Characteristics of chlamydia-like organisms pathogenic to fish

Bacteria from the Chlamydiales order have been long known, especially as pathogenic bacteria to humans and many animal species, principally including birds and mammals. But for slightly over 20 years, they have been identified in the aquatic environment as endosymbionts of amoebas and sea worms. For several years, they have also been recorded as a cause of diseases among fish, causing respiratory system infections in the form of epitheliocystis of the gill. At present, 11 chlamydia-like organisms pathogenic to fish have been described, including nine new ones, classified into six families, four of which are already known (Parachlamydiaceae, Rhabdochlamydiaceae, Candidatus Parilichlamydiaceae, Candidatus Clavichlamydiaceae) and two newly created families, namely Candidatus Actinochlamydiaceae and Candidatus Parilichlamydiaceae. This paper characterises 11 chlamydia-like organisms, as well as seven isolates not classified into families, which are pathogenic to fish, presenting their genetical properties allowing for their classification, as well as morphological properties and diseases caused.