Disease Facts - BSE, CJD and Scrapie

These diseases are grouped together by the name of transmissible spongiform encephalopathies (TSEs). TSEs cause widespread damage to the central nervous system. Animals die after large numbers of nerve cells in the brain are destroyed.

Scrapie was first recognised and described in 1732. It was a disease of sheep which occurred sporadically and its incidence was always low. For more than two centuries it remained a scientific curiosity, despite the fact that several other related diseases were recognised. It wasn't until BSE was discovered in 1985 that there any real interest or concern in TSEs. In 1996, with the link between BSE and vCJD in people proven by scientists at IAH, public concern rose dramatically.

 

Common name Species Year described
Scrapie Sheep 1732
Mad cow disease (BSE) Cattle 1985
Mink encephalopathy Mink 1947
Chronic wasting disease Deer 1967
Feline spongiform encephalopathy Cat 1992
Kuru Man 1990
Creutzfeld-Jacob disease (CJD) Man 1920
Variant (CJD) Man 1996

There have been several instances of spongiform encephalopathies in some animals kept in zoos, including kudus, nyalas and large cats

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Scrapie is found in many countries but is officially absent from both Australia and New Zealand. BSE is essentially a disease confined to the UK. There have been examples of the disease in several European countries, but all have been traced back to the imports from the UK. Similarly the cases involving exotic animals have all been found to have been caused by these animals being fed on contaminated meat and bone meal, the likely source of BSE.

Surprisingly, given the enormous interest in these diseases, the causal agent of scrapie and BSE is still unknown. There are two scientific theories: that it is caused by a virus-like particle (a virino) which has a very small genome of nucleic acid or that it is a unique form of life, an infectious protein (prion). Neither theory has yet been proven, though there is a lot of evidence in favour of the prion. Some scientists claim that because there are many types of TSE agent, that the virino is a more likely explanation.

Target hosts

Scrapie is a disease of sheep but not all sheep are susceptible, and there is evidence of genetic resistance. The PrP gene affects the incidence of scrapie. If sheep possess a PrP gene with the amino acid valine at codon 136, then the sheep is likely to be susceptible to scrapie, but if the codon codes for alanine then the sheep is likely to be resistant.

BSE is a disease of cattle, though it has been shown to have crossed the species barrier and affect humans.

Transmission

We do not know how TSEs are spread naturally. Although the BSE agent is known to have been present in certain meat-and-bone meals which were then fed to cattle. The oral route is therefore proved, but we don't know how much infected material has to be eaten to cause disease. Lambs may be infected with scrapie after eating infected placenta. It is also possible that lambs could be infected in the womb. Other transmission routes are possible. Studies have shown that infection can occur through cuts and abrasions.

Incubation periods of BSE and scrapie in their natural hosts range from 2 to 20 years or more. However in the mouse, not a natural host, this is reduced to between 3 months to 3 years. That is why a lot of TSE research is carried out using mice, because scientists can get results quickly.

Why did BSE occur?

There is no clear answer to this as there were probably several factors involved. These include the fact that:

  • the UK has a large population of sheep in which scrapie is a common disease
  • there were changes taking place in the rendering processes used to produce meat-and-bone meal
  • the demands on the dairy industry resulted in larger quantities of meat-and-bone meal being fed to cattle.

It is possible that the combination of these factors resulted in an initial infection in cattle that was then recycled through meat-and-bone.

Feeding meat-and-bone meal to cattle and sheep was banned in July 1988.

Diagnosis

You may remember the pictures shown on television of cows falling over when they have BSE. Similarly shepherds recognise scrapie in sheep by the animal's persistent scratching. But at present there is no test available that allows a TSE to be diagnosed accurately before the onset of clinical signs. On a more promising note, a test currently under development at the Institute which can detect the abnormal form of the prion protein.

In the mean time, the only reliable methods to diagnose TSE infection are to cut sections of preserved brain samples and examine them under the microscope or inject brain from suspect animals into mice and wait for the disease to develop.

Treatment

There is no treatment. The possibility of developing a vaccine is remote because these diseases do not produce an immune response in the host. Susceptibility to scrapie in sheep has been found to be genetically determined (but BSE is not) and a test is now available which allows the degree of susceptibility (or resistance) to be estimated. Selective breeding based on such results is now underway. It is hoped that targeted breeding will remove scrapie from the UK flock.

BSE is being controlled by slaughter of all suspect cases and by a ban on feeding meals containing various cattle parts, which are known or thought likely to carry infectivity. This policy is proving successful and it is believed that the infection will be eliminated from the UK herds in the near future.

At present there is no way to diagnose infected animals before clinical signs appear. Decontamination of equipment and premises is difficult because the agents are extremely resistant to the procedures that are commonly used to kill bacteria and viruses.

On-going research

There has been a massive amount of TSE research carried out since BSE was first recognised in 1985. IAH is a major centre for this work and spends over £3 million each year in this area. But advances are slow because of the long incubation times involved and a general lack of knowledge about these perplexing diseases. However, the first steps have already been taken in the search for a treatment for CJD; the outlook is promising for a test for TSEs that can be applied before clinical signs become apparent; and a genetic test for identifying sheep resistant to scrapie now is available. Important advances are being made, and the incidence of BSE is falling rapidly, which means that the research is paying off.