BSE and CJD disease

Bovine spongiform encephalopathy (BSE) and Creutzfeldt-Jakob Disease (CJD) are ailments in which the functioning of the brain is progressively impaired. Both diseases are associated with visually abnormal pinpoints (or plaques) in the brain, and in a changed texture of the brain tissue. The brain tissue, particularly in the cortex and cerebellum, becomes filled with large open spaces (vacuoles) and becomes spongy in texture. The "spongiform" part of BSE comes from this texture characteristic.

BSE is a disease of animals such as cattle and sheep, while CJD is associated with humans. However, the two diseases may have a common cause. The cause of BSE and CJD, and of other diseases such as scrapie, transmissible mink encephalopathy, fatal familial insomnia, and kuru, are prions . Prions are particles that are made solely of protein. Even though they lack genetic material, they are infectious.

Both BSE and CJD are characterised by a loss of coordination and the control over functions such as grasping and holding, dementia, paralysis, eventually leading to death. There is no cure for either disease, and no are vaccines available.

CJD derives its name from its discoverers. Progressive and ultimately fatal dementia that was accompanied by other neurological abnormalities was described in six patients in the 1920s by two German neuroscientists, Hans Gerhard Creutzfeldt and Alphons Maria Jakob. In the 1960s, the neurological changes associated with the development of CJD had become accepted by the medical community.

The average incidence rate for CJD over time is about one person per million. Clusters of CJD do occur. The most recent example is the 48 confirmed cases that were diagnosed in Britain between 1996 and 2001. There is no evident predilection for a gender, any ethnic group, or for geographical location. However, the incidence in those over 55 years of age is far higher (over 30 times) than for those under 55 years.

Three means of acquiring CJD have been identified. First, the disease can be genetically inherited. This is also described as familial CJD. Secondly, the disease can appear with no exact origin being known. About 85% of CJD cases are of this unknown variety. Lastly, the disease can be acquired during surgery. This so-called iatrogenic form is typically a result of CJD-contaminated equipment or tissue (brain and corneal grafts are two examples).

There is no cure for CJD. Treatment consists of managing the patient so that his/her increasingly impaired mental faculties do not result in injury, and in personal care as these functions become impossible for the person to perform themselves.

BSE causes a progressive neurological deterioration in cattle that is similar to the course of CJD in humans. Cattle with BSE are more temperamental, have problems with their posture and coordination, have progressively greater difficulty in rising off the ground and walking, produce less milk, have severe twitching of muscles, and loss weight even though their appetite is undiminished. The so-called incubation period, the time from when the animal is first infected until symptoms appear, ranges from two to eight years. After appearance of symptoms, deterioration is rapid and the animal dies or is destroyed within six months. The disease is one of several so-called transmissible spongiform encephalopathies (TSEs) in animals.

BSE was confirmed as a disease of cattle in November of 1996. Since then, almost all reported cases have been in cattle born in the United Kingdom. Other countries in Europe and Asia have reported BSE, but in far fewer numbers than in Britain. No cases have been detected in the United States (the U.S. has not imported U.K. beef since 1985 and maintains a rigorous surveillance program). As of November 2001, the total number of confirmed cases of BSE in U.K. cattle was just over 181,000. In 1993, a BSE epidemic in the U.K. peaked at almost 1,000 new cases per week. While the cause of this near-exclusivity has yet to be conclusively determined, the 2001 outbreak of hoof and mouth disease in the United Kingdom revealed that a common practice has been to feed cattle "offal," the ground up waste from the slaughter process. Experience with kuru has shown that consumption of prioninfected tissue is a means of spreading the disease.

This method of transmission is suspected of being important, if not principle, for BSE. The exact origin of the prions is not known. Sheep are considered a likely source.

Until the 1900s, scientists believed that the transmission of the BSE agent to humans did not occur. However, several studies conducted in the latter years of the 1990s has cast doubt on this assumption. Studies using mice showed that the brain injuries caused in BSE and CJD are identical. Moreover, these brain alterations occurred in mice injected with either brain tissue from BSE-diseased cattle, which was expected, or with brain tissue from CJD-diseased animals, which was unexpected. Thus, development of CJD could be due to human consumption of BSE-diseased meat.

The currently held view is that prions from cattle infected with BSE are capable of infecting humans and causing what is termed a variant CJD (vCJD) disease in humans. There is evidence that the suspect vCJD has a different infectious behavior than CJD. For instance, younger people can be infected, and the neurological symptoms differ.

The existence of a vCJD is based mainly on epidemiological evidence. If the existence of vCJD is proven, then the species barrier for the transmission of BSE and CJD does not exist. However, the possibility still remains that the contaminating agent in the meat is really a prion that causes normally CJD, and that this prion is naturally present in cattle but has escaped detection until now. If so, then BSE and CJD infections could indeed be confined to non-human and human mammals, respectively.

See also BSE and CJD disease, advances in research; BSE and CJD disease, ethical issues and socio-economic impact; Latent viruses and diseases