Confirmed cases of atypical BSE to date have usually occurred in older cattle and have been detected through routine surveillance programs of healthy animals or fallen stock around the world. In a laboratory setting, both L- and H- forms are transmissible to other species by direct intracerebral inoculation. The L-type has been transmitted into wild-type, bovinized and ovinized transgenic mice, to cattle and cynomolgus macaques. The H-type has been transmitted to wild-type, bovinized and ovinized transgenic mice. Transmission studies in cattle and cynomolgus macaques are underway. No horizontal transmission (animal-to-animal by direct contact) has been reported so far.