Heartwater (also known as cowdriosis) is an acute, fatal, non-contagious, infectious, tick-borne rickettsial disease of ruminants caused by Ehrlichia ruminantium (formerly Cowdria ruminantium) and transmitted by Amblyomma ticks. The disease can cause high mortality (up to 90%) in susceptible domestic ruminants. Goats and sheep are more susceptible than cattle, and European breeds are generally more susceptible than indigenous African breeds. In most cases, heartwater is an acute febrile disease, with a sudden rise in body temperature, which may exceed 41°C within 1–2 days after the onset of fever. Fever is followed by inappetence, sometimes listlessness, diarrhoea, particularly in cattle, and dyspnoea indicative of lung oedema. Nervous signs develop gradually. The animal is restless, walks in circles, makes sucking movements and stands rigidly with tremors of the superficial muscles. Cattle may push their heads against a wall or present aggressive or anxious behaviour. Finally, the animal falls to the ground, pedalling and exhibiting opisthotonos, nystagmus and chewing movements. The animal usually dies during or following such an attack. Acute and peracute clinical forms of the disease occur: in the former, there are high death rates without many clinical manifestations and, in the latter, there is a higher recovery rate. Recovered animals become carriers of infection. Certain wild animals can play a role as reservoir; Rusa deer, white-tailed deer, and springbok are susceptible to this infection and can experience high mortality. Experimental vaccines exist but none has been fully validated under field conditions. Ehrlichia ruminantium has been strongly suspected in several cases of rapidly fatal encephalitis in humans. However, in all cases, evidence of E. ruminantium infection was based on molecular detection. Isolation and characterisation of the infectious agent is necessary before E. ruminantium can be considered an emerging pathogen in species other than ruminants and especially in humans. Since then, no other clinical human case associated with heartwater has been observed.