They are degenerative central nervous system diseases caused by infectious agents called "unconventional transmissible agents" (UCTA) or "pathogenic prions". They lead to the development of cavities in cerebral nerve cells (which can be seen under a microscope) making the brain look like a sponge (hence the term "spongiform").
BSE causes nervous symptoms in adult animals which progresses gradually (between one and six months) and inevitably towards death.
The incubation period is not known precisely but appears to be long, in the region of 5 years.
The characteristic symptoms of the disease are changes in behaviour and locomotor problems. The diagnosis, however, can only be made after the animal has died. Affected animals begin by becoming nervous, anxious, fearful or even aggressive. They tend to isolate themselves from the herd, and may push out their tongue to lick their muzzles. Cattle have a hesitant, vacillating gait, together with tremor. The animal's general state deteriorates. Death occurs six to eight weeks, after the onset of symptoms.