THE DISTRIBUTION OF HEARTWATER IN THE HIGHVELD OF ZIMBABWE, 1980-1997

Heartwater, the tick-borne disease caused by the rickettsia Cowdria ru minantium has historically been confined to the southern and western l owvelds of Zimbabwe. Since 1986, however, cases of heartwater have bee n diagnosed with increasing frequency in the central and eastern regio ns of the previously heartwater-free highveld plateau. During the same period, collections of the two major tick vectors of heartwater in Zi mbabwe, Amblyomma hebraeum and Amblyomma variegatum, were made for the first time in these areas, suggesting that spread of these ticks was responsible for the changed distribution of the disease. The factors a ssociated with this spread have not been determined, but increased cat tle and wildlife movement and reduced intensity of dipping undoubtedly play important roles. Currently, the distribution of heartwater and i ts vectors in the highveld is still largely restricted to the central and eastern regions. The northern regions of the highveld appear to be predominantly uninfected, though it is likely that, eventually, heart water will spread further with considerable impact on livestock produc tion in Zimbabwe.