Molecular epidemiology of heartwater (Ehrlichia ruminantium infection) in The Gambia

Heartwater is caused by Ehrlichia ruminantium and transmitted by ticks of the genus Amblyomma. It occurs in sub-Saharan Africa and in the Caribbean and affects domestic ruminants. There is general lack of information on the epidemiology of the disease in The Gambia. Results of a countrywide serological survey (chapter 2) using MAP1-B ELISA showed the infection is widespread throughout the country and showed a gradient of increasing heartwater risk for susceptible livestock from the east to the west of The Gambia. In sheep, the highest seroprevalence was detected in the western part of the country, 88.1% in the Western Division and 62.1% in the Lower River Division. Similarly in goats, the highest seroprevalence was detected in the westerly part of the country, North Bank Division (59%) and Western Division (44.1%). Sheep and goats in the two easterly divisions (Central River and Upper River divisions) showed the lowest seroprevalence rates. In Chapter 3 the nested pCS20 PCR showed the highest detection performance for detection of E. ruminantium in Amblyomma variegatum ticks compared to the nested map1 PCR and reverse line blot hybridization. Application of the pCS20 PCR in a subsequent study of E. ruminantium tick infection rates in The Gambia showed infection rates ranged from 1.6 % to 15.1 % with higher prevalences detected at sites in the westerly divisions (Western, Lower River and North Bank) than sites in the easterly divisions (Central River and Upper River) of the country. The RLB, in addition, demonstrated molecular evidence of Ehrlichia ovina, Anaplasma ovis and Anaplasma marginale infections in The Gambia. Chapter 4 described longitudinal monitoring of E. ruminantium infection in extensively managed newborn lambs and kids using pCS20 PCR and MAP1-B ELISA. Infection rates detected by pCS20 PCR increased with increasing age. Half (7/14) of the newborn PCR-positive animals tested positive in the first 3 days after birth, suggesting that transmission other than by tick feeding had played a role. In contrast, seropositivity detected by MAP1-B ELISA declined with increasing age suggesting the presence of maternal antibodies. It is concluded that both the vector tick and vertical transmission may play a vital role in the epidemiology of heartwater in young animals; and showed the age range of 4 and 12 weeks corresponds to the period of highest susceptibility to heartwater in traditionally managed small ruminants. The study of genetic diversity of E. ruminantium by restriction fragment profile analysis of map1 gene is described in chapter 5. The results showed the presence of multiple genotypes (at least 11) of the E. ruminantium in the three agroecological zones of The Gambia, Sudano-Guinean (SG), Western Sudano-Sahelian (WSS) and Eastern Sudano-Sahelian (ESS) with the greatest number of diversity detected in the WSS and SG zones. Chapter 6 described the evaluation of an inactivated vaccine, prepared from E. ruminantium (Gardel stock), and a live attenuated vaccine from E. ruminantium (Senegal stock) against heartwater in Sahelian sheep. A local stock of E. ruminantium (Kerr Seringe) was used as challenge material. Inactivated and live attenuated vaccines provided 43 % and 100% protection, respectively, against virulent needle challenge. In a subsequent field trial, the attenuated vaccine protected 75% of sheep against virulent tick challenge, which was fatal for all control sheep