Tetracapsuloides bryosalmonae. It is an emerging threat to wild brown trout Salmo
trutta fario populations in Switzerland. Here we examined (1) how PKD prevalence and pathology
in young-of-the-year (YOY) brown trout relate to water temperature, (2) whether wild brown trout
can completely recover from T. bryosalmonae-induced renal lesions and eliminate T. bryo -
salmonae over the winter months, and (3) whether this rate and/or extent of the recovery is influenced
by concurrent infection. A longitudinal field study on a wild brown trout cohort was conducted
over 16 mo. YOY and age 1+ fish were sampled from 7 different field sites with various
temperature regimes, and monitored for infection with T. bryosalmonae and the nematode Raphidascaris
acus. T. bryosamonae was detectable in brown trout YOY from all sampling sites, with
similar renal pathology, independent of water temperature. During winter months, recovery was
mainly influenced by the presence or absence of concurrent infection with R. acus larvae. While
brown trout without R. acus regenerated completely, concurrently infected brown trout showed
incomplete recovery, with chronic renal lesions and incomplete translocation of T. bryosalmonae
from the renal interstitium into the tubular lumen. Water temperature seemed to influence complete
excretion of T. bryosalmonae, with spores remaining in trout from summer-warm rivers, but
absent in trout from summer-cool rivers. In the following summer months, we found PKD infections
in 1+ brown trout from all investigated river sites. The pathological lesions indicated a reinfection
rather than a proliferation of remaining T. bryosalmonae. However, disease prevalence
in 1+ trout was lower than in YOY.