a chronic pneumonia of sheep caused by a lentivirus,which also causes visna when it invades the brain of sheep.In maedi the characteristic features are a prolonged incubation period of more than two years,a progressive pneumonia which lasts for about six months,and,at the inevitable death,an abnormally high density and heaviness of the lungs

a chronic pneumonia of sheep caused by a lentivirus,which also causes visna when it invades the brain of sheep.In maedi the characteristic features are a prolonged incubation period of more than two years,a progressive pneumonia which lasts for about six months,and,at the inevitable death,an abnormally high density and heaviness of the lungs