Infectivity, transmission and 16S rRNA sequencing of a rickettsia, Coxiella cheraxi sp. nov., from the freshwater crayfish Cherax quadricarinatus

Abstract

A rickettsia-like organism isolated from infected, farm-reared Cherax quadricarinatus was cultured in the yolk sac of developing chicken eggs, but could not be cultured in 3 continuous cell lines, bluegill fry (BF-2), fathead minnow (FHM), and Spodoptera frugiperda (Sf-9). The organism was confirmed by fulfilling Koch's postulates as the aetiological agent of mortalities amongst C. quadricarinatus. When C. quadricarinatus was inoculated with the organism, mortality was 100% at 28 degrees C and 80% at an ambient temperature of 24 degrees C. Horizontal transmission with food and via the waterborne route was demonstrated, but mortalities were lower at 30 and 10% respectively over a 4 wk period. The 16S rRNA sequence of 1325 base pairs of the Gram-negative, obligate intracellular organism was 95.6% homologous to Coxiella burnetii. Of 18 species compared to this rickettsia, the next most closely related bacterium was Legionella pneumophila at 86.7%. The suggested classification of this organism is Order Rickettsiales, family Rickettsiaceae, tribe Rickettsieae, within the genus Coxiella. We suggest it should be named Coxiella cheraxi sp. nov.