Transmission of the relapsing fever spirochete, Borrelia miyamotoi, by single transovarially-infected larval Ixodes scapularis ticks

The relapsing fever spirochete, Borrelia miyamotoi, is increasingly recognized as a cause of human illness (hard tick-borne relapsing fever) in the United States. We previously demonstrated that single nymphs of the blacklegged tick, Ixodes scapularis, can transmit B. miyamotoi to experimental hosts. However, two recent epidemiological studies from the Northeastern United States indicate that human cases of hard tick-borne relapsing fever peak during late summer, after the spring peak for nymphal tick activity but coincident with the peak seasonal activity period of larval ticks in the Northeast. These epidemiological findings, together with evidence that B. miyamotoi can be passed from infected I. scapularis females to their offspring, suggest that bites by transovarially-infected larval ticks can be an important source of human infection. To demonstrate experimentally that transovarially-infected larval I. scapularis ticks can transmit B. miyamotoi, outbred Mus musculus CD1 mice were exposed to 1 or 2 potentially infected larvae. Individual fed larvae and mouse blood taken 10 d after larvae attached were tested for presence of B. miyamotoi DNA, and mice also were examined for seroreactivity to B. miyamotoi 8 wk after tick feeding. We documented B. miyamotoi DNA in blood from 13 (57%) of 23 mice exposed to a single transovarially-infected larva and in 5 (83%) of 6 mice exposed to two infected larvae feeding simultaneously. All 18 positive mice also demonstrated seroreactivity to B. miyamotoi. Of the 11 remaining mice without detectable B. miyamotoi DNA in their blood 10 d after infected larvae attached, 7 (64%) had evidence of spirochete exposure by serology 8 wk later. Because public health messaging for risk of exposure to Lyme disease spirochetes focuses on nymphal and female I. scapularis ticks, our finding that transovarially-infected larvae effectively transmit B. miyamotoi should lead to refined tick-bite prevention messages.