In vivo tissue distribution of the Ehrlichia muris-like agent in Ixodes scapularis ticks

The blacklegged tick (Ixodes scapularis) is an important vector of tick-borne diseases in North America, transmitting the agents of Lyme disease, anaplasmosis, babesiosis and Powassan encephalitis. A newly recognized intracellular pathogen referred to as the Ehrlichia muris-like agent (EML) has been identified as the cause of human cases of ehrlichiosis in Minnesota and Wisconsin.  EML has been isolated from a field-collected I. scapularis and successfully transmitted to mice by this same tick species under experimental conditions.  The biological interactions between vector and microbe have yet to be described.  Here we used in situ hybridization to show distribution of EML infection internally throughout whole, sectioned ticks, and electron microscopy to confirm the results.  We identified ehrlichiae in an unexpectedly broad range of tick tissues and cell types, including extensive colonization of salivary glands, epithelial cells, neuronal cells of the synganglion, and male accessory glands.  EML was not observed in ovarian tissues and spermatids. These results suggest that EML is likely maintained in nature through enzootic, horizontal transmission.