Mad cow disease (also known as Bovine spongiform encephalopathy [BSE]) is a neurodegenerative, fatal affliction of cows. It’s caused by prions, which are proteins that are normally present in cows, but they have been mis-folded. Think of a paper clip as a prion, and its unwound form as a straight strip of metal is the original protein. The mishapen prions can also induce other normal proteins around them to become prions.

Prions are not alive – they are errant proteins produced in an animal, much like cancer cells are regular human cells gone wrong.

When prions form in a cow’s brain, they aggregate and damage the nerves, resulting in a spongy brain and spinal cord, with holes where the nerves have died. This doesn’t only happen in cows: scrapie is a prion disease in sheep, named because the suffering animals itch and scrape off their fleece on trees, fences, anything they can find.

So what causes these prions to form? Mammals have a prion gene which makes normally folded prion proteins; it’s mutations in this gene that result in aberrantly-folded proteins, which are the disease-causing ones (for the protein-lovers out there, it causes alpha-helices to convert to beta-sheets).

Most cases of prion disease are sporadic, meaning that they occur without any known risk factors. Some cases are genetic, and the mutated genes can be passed from parents to their children, but this is very rare. It can also be caught if animals are exposed to prion-contaminated tissues obtained from animals with prion disease. It is thought that cows contracted BSE from scrapie in sheep, and this then spread exponentially when affected meat and bone meal from cattle was fed to other cattle.

The first report of BSE was in the UK in 1986. Around 180,000 cows in the UK were suspected to have been infected in the years after that, but since the incubation period of this disease is 3 to 8 years, the government couldn’t risk waiting: almost 4.5 million cows were killed, without having visible symptoms. These days, cows are tested before entering into the human food chain.

There was a massive reaction to this tainted British beef – the EU banned all imports between 1996 and 2006. Some thought this ban was excessive and punitive, but governments were rightly worried: prions are so stable that high temperatures (in well-done beef) don’t destroy them. BSE first showed up in Canada and the USA in 2003, after which Japan stopped all USA beef imports.

Soon after 1986, cases of a similar disease in humans in the UK were reported, and by October 2009, 166 people had acquired and died from variable Creutzfeldt-Jakob disease (vCJD). This incurable ailment can’t be transmitted between people, but is thought to come from consuming prion-containing beef. It has only been observed in countries where BSE is found, and those afflicted are young (median age 28 years).

The symptoms include dementia, memory loss and seizures, which may show up weeks, months or even years after transmission, and it is almost always fatal.

Microscopic holes in the grey matter gives a BSE-affected cow’s brain a sponge-like appearance.

Did the incidence decrease?  Is it still around? Around 5 people per year are still diagnosed with vCJD in the UK, and it is thought that the appearance of vCJD can be due to exposure years or decades earlier. Many countries still ban blood donations from people who lived in the UK for 6 months before 1996. As of March 2011, there had been 221 cases of vCJD worldwide (172 in the UK, 3 in the USA), and only five of those sufferers are still alive.

Was it determined that BSE wasn’t involved in Creutzfeldt–Jakob disease?
BSE isn’t involved in ‘classical’ CJD, which has been around since the 1900s. Five to 10% of cCJD is genetic, some is sporadic, and older people are normally affected (median age 68 years). cCJD can be transmitted between humans through affected blood products: the recent scare in the UK was in a hospital in Wales where tools used in the surgery of a patient at possible risk of having cCJD were sterilised (a process which kills germs but not prions) and then used on 37 other patients, unknowingly putting them at risk.

So yes, BSE isn’t involved in cCJD, but it is most likely the source of vCJD. And while the incidence of vCJD has decreased, it is likely to continue at very low levels, as people who were exposed to BSE meat a long time ago develop symptoms years later.