Mapping heartwater (cowdriosis) in Tanzania using national tick survey and cattle seroprevalence data from 1998–2001
Abstract
Heartwater (ehrlichiosis or cowdriosis) is a domestic and wild ruminant disease caused by a bacterium called Ehrlichia ruminantium. The pathogen is transmitted among susceptible hosts by ticks of the genus Ambylomma. The enzootic areas of the disease correspond to the distribution of vector ticks in southern, eastern, central, and western African countries and in Madagascar. Despite its importance, information on its epidemiology in Tanzania is limited. A national-wide tick and cattle serosurvey was conducted between 1998 and 2001, encompassing 21 administrative regions of Tanzania, to determine the seroprevalence of E. ruminantium and develop a risk map to guide surveillance and control measures. A total of 9400 indigenous cattle aged 6–12 months randomly selected from 825 villages were restrained and examined for tick infestations. Serum samples were subsequently collected and examined for E. ruminantium antibodies by indirect MAP 1B ELISA. The geographic database was managed with ArcView 3.1 (ESRI). Among the 9,400 cattle examined, 9,142 (97.3%) were infested by one or more tick species. Rhipicepahalus appendiculutus (48%) was the most prevalent, followed by Amblyomma variegatum (37%). The remaining (15%) included Boophilus decolaratus, Hyalomma spp and Rhipicephalus evertsi evertsi. The overall seroprevalence of E. ruminantium antibodies was 70.7% (95% CI: 69.8–71.7), with the highest prevalence observed in the Lake Zone (81%), followed by the Central (77.9%), Southern Highlands (73.9%), and Western Zone (70.9%), while the lowest was recorded in the Southern Zone (51.9%). Overlaying maps of modeled Heart water (HW) risk assessments with sampling locations and herds indicated positive HW serology in many places within the country. The geographical distribution of HWs closely follows the distribution of its vector, A. variegatum. Our results establish the need to continue and expand tick surveys and serosurveillance of other species (small ruminants and wildlife) to further calibrate and improve the HW risk model and better understand the dynamics of E. ruminantium transmission.