Heartwater (cowdriosis) is a tick-borne disease of cattle, sheep, goats and some wild ruminants which is caused by a rickettsia, previously known as Cowdria ruminantium, but recently reclassified as Ehrlichia ruminantium. It is now known that there is far more genetic variability among E. ruminantium organisms than had ever been suspected. This has hampered the development of a safe and effective vaccine and the disease remains one of the major causes of stock losses in sub-Saharan Africa, as well as some tropical Pacific and Indian Ocean islands.