Summary
The phylum
Chlamydiae
contains obligate intracellular bacteria, several of which cause disease in their hosts. Morphological studies have suggested that this group of bacteria may be pathogens of fish, causing cysts in epithelial tissue – epitheliocystis. Recently, the first genetic evidence of a chlamydial aetiology of this disease in seawater reared Atlantic salmon from Norway and Ireland was presented, and the agent was given the name ‘
Candidatus
Piscichlamydia salmonis’. In this article we present molecular evidence for the existence of a novel
Chlamydiae
that also may cause epitheliocystis in Norwegian salmonids. This novel
Chlamydiae
has been found in salmonid fish from freshwater, and based on its partial 16S rRNA gene, it may constitute a third genus in the family
Chlamydiaceae
, or a closely related sister family. By using whole‐mount RNA–RNA hybridization we demonstrate how infected cells are distributed in a patchy manner on a gill arch. The morphology of the novel
Chlamydiae
includes the characteristic head‐and‐tail cells that have been described earlier from salmonid fish suffering from epitheliocystis. We propose the name ‘
Candidatus
Clavochlamydia salmonicola’ for this agent of epitheliocystis in freshwater salmonids.