Identification of a target population for immunisation against East Coast fever in coastal Kenya

Two experiments were carried out to identify the target population of cattl e for immunisation against East Coast fever (ECF) using the infect ion-and- treatment method. Firstly, a sentinel-calf study was used to determine the age window for ECF immunisation by determining ages at clinical detection o f infection with Theileria parva. Six groups of five naive cross-bred (Bos taurus/Bos indicus) male calves, introduced at intervals of 2 months at a m ean age of 26 days, were exposed to natural tick challenge on a high ECF-ri sk, small-holder farm in the coastal lowland, coconut-cassava agro-ecologic al zone of coastal Kenya. Secondly, a challenge study evaluated the relatio nship between the presence of T parva antibodies and immunity. Ten indigeno us adult Zebu cattle and nine Zebu young stock purchased from farmers in th e same zone, and eight cross-bred calves (survivors of the sentinel-calf st udy) were challenged with 10 times the immunising dose of T. parva Marikebu ni stock. Twenty-four of these 27 cattle had high antibody titres before ch allenge. Two cross-bred calves, obtained from an ECF-free area and seronega tive to T. parva schizont antigen, also were challenged and used as suscept ible controls. Twenty-five (83%) of the 30 sentinel calves contracted ECF over an age rang e of 36-116 days (mean 72 days). The remaining five calves died of other ca uses within 2 months of arrival on the farm. Fourteen of the 25 calves surv ived the infection and developed antibodies to T parva. Despite tick contro l, seven of these 14 calves had a second episode of ECF and two died. In to tal, 13 of the 25 calves that contracted ECF died. Only one of 19 indigenous Zebu animals developed clinical ECF when challeng ed with T parva Marikebuni (mild clinical signs with spontaneous recovery). Of the eight cross-bred survivors from the first experiment, only one succ umbed to ECF when challenged and it died. Both susceptible cross-bred calve s developed severe clinical signs of ECF and one died. The experimental studies show that in the high ECF-risk areas of the coconu t-cassava zone of coastal Kenya, immunisation against ECF in cross-bred (B. taurus/B. indicus) cattle should be targeted at an early age (preferably w ithin 1-2 months of birth). (C) 2001 Elsevier Science B.V. All rights reser ved.