The tick-borne protozoan parasite Theileria parva causes an acute, often fatal disease in cattle throughout a large part of eastern and southern Africa. Infection of African buffalo (Syncerus caffer) is also widespread in this region but does not cause clinical disease in this species. This difference most likely reflects the evolutionary history of the parasites in these species, in that cattle were only introduced into Africa within the last 8000 years. In both hosts, T. parva establishes a carrier state, involving persistence of small numbers of parasites for many months following the acute phase of infection. This persistence is considered important for maintaining the parasite populations. Although cattle and buffalo parasites both produce severe disease when transmitted to cattle, the buffalo-derived parasites are usually not transmissible from infected cattle. Recent studies of the molecular and antigenic composition of T. parva, in addition to demonstrating heterogeneity in the populations in both host species, have revealed that infections in individual animals are genotypically mixed. The results of these studies have also shown that buffalo T. parva exhibit much greater genotypic diversity than the cattle population and indicate that cattle parasites represent a subpopulation of T. parva that has adapted to maintenance in cattle. The parasites in cattle and buffalo appear to be maintained largely as separate populations. This insight into the genotypic composition of T. parva populations has raised important questions on how host adaptation of the parasite has evolved and whether there is scope for further adaptation of buffalo-maintained populations to cattle.

在非洲东部和南部的大部分地区，蜱传播的原生动物寄生虫细小泰勒虫会在牛中引起一种急性的、通常是致命的疾病。非洲水牛 ( Syncerus caffer ) 的感染也在该地区广泛传播，但不会引起该物种的临床疾病。这种差异很可能反映了这些物种中寄生虫的进化历史，因为牛只在过去 8000 年内才被引入非洲。在两个宿主中，T. parva建立携带者状态，涉及在感染的急性期后持续数月的少量寄生虫。这种持久性被认为对维持寄生虫种群很重要。虽然牛和水牛寄生虫在传播给牛时都会产生严重的疾病，但水牛寄生虫通常不会从受感染的牛身上传播。最近对T. parva的分子和抗原组成的研究，除了证明两种宿主物种种群的异质性外，还揭示了个体动物的感染是基因型混合的。这些研究的结果还表明，水牛T. parva表现出比牛群更大的基因型多样性，并表明牛寄生虫代表了T. parva 的一个亚群，它已经适应了牛的维持。牛和水牛体内的寄生虫似乎主要作为单独的种群维持。对T. parva种群基因型组成的这一见解提出了重要的问题，即寄生虫的宿主适应是如何进化的，以及水牛饲养的种群是否有进一步适应牛的空间。