During epizootics, mechanical infection via other biting insects may also occur. In addition to sheep, goats, cattle, and humans, subclinical infection has been reported in horses, pigs, a camel and a Namaqua gerbil. Human infection may produce a nonfatal, influenza- like illness. In countries of southern costal Africa with year-round warm, moist weather, infection and passage of maternal immunity occur continuously, but clinically evident disease is rare. South Africa experiences periodic outbreaks due to a lower prevalence of immunity and periodic heavy rains.
Outbreaks end abruptly after the first winter frosts.
Clinical signs are similar in newborn lambs and kids, are nonspecific and may progress to death within 72 hours. Gross findings include icterus, hepatomegaly, widespread petechiae, gastrointestinal hemorrhage and enlarged, edematous lymph nodes. Infected adult animals lack gross lesions and are rarely febrile or jaundiced. Infection of pregnant animals may result in embryonic death, fetal mummification or abortion. In pregnant sheep, natural and experimental infection and vaccination with attenuated modified live vaccine has produced abortions and congenital defects including hydranencephaly, porencephaly and arthrogryposis.