The organism designated the SF agent was originally isolated in Japan in 1962 from metacercaria parasitic on gray mullet fish. The SF agent resembles members of the genus morphologically and exhibits weak antigenic cross-reactivity with . This organism causes mild clinical signs in dogs, but severe splenomegaly and lymphadenopathy in mice. This suggests that the SF agent may be similar to either , an intracellular parasite of a fluke and the cause of salmon poisoning disease in dogs, or , the causative agent of human sennetsu ehrlichiosis in Japan and Malaysia. In order to determine the phylogenetic relationship between the SF agent and other ehrlichial species, the 16S rRNA gene was amplified by the PCR and sequenced. The SF agent sequence was most closely related to the sequences of (level of sequence similarity, 99.1%), the causative agent of Potomac horse fever, and (level of sequence similarity, 98.7%). The next most similar sequence was that of , but the level of sequence similarity was only 93.7%. , the SF agent, and formed a distinct cluster that was separated from all other ehrlichial species. As determined by immunofluorescence labeling, antiserum against the SF agent cross-reacted strongly with , and . When three genetically distinct ehrlichial isolates obtained from horses with Potomac horse fever were compared with the SF agent, we found that the SF agent was most closely related to Ohio isolate 081, followed by Illinois (T = type strain) and a Kentucky isolate. We observed strong antigenic cross-reactivities and similarities in Western blot (immunoblot) reaction profiles when we compared the SF agent, , and however, weaker antigenic cross-reactivity was observed when the SF agent and were compared. Our results indicate that the SF agent is antigenically more closely related to and than to . The biological and antigenic characteristics and the 16S rRNA sequence data suggest that the SF agent is a new species that belongs to the genus .


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