Serum samples collected in a cross-sectional survey of grazing cattle on Manyara Ranch, Monduli district, Tanzania, were tested by indirect major antigenic protein 1 fragment B (MAP 1-B) ELISA to determine the seroprevalence of Ehrlichia ruminantium and to assess ranch-level risk factors for heartwater. Heartwater-exposed cattle were widespread on the ranch and overall seroprevalence was 50.3 % (95% CI, 44.9 -55.6), enough to indicate an endemically unstable situation. Multivariate logistic regression modelling was used to identify risk factors associated with seropositivity. Two factors appeared to increase the herd's risk for contracting heartwater. Seroprevalence increased significantly with age (β= 0.19 per year of age, P < 0.001) and animals carrying ticks of any species were associated with an increased risk of infection with E. ruminantium (Odds ratio, OR = 3.3, P < 0.001). The force of infection based on the age seroprevalence profile was estimated at 18 per 100 cattle year-risk. The current tick control measures on the ranch were associated with a decreased risk of infection with E.ruminantium (OR=0.25 for no dipping and OR=0.31 for low dipping, P < 0.001). Six tick species were identified; in order of frequency these were: Ambylomma variegatum 59.9 %, Rhipicephalus evertsi evertsi 13.9 %, Rhipicephalus pulchellus 12.5 %, Hyalomma truncatum 7.03 % and Rhipicephalus appendiculatus 6.07 %. The least encountered tick was Rhipicephalus simus, which accounted for 0.38 %. The cattle seemed well adapted to their environment and capable of resisting the tick burden under this extensive wildlife / livestock grazing and interaction system

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