INTRODUCTION

Arthropod-borne spirochaetes have long caused human suffering and disease. Louse-borne relapsing fever (LBRF), caused by Borrelia recurrentis and transmitted by the human body louse (Pediculus humanus), was once widespread in the extensive areas where human body lice were found. Today, LBRF is reported mainly from northeastern and central Africa including the countries of Ethiopia, Somalia and Sudan, in discrete foci where human body lice remain prevalent (Porcella et al., 2000). Tick-borne relapsing fever (TBRF) was first described in Africa where the argasid (soft) tick Ornithodoros moubata was found to transmit Borrelia duttoni (see historical review by Burgdorfer, 2001). Isolated endemic cycles of TBRF caused by individual species of relapsing fever spirochaetes and their matching argasid vector species have been described in Asia, Europe and the Americas (Felsenfeld, 1979). Recent reports detailing the epidemiology and biology of relapsing fever include studies in Tanzania, where B. duttoni frequently causes human disease (Melkert & Stel, 1991; Fukunaga et al., 2001), as well as studies in North America where Borrelia hermsii is the primary aetiologic agent of relapsing fever (Dworkin et al., 2002). Although Borrelia spp. were known to cause human disease in isolated pockets, scant attention was directed towards the study of these organisms in the latter half of the twentieth century until an epidemic of arthritis was described in Lyme, Connecticut (Steere et al., 1977b).