The isolation of a rickettsia‐like organism causing disease and mortality in Chilean salmonids and its confirmation by Koch's postulate

TLDR

The organism is a Gram‑negative, obligate intracellular pathogen found in host cell vacuoles, belonging to the Rickettsiales order, Rickettsiaceae family, and possibly the Ehrlichiae tribe. The study describes the organism, its clinical signs, and the pathology it produces. The disease, first seen in coho salmon in seawater net pens, was experimentally reproduced in fish housed in both freshwater and seawater aquaria. RLO was isolated from infected coho salmon, confirmed by Koch's postulates as the cause of systemic disease with significant mortality, shown to infect Atlantic, Chinook, and rainbow trout, with horizontal transmission observed, and the disease proposed to be named salmonid rickettsial septicaemia. Abstract.

Abstract

Abstract. A rickettsia‐like organism (RLO) was isolated from infected coho salmon, Oncorhynchus kisutch (Walbaum), in cultures from six fish cell lines, but could not be cultured on 41 artificial media. The organism was confirmed by Koch's postulates as the aetiological agent of a systemic disease causing signifieant mortality among coho in Chile. The organism was a Gram‐negative, obligate intracellular pathogen frequently observed within intracyto‐plasmic vacuoles or inclusions of host cells, and appears to belong to the order Rickettsiales, family Rickettsiaceae and possibly the tribe Ehrlichiae. The organism, as well as the clinical signs and pathology it produces, is described. The disease, originally observed only in coho raised in seawater net pens, was reproduced experimentally in fish in both freshwater and seawater aquaria. Horizontal transmission without parasite vectors was observed in fish in both aquaria. The organism was shown, for the first time, to cause disease and mortality in Atlantic, Salmo salar L., and Chinook, O. tshawytscha (Walbaum), salmon and rainbow trout, O. mykiss (Walbaum). Because of the systemic nature of the disease, it is proposed that it be called ‘salmonid rickettsial septicaemia’.