Results of a 10-year survey of louse-borne relapsing fever in southern Ethiopia: a decline in endemicity

When transmitted from person to person by the human body louse, Pediculus humanus humanus, the spirochaete Borrelia recurrentis can cause an acute febrile illness. In the highlands of Ethiopia, this illness, which is known as louse-borne relapsing fever (LBRF), has probably been endemic for centuries and, until the last few years, epidemics of the disease have been common (Borgnolo et al., 1993). The outbreaks that occurred in northern Ethiopia in the 1990s were attributed to the return of soldiers to their original recruitment areas, after the end of 30 years of fighting against Eritrea (Borgnolo et al., 1993; Sundnes and Haimanot, 1993). In the catchment area for the Gambo General Rural Hospital (GGRH), which lies, at about 2200 m above sea level, on the eastern escarpment of the Great Rift Valley, about 250 km to the south of Addis Ababa, LBRF has been endemic for many years. Ramos et al. (2004) described the 197 cases identified in this area between 1997 and 2002. Encouragingly, however, no new cases of the disease have been detected in this area since early in 2005, raising the possibility that the disease in no longer locally endemic. The trends in LBRF seen in this region in the last decade are described and discussed below. During the study period (11 September 1997–10 September 2007), 579,136 patients attended the outpatients’ department of the GGRH, of whom 251 were found to have Borrelia infection (by microscopical examination of their bloodsmears) and were therefore recorded as cases of LBRF. Although this gives an overall incidence, over the study decade, of 4.3 cases/10,000 outpatients, this value does not show the remarkable fall seen in annual incidence. Over half (58%) of all the cases detected in the decade occurred in the first 3 years but only four (1.6%) occurred in the last 3 years (see Table). The highest monthly numbers of LBRF cases were seen in February–November 1998 (see Figure). In each of the months between this peak and February 2005 (except for six scattered months when no cases were seen), between one and eight cases of the disease presented at the GGRH. Since March 2005, however, no new cases have been diagnosed. The seasonality of LBRF in Ethiopia is unclear. While Parry et al. (1970) observed a peak in incidence during the ‘big rains’ (known locally as kiremt), which fall between June and September, Daniel et al. (1992) subsequently found incidence to peak during the ‘small rains’ (belg), which fall between March and May. No marked seasonality was observed in the present study, with 53 (21.1%) of the cases diagnosed during the big rains and 80 (31.9%) during the small rains. In Ethiopia, LBRF has been both an epidemic problem, with outbreaks associated with war, military personnel and refugee camps (Borgnolo et al., 1993; Sundnes and Haimanot, 1993), and an endemic problem, with a relatively small but steady stream of cases presenting for treatment (Mitiku and Mengistu, 2002). Until 2005 — assuming that the 1998 peak in the numbers of cases presenting can be considered an outbreak — this was the picture at the GGRH. Since mid-2005, however, no cases of LBRF have been recorded at the GGRH, possibly indicating Annals of Tropical Medicine & Parasitology, Vol. 102, No. 5, 467–469 (2008)