Cafiso et al.
Midichloria mitochondrii, endosymbiont of Ixodes ricinus: Evidence for the transmission to the vertebrate host during the tick meal.
Ticks Tick Borne Dis., in press, doi.org/10.1016/j.ttbdis.2018.08.008

Midichloria mitochindrii is a tick-borne intracellular bacterium of the order Rickettsiales and is found with high prevalence in Ixodes ricinus ticks. M. mitochondrii is located in the ovaries and salivary glands of ticks and can horizontally be transmitted to tick hosts. Consequently, hosts (e.g. dogs and humans) develop antibodies to these bacteria (e.g. to the flagellar protein FliD) and bacterial DNA can be detected in hosts. While prevalence of M. midichlorii is highly variable in adult males of ticks, this bacterium is present to 100% in wild female ticks. However, under laboratory conditions, the percentage of ticks harboring this bacterium decreases. The function of M. midichlorii in ticks is yet unknown. Seroconversion against M. midichlorii in rabbits infested with wild I. ricinus was observed one week post-infection and also occurred when the bacterial load was extremely reduced (after infesting with laboratory ticks). The authors verified that M. midichlorii can replicate in the mammalian host (e.g. in rabbits infested with wild I. ricinus). The loss of M. mitichondrii is not a rare event in natural tick populations, but reacquisition can occur via horizontal transmission during co-feeding on a shared host. So far, no pathogenic role of M. midichlorii to the mammalian host has been reported.

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