Bovine Spongiform Encephalopathy - Cause

Cause

The infectious agent in BSE is believed to be a specific type of misfolded protein called a prion. Prions will not disappear even if the beef containing them is cooked. Prion proteins carry the disease between individuals and cause deterioration of the brain. BSE is a type of transmissible spongiform encephalopathy (TSE). TSEs can arise in animals that carry an allele which causes previously normal protein molecules to contort by themselves from an alpha helical arrangement to a beta pleated sheet, which is the disease-causing shape for the particular protein. Transmission can occur when healthy animals come in contact with tainted tissues from others with the disease. In the brain, these proteins cause native cellular prion protein to deform into the infectious state, which then goes on to deform further prion protein in an exponential cascade. This results in protein aggregates, which then form dense plaque fibers, leading to the microscopic appearance of "holes" in the brain, degeneration of physical and mental abilities, and ultimately death.

Different hypotheses exist for the origin of prion proteins in cattle. Two leading hypotheses suggest it may have jumped species from the disease scrapie in sheep, or that it evolved from a spontaneous form of "mad cow disease" that has been seen occasionally in cattle for many centuries. In the fifth century BCE, Hippocrates described a similar illness in cattle and sheep, which he believed also occurred in man. Publius Flavius Vegetius Renatus recorded cases of a disease with similar characteristics in the fourth and fifth centuries CE. The British Government enquiry took the view the cause was not scrapie, as had originally been postulated, but was some event in the 1970s that was not possible to identify.

Findings published in PLoS Pathogens (12 September 2008) suggest mad cow disease also is caused by a genetic mutation within a gene called the prion protein gene. The research shows, for the first time, that a 10-year-old cow from Alabama with an atypical form of bovine spongiform encephalopathy had the same type of prion protein gene mutation as found in human patients with the genetic form of Creutzfeldt–Jakob disease, also called genetic CJD, for short. Besides having a genetic origin, other human forms of prion diseases can be sporadic, as in sporadic CJD, as well as foodborne. That is, they are contracted when people eat products contaminated with mad cow disease. This form of Creutzfeldt-Jakob disease is called variant CJD.