SPONGIFORM encephalopathy is a fatal degeneration of the brain in which the organ takes on a spongy appearance due to the presence of many holes. The variety found in cattle is bovine spongiform encephalopathy (BSE) "mad cow" disease.
BSE is a particular problem in the UK, where it is estimated that millions of people have eaten food products from animals who had the disease. We don't yet know the full human health implications. However, at this stage it seems almost certain that BSE has the capacity to kill humans.
A particular form of spongiform encephalopathy that afflicts humans is CJD, Creutzfeldt Jakob Disease. CJD is rare, but everyone is at low risk of contracting it in later life (lifetime risk of one in 50,000). It can be classified under three headings: (a) About 85 per cent of all CJD is sporadic CJD (no known cause). It occurs with similar low frequency all over the world; (b) A very small number of people have developed CJD as a result of intimate exposure to something contaminated with CJD; (c) There are rare inherited forms of CJD.
None of these three forms has anything to do with BSE (as far as I know)
There has only been one well documented outbreak of human spongiform encephalopathy. It began early this century in Papua, New Guinea, and was called Kuru.
In 1957, a team of scientists studied the disease. The distribution of Kuru was peculiar - it affected women and children of both genders. The victims experienced hallucinations and trembled with fear. The symptoms progressed inexorably from uncoordinated movement, to staggering, to inability to walk, to inability to stand. Death occurred within nine months.
Kuru had a simple explanation. These people were cannibals and ate dead relatives as a mark of respect. The men got the best parts (i.e. the muscle), while the women and children ate the offal (brain, liver, kidney, etc.). The causative agent of the disease concentrates in the parts (especially the brain), eaten by the women and children, and they developed Kuru. Kuru died out when cannibalism ended in the 1960s.
Spongiform encephalopathy in sheep is called scrapie. It has been present in UK sheep for hundreds of years and, although fatal in sheep, is harmless to humans.
In 1986, a new form of the disease (BSE) was noted in UK cattle. This raised concern regarding possible effects in humans, as people had been eating the brains of cattle. Two questions needed urgent answers: What is the nature of the agent that causes BSE and can BSE transfer to humans? However, long experience with scrapie made it seem unlikely that BSE would cause problems in humans.
The agent that causes scrapie can cross into cattle. BSE apparently developed when cattle were fed meat and bone meal containing rendered sheep offal. For the first two years of the BSE outbreak in the UK, affected cows could be slaughtered and used for human consumption. Brains ended up in burgers, sausages, meat pies etc.
In 1988, a UK scientific committee was convened to advise on the BSE. It said that cattle dying of BSE should be slaughtered and incinerated. However, because of the long incubation period for BSE, during much of which cattle remain symptom free, this advice allowed animals that were incubating the disease to enter the food chain. The only way to avoid this would have been to remove the brain and spinal chord (the most dangerous organs), from all cattle carcasses. This was not called for, except for baby foods.
Nine months later, the government extended the offal ban to all foods. Some abattoirs apparently did not fully enforce the ban and infected material got into the human food chain until the end of 1995. Millions of people ate infected beef in the UK.
In hindsight, optimism based on experience with scrapie was unjustified. Research had shown that when scrapie enters a new species it can change its infective characteristics. Although scrapie does not directly cause CJD, when scrapie is passaged through cattle it may change. An ominous warning was sounded in 1990 when cats, never susceptible to scrapie, died of BSE.
As an infectious agent, BSE is quite promiscuous and it can cross into almost every species exposed to it - mice, cats, monkeys, etc. Could BSE cross into humans and cause a new variety of CJD? A study was established in 1990 to search for emerging new varieties of CJD.
Conventional CJD, familiar to medicine, affects people between the ages of 50 and 75. CJD brains examined in the microscope show holes and clumps of protein. In 1994, scientists noticed the appearance of an apparently new variety of CJD. These cases occurred in younger people - late teenagers, 20 and 30 year olds.
Their brains looked distinctly different to conventional CJD under the microscope. The timing of these new cases was also suspicious.
Exposure to BSE infected food in the mid 1980s could cause the first new cases of atypical CJD to appear in the mid 1990s. When analysed chemically, brain samples from the new CJD cases had an identical molecular signature to BSE. This was close to proof that BSE had crossed into humans to cause this fatal disease.
Most diseases are caused by viruses or bacteria, which must be present in large numbers to cause harm. They increase in numbers by dividing. The infection can be combated by killing the micro organisms or by preventing them from dividing. The ability to divide depends on the genetic material, DNA.
The agent that causes spongiform encephalopathy is unique. It is incredibly resistant to the methods commonly used to kill bacteria and viruses - boiling, radiation, alcohol, autoclaving, etc. Ordinary cooking of food will not kill this agent, infected samples must be incinerated to destroy the agent.
There is considerable evidence that the infectious agent in spongiform encephalopathy is a protein called a prion. It is something of a scientific heresy to claim that a protein is an infectious agent. Proteins contain no DNA, so how can they replicate and be infectious? In 1967, J.S. Griffith, in a theoretical paper, suggested a way in which protein alone could replicate and cause disease.
Proteins are composed of amino acids joined together to form long strings. The string coils into a unique three dimensional configuration and this overall shape confers a unique biological personality on the protein. If the protein folds into a different shape, it will behave differently.
Imagine you have two forms of the same protein, one folded normally and the other folded abnormally. Further imagine that if the abnormally folded protein comes into contact with the normally folded protein, it triggers the normal protein to fold itself into the abnormal shape. So, one abnormal protein placed into a collection of normal proteins will quickly convert all the normal into abnormal protein. This seems to be what happens in spongiform encephalopathy.
The abnormally folded protein is extremely chemically stable and can survive huge doses of treatments that would kill any virus or bacteria. The normal healthy body contains the normal form of prion. However, infection with abnormal prions can corrupt the normal prions and the abnormal prions link together to cause the clumps and holes seen in the CJD brain.
The first case of BASE in Ireland was diagnosed in 1989. The annual average was about 17 cases from 1989 through 1995. In 1996 had 73 cases were identified.
Ireland has only a tiny BSE problem - 188 cases since 1989 in a national cattle herd of 7.5 million. Britain has recorded 165,000 cases of BSE since 1986 in a national cattle herd of about 10 million.
Ireland took strict measures to control BSE right from the start. Feeding meat and bone meal to cattle has been banned since 1990. All animals in a herd where BSE has been diagnosed are slaughtered and removed from the food chain. Importation of cattle or beef products from the UK is prohibited. Overall, it seems highly unlikely that BSE will cause any significant ill health effects in Ireland.