Spongiform Encephalopathies

These diseases were once referred to as ' slow virus infections'; what are the criteria for 'slow infections'?

1. A prolonged incubation period, measured in months or years, rather than in days.

2. A regular clinical course of infection, leading inevitably to serious disease and, most likely, death.

3. Pathology is limited to a single organ system - the brain.

4. Limitation of the disease to a single host species.

5. Lack of an inflammatory reaction.

Slow infections of the central nervous system are associated with two, broadly classified groups of agents.

The first group are viruses with conventional properties, mostly associated with an acute illness that, under the right conditions, can cause a chronic and protracted disease. An example is the association of the measles virus with progressive CNS disease, subacute sclerosing panencephalitis, (SSPE). The SSPE agent is clearly not the wild-type measles virus but, rather, is most likely a mutant, defective or recombinant form of the virus. Other examples include Progressive Rubella Panencephalitis (PRP) associated with the rubella virus and Progressive Multifocal Leukoencephalopathy (PML) associated with the polyomavirus JC.

The second group of agents associated with slow infections are the ill-defined agents of the subacute spongiform encephalopathies. These include the agents of Scrapie, BSE, Creutzfeldt-Jakob disease, kuru, etc.

Slow virus infections caused by conventional viruses

Subacute Sclerosing Panencephalitis

SSPE is an encephalitis involving both the grey and the white matter of the brain. The course of the disease is slowly progressive and the patient usually dies within 6-12 months of the onset of the disease. Measles virus typically causes an acute febrile illness but very rarely, 1 in every 10⁵-10⁶ cases, the virus, which has remained dormant in the brain, reactivates after a period of between three and seven years to cause the symptoms of SSPE.

Half of all patients who develop SSPE had their acute measles infection before the age of two years. The disease is three times more common in boys than in girls. It is not known in which form the virus remains dormant in the brain, nor is it known what reactivates the infection to lead to SSPE.

Measles virus can only be recovered with difficulty from pathological material in SSPE and hence the disease is considered non-transmissible. SSPE is not infectious for contacts of the sufferer. The virus that can be recovered is defective, having lost part of its genome, and is thus not released from cells in the normal way. It can spread, however, by cell-to-cell contact.

The frequency of SSPE appears to be reduced at least ten-fold when using a live measles virus vaccine to prevent the occurrence of the wild virus infection. This is probably because the vaccine strain has a much lower infectivity for the CNS.

Progressive Rubella Panencephalitis

This unusual disease shows many similarities to SSPE. The rubella virus causes it. This virus causes congenital abnormalities in the babies of women infected during pregnancy and these most often affect the CNS. It particularly affects the offspring of women infected in the first trimester of pregnancy, when the foetal tissues are at their most plastic. PRP typically manifests between the ages of five and twenty years. It is a progressive, fatal disease with death typically occurring within a year of onset of the symptoms.

Progressive Multifocal Leucoencephalopathy

PML occurs primarily as a complication of severe disease of the lymphatic system, such as Hodgkin's lymphoma. The underlying disease causes severe immunosuppression, allowing reactivation of a previously silent persistent JC virus infection. The JC virus is a polyoma virus, related to wart viruses and the BK virus that affects kidneys.

In PML, the virus attacks oligodendroglial cells predominantly. Virus particles accumulate in the nucleus of these cells where they cause large inclusions. Destruction of these cells leads to the formation of demyelinated plaques disseminated through the brain and this leads relentlessly to death.

Before the AIDS pandemic, PML was a very rare condition but within a year of the first description of AIDS, it became a recognised associated disorder. Current estimates are that 4-5% of all HIV-infected individuals will develop PML. This formerly rare disease, once regarded as a clinical curiosity by most neurologists, is now alarmingly common.

The Unconventional Agents of Slow Infection:
The (Subacute) Transmissible Spongiform Encephalopathies

The term 'spongiform' refers to the vacuolating changes found in brain tissue after the disease becomes apparent. The brain becomes spongy, with holes in the substance of the brain. These diseases are slowly progressive and inevitably fatal through the destruction of the brain substance. They all share common pathological effects and they are all always fatal. The spongiform encephalopathies include the following:

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Human Infections - Kuru

Kuru was the first human spongiform encephalopathy to be described and has a fascinating epidemiological course. It is a subacute, endemic, fatal degenerative disease of the central nervous system. It is restricted to certain Melanesian tribes, particularly the Fore people of Papua New Guinea.

In the 1950's, the disease was the most common cause of death among the Fore people, with a death rate of 2-3% annually. At that time, the rate of attrition was scarcely balanced by the birth rate in the Fore tribe. The disease affected predominantly children and adult women, progressing to death within six months from the first appearance of symptoms.

In the Fore Language, 'kuru' means 'to tremble', describing the initial symptoms of the disease: a mild cerebellar ataxia, which progresses to a progressive dementia. There is loss of muscle control and balance as the disease progresses.

Transmission of kuru was shown to be related to ritual cannibalism: a mark of respect for the dear departed rather than as an act of vengeance against an enemy. This is why the disease is largely confined to a single tribe. At a funeral, mourners, having prepared the corpse of the deceased individual, consumed the body, which was prepared by the women and children of the tribe. Adult men did not take part in these rituals and thus escaped the disease. This explains the age and sex distribution of this condition.

In 1957, ritual cannibalism ceased and nobody born after this year has ever been affected by kuru. The last cases, however, did not manifest until the late 1980's showing that the incubation period for this disease may be as long as thirty years. Within another generation, kuru should be extinguished completely. Its infectious agent has been experimentally transmitted to animals.

Human Infections - Creutzfeldt-Jakob disease (CJD)

CJD is a rare form of presenile dementia that is rapidly fatal. There is premature ageing of the brain with loss of function before physiological ageing has taken place. It occurs worldwide and is considered to be the industrialised variant of kuru, from which it is different.

The mean age of patients with CJD is 56 years and it is VERY rare in people younger than 30 years of age. The new-variant or variant form of the disease (nvCJD or vCJD) typically affects people under 30 years of age. CJD affects men and women in approximately equal numbers and has an incidence of one case per million people. The patient dies within 3-4 months of onset, from atrophy of the brain.

Typically, CJD occurs in sporadic cases but in 10-15% of cases it occurs in a familial form suggesting that in these cases, it is a genetic disease with an autosomal dominant inheritance. CJD has been transferred to chimpanzees and other experimental animals using infected brain tissue. Inadequately treated neurological instruments and corneal grafts have been responsible for iatrogenic transmission.

Human Infections - Gerstmann-Sträussler-Scheinker Syndrome

GSS is a disease that is similar to CJD and is considered a variant form of the disease. It is particularly noted for the deposition of amyloid plaques in the brain and has a longer progression time to death than is typical of CJD.

Animal Infections - Scrapie

Scrapie is a natural infection of sheep also seen in goats. It is usually spread maternally but can spread horizontally. It is the oldest known spongiform encephalopathy, first recognised several centuries ago. It was not properly characterised until the 1930's.

Scrapie has a worldwide distribution, probably due to the export movement of sheep incubating the disease. Its name derives from the habit of infected animals, which scrape against objects incessantly, presumably to relieve itching of the skin.

The incidence of the disease increases with the age of the animals and the disease is not seen in animals less than eighteen months of age. This indicates that the incubation period is at least this long.

Some countries, notably Australia, have eliminated Scrapie by a policy of slaughtering affected flocks and the imposition of rigorous import controls. The United Kingdom does not have a slaughter policy for Scrapie.

Animal Infections - Transmissible Mink Encephalopathy (TME) and Feline Spongiform Encephalopathy (FSE)

These are Scrapie-like diseases of mink and cats caused by feeds that were derived from Scrapie-infected sheep carcasses. FSE was first reported at the Bristol Veterinary College, Langford. FSE has now been shown to affect ocelots, pumas, cheetahs and tigers; all by using infected feedstuffs.

Animal Infections - Chronic wasting disease

This is a disease similar to Scrapie, which affects captive deer (Journal of General Virology [1999] 80, 2265-9).

Animal Infections - Bovine Spongiform Encephalopathy (BSE)

BSE suddenly appeared in dairy cattle in the UK during the 1980's and was first recognised officially in 1986 as a spongiform encephalopathy. It is a disease of the central nervous system that induces changes in mental state, movement and sensation. It lasts for several weeks and is always fatal. Affected cattle become nervous and have a staggering gait. This has earned the name 'mad cow disease'.

On microscopic examination, the brains of affected cattle show extensive spongiform lesions. It is almost certain that BSE resulted from the feeding of Scrapie-infected sheep carcasses to cattle although the lesions seen in the two diseases are distinct. This has led to the suggestion that the Scrapie agent has adapted to infect cattle.

In the 1980's, the regulations controlling the rendering industry were relaxed to permit the introduction of a less intensive, low-temperature treatment of animal products destined for the animal feed industry. There are reports that the agent of BSE can survive 360^oC for one hour. It is considered that these changes allowed the infectious agent present in infected sheep material to infect cattle via contaminated feeds.

Introduction of stricter controls on the animal feeds industry did not see a drop in the cases of BSE because of the lag of a long incubation period. This did follow and later, there was a policy of slaughtering dairy cattle at 30 months of age. There are still cases of animals born after these controls, suggesting that the agent is contaminating the environment and that it is capable of horizontal transmission. BSE could have been controlled by a rigorous slaughter policy but, at the time, the economic cost was considered too great.

The Infectious Agent of BSE

All spongiform encephalopathies share the same basic histochemical abnormalities and are thus assumed to be caused by similar mechanisms. Both the gross pathological changes and the intracellular cytochemical changes in the various spongiform encephalopathies share many common features:

Pathological changes

• A defect in plasma membrane formation
• This is followed by vacuolation of neurons, particularly astrocytes and oligodendrocytes.
• At a later stage in the disease, the grey matter of the brain shows extensive neuronal loss and develops a spongiform microscopic appearance - it becomes full of holes.

Intracellular cytochemical changes

• There is an accumulation of large plaques composed of glial fibrillary acidic protein (GFAP).
• There are also fibres of another cellular protein: Scrapie-Associated Fibrils (SAF).

The pathological changes have more than a passing similarity to Alzheimer's disease - a senile dementia. There is, however, NO EVIDENCE that Alzheimer's disease is transmissible and, unlike the spongiform encephalopathies, Alzheimer's disease is not thought to be caused by an infectious agent.

Initially viruses were thought to be the infectious agents responsible for the spongiform encephalopathies. Similar neurodegenerative diseases could be induced in experimental animals, chimpanzees and mice, inoculated with infected tissues and filtration studies showed that the size of the infectious agent was comparable with that of viruses. The properties of the infectious agent have cast doubt on the virus theory. These include:

• Resistance to DNAse and RNAse treatments;
• Resistance to heat inactivation;
• Resistance to radiation damage BUT
• Sensitivity to urea, SDS, phenol and other protein-denaturing treatments.

The Scrapie agent is very resistant to both ultraviolet light and ionising radiation indicating that nucleic acid, if present, must be extremely small; probably less than 50 nucleotides. Taken as a whole, these data suggest that the infectious agent is probably a protein and that if it is associated with any nucleic acid is present it will be extremely small.

Study of infectious agents has resulted in the identification of a protein of 254-amino acids that appears to be responsible for infectivity. There is no positive evidence that any nucleic acid component is associated with this protein. In 1982, Stanley Prusiner proposed the term Prion for this protein, now referred to as PrP - Prion Protein. Prusiner acknowledged that the idea of an infectious protein devoid of nucleic acid was a heresy but problems were compounded when it was shown that PrP is encoded by a single-copy cellular gene and that the protein is present in uninfected tissue.

There are two forms of PrP. The normal cellular form is called PrP^C and is found predominantly on the surface of neurons. PrP^C is sensitive to and is completely degraded by protease digestion. The second form of the protein, PrP^SC is found in diseased tissue and, unlike PrP^C, PrP^SC is partially resistant to protease digestion, resulting in the formation of a 141-amino acid protease-resistant fragment that accumulates as fibrils in the infected cells of the brain associated with areas of neuronal damage. Only a proportion of the total PrP in diseased tissue is present PrP^SC but extensively purified PrP^SC has been shown to be infectious when inoculated into experimental animals.

The term prion is defined as a 'small proteinaceous infectious particle that resists inactivation by agents that destroy nucleic acid and that contains an essential modified isoform of a cellular protein". Extensive studies have revealed no obvious differences, such as alterations in amino acid sequence, between PrP^C and PrP^SC that could account for the different properties of the two forms. It has been suggested, therefore, that the functional differences must be the result of conformational changes but this has not been proven and the 3-dimensional structure of PrP still awaits elucidation. Neither form of the protein elicits antibody formation or other immunological reaction since they are recognised as 'self' antigens.

Several families have shown a high incidence of GSS, and in family studies, mutations in the gene encoding PrP have been observed. Even in these cases, however, there are no detectable differences in the PrP^C and PrP^SC. This has been explained by suggesting that mutation facilitates the spontaneous conversion of PrP^C to PrP^SC, placing individuals who carry the mutation at greatly increased risk of developing GSS. It has been suggested that PrP^SC autocatalyses the conversion of PrP^C into PrP^SC and that the symptoms of disease result from the loss of normal PrPM^C function. Unfortunately for this theory, transgenic mice that completely lack PrP through a genetic knockout appear entirely normal. An alternative theory suggests that PrP^SC triggers apoptosis of neurons.

The major problem with the prion theory is the need to account for the existence of many different strains with different incubation times, different clinical targets within brain tissue and different patterns of lesions. The prion protein would require at least twenty different stable conformations to account for the number of different Scrapie agents that can be propagated stably in mice. This is difficult for some experts to accept.

Three explanations have been proposed to overcome this difficulty. PrP^SC could be associated with an accessory molecule, such as a small nucleic acid, that modulates the phenotype of the prion without being essential for infectivity. No evidence has been found that supports the existence of an accessory molecule. Alternatively, the PrP^SC could undergo secondary modification such as glycosylation. Different polysaccharides could then target the molecule to different tissues or cells, bringing about different pathologies. This is the target-cell hypothesis. Finally, it has been proposed that each form of the disease is associated with its own specific conformational change. More than one protein chemist has poured scorn on this idea but a growing number of studies imply that this is the case.

An alternative hypothesis for the nature of the infectious agents of the spongiform encephalopathies has been proposed. This is the virino hypothesis. A virino is an 'informational hybrid' consisting of a host protein coat surrounding a host-independent genome. PrP^SC is the only protein that co-purifies with infectivity, thus, this protein is likely to form the protein coat of the putative virino. The host-independent genome has to be very small, but this is possible since it does not have to code for its own coat. Furthermore, although it does not have to be a nucleic acid, it is likely to be one since strains of the agent do mutate and this is explained biologically most easily if the genome is a nucleic acid. Although this hypothesis is very attractive, there have been no reliable reports of the isolation of Scrapie-associated virinos. It should not, however, be discounted as an idea.

Yet another hypothesis is that an as yet unidentified virus binds to its cell receptor, which happens to be PrP^C, before entering the cell and replicating. This binding brings about the change to the protease-resistant isoform PrP^SC. Supporting this theory, there are reports of transmission of BSE to mice in the absence of the prion protein.

Some additional notes on human spongiform encephalopathies

CLASSIFICATION OF THE HUMAN TSEs

Idiopathic:

sporadic CJD

Acquired:

Iatrogenic CJD
Kuru
New-variant CJD - nvCJD also latterly referred to as vCJD for variant CJD

Inherited:

Familial CJD
GSS

DIAGNOSIS OF THE HUMAN TSEs

• Clinical features

• Neuropathology (spongiform changes and accumulation of prion protein)

• Western blots

Sporadic CJD has an incidence of about 1 per 1,000,000 persons. There is no link with Scrapie since the incidence is no different in Australia, where Scrapie has been eradicated. The average age of patients is between 55 and 70 years old although the disease may be more common than presently recognised in older people. Symptoms are of a rapidly progressive dementia with most people affected dying within six months of onset of the disease.

The new variant form of CJD typically affects people from teenage to early fifties. There are deposits of amyloid plaques not seen in sporadic CJD. There is a massive accumulation of PrP, which is different in pattern from the sporadic CJD. The early disease is characterised by psychiatric and sensory symptoms rather than dementia, which only develops later.

There is an interesting distribution of polymorphism at codon 129 in the prp gene. It can encode either methionine or valine. Humans are diploid and can be either homozygous for methionine (M - M) or for valine (V - V) or may be heterozygous (M - V).

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This may represent a risk factor for CJD but consumption of contaminated beef before the statutory ban on offal in 1989 still represents the most likely cause of vCJD. However… see: Peden, A.H., Head, M.W., Ritchie, D.L., Bell, J.E., Ironside, J.W. (2004) “Preclinical vCJD after blood transfusion in a PRNP codon 129 heterozygous patient.” Lancet. 364 (9433), 477-9. This comment reports the case of preclinical variant Creutzfeldt-Jakob disease (vCJD). This occurred in a patient who died from an unrelated illness five years after receiving a blood transfusion from a donor who subsequently developed vCJD. This has major implications regarding the pool of people susceptible to vCJD.

The epidemiology of vCJD is under constant review. Three clusters of cases have been reported though the cause is unknown. There are still important questions to be answered, not the least of which is how many people will be affected and what the incubation period is.

Data as of 29 September 2006 regarding deaths from vCJD are presented below and a text version is provided for browsers that cannot handle tables.

* Data correct as of 3rd November 2008

A summary of the information on the CJD Surveillance Unit, Edinburgh, is provided and is considered correct as of 5 November 2007.

Dead

Deaths from definite vCJD (confirmed): 115

Deaths from probable vCJD (without neuropathological confirmation): 49

Deaths from probable vCJD (neuropathological confirmation pending): 0

Number of deaths from definite or probable vCJD (as above): 164

Alive

Number of probable vCJD cases still alive: 3

Total number of definite or probable vCJD (dead and alive): 167.

Diagnostic criteria for vCJD

Definite vCJD is based on: I) A (progressive neuropsychiatric disorder) and neuropathological confirmation of vCJD ****

Probable vCJD diagnosis is based on: I and 4/5 of II and III A and III B or I and IV A

Experimental therapy

During 2003, interest has focused on a 19-year old victim who has, following a High Court ruling by Dame Elizabeth Butler Sloss, been receiving pentosan polysulphate as a direct injection into the brain. He has survived significantly longer following the initiation of treatment than would have been expected in the absence of therapy and his family claim to see an improvement in his condition. Nevertheless, he remains heavily disabled by the illness. In the past, pentosan polysulphate has been used to treat inflammation in other parts of the body but it does not cross the blood-brain barrier; hence the need for direct intrathecal injection.

Page edited:  
October 2006