Transmission of Members of the “Candidatus Midichloriaceae” Family to Vertebrates and Possible Involvement in Disease Pathogenesis

“Candidatus Midichloriaceae” (order Rickettsiales) is a family that encompasses obligate intracellular bacteria present in a wide range of hosts, from parasitic arthropods (ticks, fleas, bedbugs) to aquatic animals (e.g. sponges and corals) and protists, including pathogenic amoebae (genus Acanthamoeba). In addition, multiple studies have detected evidence of the presence of “Candidatus Midichloriaceae” in vertebrates, suggesting a capacity to cause infections, with possible pathogenic effects. Two members of the family that are described as symbionts of ticks, i.e. “Candidatus Midichloria mitochondrii” and “Candidatus Lariskella arthropodarum” have also been found in the blood of humans and other mammals, using molecular and serological methods. Consequently, it has been hypothesized that these arthropods could transmit the microorganisms during the blood meal, as is known to happen with pathogenic members of the order Rickettsiales; for example, ticks can be vectors of rickettsiosis or ehrlichiosis. Indeed, clinical cases compatible with these pathologies in humans or animals parasitized by ticks, but characterized by the absence of antibody titers for known pathogenic rickettsiae, could be reinvestigated for the presence of “Candidatus Midichloriaceae”. A third member of the family, a “Candidatus Midichloria”-related organism, has repeatedly been found in association with red mark syndrome in the rainbow trout Oncorhynchus mykiss. Although the etiology of this economically relevant disease is currently unknown, multiple independent studies suggest that this member of “Candidatus Midichloriaceae” is the most probable causative agent.