Midichloria mitochondrii transmitted to the vertebrate host by Ixodes ricinus: a transient passenger or an infectious agent?

INTRODUCTION. Ticks are important vectors for a variety of pathogens affecting humans and other animals. They also harbor numerous commensal and symbiotic microorganisms, whose role is still less investigated. Ixodes ricinus harbors the intracellular bacterium Midichloria mitochondrii, which is present in ovaries and in salivary glands (Sassera et al., 2008, Appl. Environ. Microbiol., 74:6138-6140; Epis et al., 2013, Ticks Tick Borne Dis., 4:39-45). The bacterium is vertically transmitted and shows a prevalence of 100% in wild adult females. M. mitochondrii prevalence reduces after some generations in laboratory conditions (Lo et al., 2007, Environ. Microbiol., 8:1280-1287). Molecular (e.g. in sheep, horse, cattle; Bazzocchi et al., 2013, Parasit. Vectors 12:350) and serological (e.g. in human, roe deer; Mariconti et al., 2012, Pathog. Glob. Health, 106:391–396; Serra et al., 2018, J. Wildl. Dis., ePub) evidences showed that M. mitochondrii is transmitted to the vertebrate hosts by I. ricinus. The aims of this work are thus: 1) understanding the seroconversion onset and the kinetic of antibody response against M. mitochondrii in a vertebrate model; 2) investigating the presence of circulating M. mitochondrii DNA in blood samples. MATERIALS AND METHODS. Two groups of rabbits were experimentally infested respectively with wild (naturally infected with the symbiont) and with lab strain (lab; with a highly reduced amount of the symbiont) I. ricinus females. Blood and serum were sampled before the infestation and for eight post-infestation time points, over a 16-weeks time span. ELISA assays were performed to assess the immune response against M. mitochondrii flagellar protein FliD. DNA was extracted from blood samples and circulating M. mitochondrii DNA was detected using a nested PCR approach. RESULTS AND CONCLUSIONS. M. mitochondrii presence was detected in rabbits infested by both W and LS I. ricinus ticks, suggesting that the transmission occurs even at low bacterial loads. Seroconversion against M. mitochondrii FliD antigen was observed around the first/second week post-infestation. The immune response showed O.D. values above the established cut–off around the first-fifth week post-infestation, then decreasing by the end of the experiment. Circulating DNA was detected in the blood of infested animals up to the end of the experiment, suggesting both a replication of the symbiont inside the vertebrate host and a possible true infection.