Comparative studies on the vector capacity of the different stages of Amblyomma variegatum Fabricius and Hyalomma rufipes Koch for Congo virus, after intracoelomic inoculation

Groups of nymphs and adults of Hyalomma rufipes and larvae, nymphs and adults of Amblyomma variegatum were infected by intracoelomic inoculation with Congo virus at concentrations of 10 3.5LD 50 (Group A), 10 1.5LD 50 (Group B) and 10 0.5LD 50 (Group C). The infection rates for the different groups were: Groups A, 92–100% for all stages of ticks, except for adult H rufipes which had 87%; Group B, 56.8, 68.1 and 50.7% for larvae, nymphs and adults of A. variegatum, respectively, and 96.3 and 84.4% for nymphs and adults of H. rufipes, respectively; Group C, 0% for larvae, nymphs and adults of A. variegatum and 8 and 1.7%, respectively, for nymphs and adult of H. rufipes. The 1–5% infection threshold was 10 0.5LD 50 for nymphs of H. rufipes and a dose greater than this for all the other stages of the two species of ticks. The 50% infection threshold was 10 1.5LD 50 for larvae and adults of A. variegatum, but less than 10 1.5LD 50 for nymphs of A. variegatum and nymphs and adults of H. rufipes. Peak virus titres obtained for H. rufipes were 10 6.0 for Group A and B nymphs and 10 3.0 for Group A and B adults, and 10 1.0 each for Group C nymphs and adults. For A. variegatum, the peak titres for Groups A, B and C were, respectively, 10 1.0, 10 0.8, and 0 for larvae; 10 3.0, 10 0.8 and 0 for nymphs and 10 1.5, 10 0.6 and 0 for adults. Sera from 100% (24/24) of rabbits used to feed adult H. rufipes and from 65% (31/48) used to feed nymphs and adult A. variegatum were positive for compliment fixation antibody to Congo virus at a titre of 1:8 from Days 15–35 postattachment of ticks. Viremia in rabbit was detected in 12.5% (3/25) of rabbits used to feed adult H. rufipes.