Response of the tick Dermacentor variabilis (Acari : Ixodidae) to hemocoelic inoculation of Borrelia burgdorferi (Spirochetales)

When Borrelia burgdorferi B31 low passage strain spirochetes were directly injected into the hemocoel of Dermacentor variabilis (Say) females, the bac teria were cleared from the hemocoel within <24 h. Viable spirochetes were not found in hemolymph, salivary gland, or ovary tissues by subculturing or by IFA. The hemocyte population increased approximate to 6 times within th e first 6 h after inoculation compared with the uninoculated controls. In c ontrast, the soluble total hemolymph protein content decreased inversely wi th the increase in hemocytes. Borreliacidal activity was demonstrated with cell-free hemolymph from D. variabilis. In vitro antimicrobial assays using hemolymph from borrelia-challenged and nonchallenged ticks resulted in 72% spirochete reductions compared with only 11.5%, respectively, within 1 h. Additional evidence of induced antimicrobial hemolymph protein activity was demonstrated by SDS-PAGE, which revealed upregulation of a lysozyme-like p eptide (approximate to 15 kDa) (22% increase) and the induction of a approx imate to 5.8 kDa peptide in the B. burgdorferi-challenged ticks. In contras t with the nonvector borne Bacillus subtilis, D. variabilis presented a rap id and robust response to challenge with cultured B. burgdorferi spirochete s and lead to their early elimination. The role of the tick immune system, including possible differences between vector and nonvector ticks, in deter mining the success of invasive bacteria is discussed.