A serologic relationship between Rickettsia sennetsu, the etiologic agent of human sennetsu rickettsiosis in western Japan, and Ehrlichia canis, the agent of canine ehrlichiosis, has been demonstrated. Using the indirect fluorescent antibody test, convalescent sera from patients with sennetsu fever reacted with Ehrlichia canis antigen. In the direct fluorescent antibody test, immunoglobulins from four patients with sennetsu rickettsiosis stained E. canis morulae in canine monocytes. This finding is significant in view of the morphologic uniqueness of the two agents and a lack of serologic relatedness with other major rickettsial agents.
Present address: HQ U.S. Army Medical Research and Development Command, Fort Detrick, Frederick, Maryland 21701.