Bovine spongiform encephalopathy (commonly called “mad cow disease”) belongs to a group of conditions known as Transmissible spongiform encephalopathies (TSEs), or prion diseases. BSE mainly affects cattle and is caused by an unconventional agent: it is now generally accepted that this agent is not a virus, but a modified form of a “non-pathological” protein, known as a “prion”. The disease is named after the brain lesions that, when examined under a microscope, appear as optically empty areas, making the brain tissue resemble a sponge. Examination of the brain tissue of cows with confirmed BSE clearly reveals the presence of the typical spongiform lesions caused by the accumulation in neurons of the pathological form (PrPsc, an acronym for Prion Protein Scrapie) of the PrPc protein, which is physiologically present in the nerve cells of cattle as well as in those of other animals and humans.
The disease occurs most frequently in dairy cows at about 5 years of age. Detectable clinical symptoms are mainly neurological, and include changes in behaviour, sensation and movement. These symptoms are usually the first to appear. The cow becomes anxious, nervous and aggressive, looks fearful when approached by humans and is hyperresponsive to external stimuli (for example, when milked or when somebody moves too close or approaches too quickly). These behaviours may be associated with symptoms revealing involvement of the autonomic nervous system, such as reduced rumination and heart rates and decreased milk production. As the disease progresses, movement and postural deficits become more noticeable (e.g. head carried low, tremors and staggering gait). The cow stumbles and often falls on its hind legs, until it loses the ability to stand upright.
As the BSE agent has proved able to make the so-called “species jump”, the description of the first case of a new variant of Creutzfeldt-Jakob disease (nvCJD) in 1996 caused renewed apprehension. Since 1996, about 100 cases of nvCJD have been reported (with a peak of 28 cases in 2000), of which only 3 occurred outside the United Kingdom. It is now generally accepted that the disease is caused by exposure to specified risk materials (SRMs), that is bovine nervous tissue and other tissues in which the presence of the BSE agent has been demonstrated.