ABSTRACT: Proliferative kidney disease (PKD) is a widespread disease of farmed and wild
salmonid populations in Europe and North America, caused by the myxozoan parasite Tetracapsuloides
bryosalmonae. Limited studies have been performed on the epidemiological role in
spread of the disease played by fish that survive infection with T. bryosalmonae. The aim of the
present study was to evaluate the persistence of T. bryosalmonae developmental stages in chronically
infected brown trout Salmo trutta up to 2 yr after initial exposure to laboratory-infected
colonies of the parasite’s alternate host, the bryozoan Fredericella sultana. Kidney, liver, spleen,
intestine, brain, gills and blood were sampled 24, 52, 78 and 104 wk post-exposure (wpe) and
tested for T. bryosalmonae by PCR and immunohistochemistry (IHC). Cohabitation trials with specific
pathogen free (SPF) F. sultana colonies were conducted to test the viability of T. bryo -
salmonae. PCR detected T. bryosalmonae DNA in all tissue samples collected at the 4 time points.
Developmental stages of T. bryosalmonae were demonstrated by IHC in most samples at the
4 time points. Cohabitation of SPF F. sultana with chronically infected brown trout resulted in successful
transmission of T. bryosalmonae to the bryozoan. This study verified the persistence of
T. bryo salmonae in chronically infected brown trout and their ability to infect the bryozoan F. sultana
up to 104 wpe.