Dynamics of Borrelia burgdorferi transmission by nymphal Ixodes dammini ticks

Groups of 25-30 nymphal Ixodes dammini infected with Borrelia burgdorferi fed on mice for 36, 42, or 48 h. Only 1 (7%) of 14 mice exposed for 36 h became infected as did 3 (25%) of 12 mice exposed for 42 h and 6 (75%) of 8 mice exposed for 48 h. The minimum transmission rate calculation for ticks attached for 36 h was 0.5%; for 42 h, 1.5%; and for 48 h, 5.8%. Homogenates derived from nonfed "flat" nymphs or nymphs fed for 12 h were not infectious to mice. Homogenates derived from nymphs attached for 24 h, however, infected 20% of inoculated mice; homogenates derived from ticks attached for > or = 36 h infected > or = 80% of inoculated mice. Incubation of nonfed ticks at 37 degrees C did not produce infectious homogenates. The ID50 for low-passage JD1 strain spirochetes in 3-week-old mice was 2.69 x 10(3) spirochetes.