Heartwater.
Authors:   UK, CABI
Published: March 2022

Heartwater (or cowdriosis) is a tickborne disease of sheep, goats, cattle and some wild ruminants caused by the rickettsia, Ehrlichia ruminantium (previously Cowdria ruminantium). It is a small pleomorphic organism (0.2-2.7 µm) and colonies named morula containing varying numbers are found in the cytoplasm of endothelial cells (Cowdry, 1926; Pienaar, 1970). According to Neitz (1968), the first record of heartwater was probably made in South Africa by the Voortrekker pioneer Louis Trichard in 1838. In an entry in his diary on the 9th of March 1838, he mentions a fatal disease, 'nintas', amongst his sheep, approximately 3 weeks after a massive tick infestation. However, it was only in 1876, almost 50 years later, that the first official report describing heartwater as a generally known disease along the coast and borders of King William's Town was presented before the Cattle and Sheep Disease Commission in Grahamstown, South Africa. According to Webb (1877), the disease was introduced into the eastern Cape region at about the same time that William Bowker found a bont tick (Amblyomma hebraeum) on a cow which was imported from northern KwaZulu-Natal (then Zululand) in approximately 1837. Due to confusion with other prevalent conditions of unknown aetiologies at that time, it is difficult to follow the introduction and spread of the disease in Africa. It is, for instance, still not clear whether heartwater is a disease indigenous to the African continent or whether it was imported at some stage, possibly from Madagascar. However, current knowledge suggests that it is a disease of the African mainland. Despite the fact that certain Amblyomma species occur on the Asian and American continents, there is no evidence that the disease exists there. The first major breakthrough in understanding the disease came when Dixon (1898) and Edington (1898) proved that it could be produced experimentally by inoculating blood from diseased to susceptible animals. Although the causative organism could not be detected in the blood or tissue of diseased animals at the time, it was believed that heartwater was caused by a micro-organism (Hutcheon, 1900), possibly a virus (Spreull, 1904). At about the same time, Lounsbury (1900) confirmed the long-standing suspicion that the bont tick (A. hebraeum) was the vector in South Africa. However, it was not until 1925 that Cowdry (Cowdry, 1925a; Cowdry, 1925b) successfully demonstrated the organism in tissue of infected animals and ticks. Cowdry named the organism Rickettsia ruminantium but this was later changed to Cowdria ruminantium (Moshkovski, 1947) and finally to E. ruminantium (Dumler et al., 2001). Traditional rickettsial taxonomy assigned Cowdria ruminantium as the sole member of the genus Cowdria in the tribe Ehrlichieae. This was one of three tribes within the family Rickettsiaceae in the order Rickettsiales which initially encompassed all intracellular bacteria but from which the Chlamydiae were later removed (Moulders, 1984). The obligate intracellular nature of E. ruminantium, coupled with morphological features suggestive of a Chlamydia-like life cycle led to confusion as to its position in the ehrlichial hierarchy (Uilenberg, 1983). After 16S ribosomal RNA and groESL gene comparisons, Dumler et al. (2001) defined that all members of the tribes Ehrlichieae and Wolbachieae should be transferred to the family Anaplasmataceae and the genus Ehrlichia was emended to include Ehrlichia ruminantium (formerly Cowdria ruminantium). The family Anaplasmataceae currently includes Ehrlichia, Anaplasma, Aegyptianella, Neorickettsia, Wolbachia, 'Candidatus Neoehrlichia' and 'Candidatus Xenohaliotis' (Thomas et al., 2016).