Pantropic, fatal disease. Predominantly infects endothelial cells and myocardium. In the acute pneumonic form, interlobular pulmonary oedema, pericarditis, haemorrhages and oedema of the viscera occur; death within 5 days. A more prolonged form of the disease has cardiac involvement. Mortality can be 90% depending on the virulence of the isolate and host susceptibility. Subclinical disease can occur in donkeys, vaccinated horses and native horses.