Aquatic Chlamydiae: A Review of Their Roles in Fish Health

Aquaculture plays a vital role in meeting the global demand for high-quality protein. However, the fish industry is challenged by infectious diseases, including gill conditions such as epitheliocystis. Epitheliocystis is characterized by cyst-like epithelial lesions, which occur in the gills of fish, and is associated with intracellular bacteria including Chlamydia-related bacteria. Although epitheliocystis was initially regarded as of low significance, attention is increasing due to its impact on commercially important fish species in intense farming conditions. This review evaluates the roles of aquatic chlamydiae as pathogens contributing to fish morbidity and mortality, and as members of fish microbiota. Additionally, Chlamydia-related bacteria are thought to be involved in complex gill disease (CGD), characterized by lamellar fusion, epithelial hyperplasia, and inflammation. Recent discoveries have expanded the diversity of Chlamydiota isolated from fish, with novel species such as Candidatus (Ca.) Panilichlamydia rohitae, Ca. Piscichlamydia trichopodus, and Chlamydia vaughanii identified in different fish hosts. Most causative agents of epitheliocystis have not yet been cultured in vitro, although C. vaughanii, the first Chlamydiaceae member isolated from fish, was successfully cultured. As C. vaughanii was recently shown to be able to propagate in mammalian cells, it raises concerns about its zoonotic potential, although a pathogenic role has yet to be described.