Abstract:
This study was conducted to assess the possibility of immunising rabbits against Rhipicephalus appendicuatu Neumann, the vector for East Coast fever and Corridor disease, the most important tick-borne diseases in East and Central Africa. The antigens whose efficacy in the control of this vector were assessed included commercially available insect moulting hormone, beta-ecdysone and tick haemolymph components. However, since antibodies play a major role in the acquisition of tick resistance, it was imperative before any immunisation attempts were made to study the passage of antibodies (specifically immunoglobulin G (IgG) across the tick midgut membrane since potentially protective antigens would be of little value if the antibodies whose production they elicit do not cross the midgut barrier. These studies were undertaken, using the enzyme linked immunosorbent assay (ELISA) and the results obtained indicated that IgG can cross the midgut barrier and retain its biological activity. In addition the study also showed that the quantities of the lgG that cross this barrier are positively correlated to the antibody titres in the host blood. Of prime importance was the finding that destruction of the midgut barrier ~th anti-tick midgut antibodies could facilitate the passage of lgG across the midgut by an eight fold concentration factor and these antibodies maybe directed against antigens entirely differemt from those of midgut origin. Based on this findings , it vvas found feasible to immunise hosts against ticks using antigens that are found in the haemolymph. Rabbits were immunised against R.appendiculatus using commercially available moulting hormone (beta-ecdysone) that was conjugated to bovine serum albumin to render it immunogenic. The rabbits were also immunised with conjugated beta-ecdysone in combination with solubilised tick midgut membrane proteins (STGMP) to increase the titres of antibodies crossing the midgut. The immunised rabbits were infested with all the developmental instar stages of R.appendiculatus. The number of ticks that fed successfully to engorgement, their feeding durations and engorgement weights were recorded along with the percentage of the immature stages that moulted. In addition, the percentage of engorged females reaching the critical engorgement weight (300mg and above), the percentage that oviposited, the weights of the oviposited eggs and egg hatchabilities were recorded. The results obtained indicated that the immunisation had little effects on the immature stages. How·ever, immunisation reduced the engorgement weights and fecundites of adult ticks. Adult ticks that had been applied onto rabbits immunised with a combination of beta-ecdysone and STGMP were the most adversely affected. The next developmental instars from these ticks when applied onto tick-naive rabbits and the same parameters recorded shOed that immunisation had no effect on the resultant larvae and nymphs. However, the percentage of the ticks that fed successfully to engorgement, the egg conversion ratio, the percentage of females that oviposited and the hatchabilities of the oviposited eggs were significantly reduced in the adults that fed as nymphs on rabbits immunised with either beta-ecdysone or beta-ecdysone in combination with STGMP. The adults that fed as nymphs on rabbits immunised with STGMP had reduced engorgement weights and hatchabilities. The efficacy of haemol)mph components in immunisation of hosts against ticks was assessed. To determine from which sex of tick haemolymph was to be used for immunisation, haemolymph was obtained from adult male and female .appendiculatus at different days of feeding (0-5), was characterised using sodium dodecyle sulphatepolyacrylamide gel electrophoresis (SDS-PAGE). The results obtained demonstrated that the haemolymph contained sex specific proteins. Since it was easier to bleed partially fed ticks compared to engorged ones, haemolymph from 5 day fed male and female ticks was used to inoculate rabbits at a dosage rate of lmg/kg intramuscularly. To facilitate the passage of antibodies across the tick midgut membrane, haemolymph components were used alone or in combination with STGMP along with STGMP control. After the last booster dose the rabbits were infested with all the developmental instar stages of R.appendiculatus and the same parameters indicated above recorded. The results obtained showed that the rabbits had been rendered resistant to all the developmental stages of. R appendiculatus. This resistance as more pronounced in adults where the number of ticks that fed successfully to engorgement were drastically reduced. In addition, the females had reduced engorgement weights and fecundities . Rechallenging the resistant experimental rabbits with all the developmental instar stages of R.appendiculatus showed that resistance was even more pronounced during subsequent infestations. These results indicated that these immunogens could be of practical use in tick control in the field as protection conferred to the hosts was maintained in subsequent infestations.