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The ixodid tick, Ixodes persulcatus, serves as a vector for the
Lyme disease spirochete, Borrelia garinii, in Japan. Transmission of the spirochete from infected nymphs to uninfected larvae during their cofeeding on mice was studied under laboratory conditions. Using feeding chambers, infected nymphs and uninfected larvae were allowed to cofeed on heads of normal BALB/c mice. In another group of mice, we separately exposed nymphs to the head and larvae to the back. The resultant engorged larvae were reared and the molted nymphs were examined for spirochetes. Spirochetal infections were found only in ticks that had fed together with infected ticks. The data strongly suggest that spirochetes can migrate from infected feeding ticks to their uninfected neighbors by way of host skin. Moreover, nymphs that had become infected through the cofeeding process could transmit spirochetes to mice.