10.1 The phenomena seen with transmissible spongiform encephalopathies and an attempt to explain what is seen
An attempt is made here to give a short description of what is seen with these diseases. They are all fatal, untreatable, diseases, for which there is no adequate method of diagnosis before death. The can be passed from one species to another by taking them in orally or by injection. The disease does not seem to be destroyed by simple cooking and produced no immunity in the body. The TSEs that we know are closely related in the way that damage is produced and the method of transmission.
The description given here is from the article 'TSEs' of the Encylcopaedia of Microbiology Volume 4 Academic Press 1992.
Alpers disease Bovine spongiform encephalopathy Chronic wasting disease of deer Creutzfeldt-Jakob disease Dose experiments Feline spongiform encephalopathy Gerstmann-Straussler-Scheinker Histopathology Immunity Infective agent Kuru Prevention Resistance of agent to destruction Scrapie Species barrier Tissue infectivity Transmissible mink encephalopathy Zoological spongiform encephalopathy
Scrapie is a natrually occurring disease of sheep found in many parts of the world, btu not everwhere. NowKNown for over 200 years, it possibly started in Spain and spread to thewhole of western Europe. The export of strictly bred sheep from Britain in the ninteenth centruy is thought to be involved in the rapid spread to other couuntries. Work in Iceland has shown that the land on which infected sheep graze may retain the condition and infect later sheep, even if all scrapie-infected sheep are removed. Although sheep that were shipped to Australia and New Zealand could not have been tested for scrapie before they were sent, the illness does not seem to exist there; eradication procedures appear to have worked. Sheep that are improted into those countries are kept separately for some years before being alloweed into contact with local sheep, and scrapie has appeared in sheep durig this quarantine period. This makes it extremely unlikely that an infected animal will gain entrance to these countries. Some countries claim to have low numbers of cases (eg. Germany) but limited outbreaks occur apparently randomly among unconnected flocks, despite this. The incidence of scrapie in a flock appears to be related to the breed of sheep with some being relatively resistant to theillness (e.g. scottish blackface) and others that are prone to it (herdwick, suffolk) attempts have been made to eradicate scrapie from certain countries (e.g. US) by slaughtere of infected flocks. This, however, has been largely unsuccessful, and theoccurence of scrapie has been claimed to be increasing in the UK. Poor statistics on the prevalence of scrapie abound, and farmers not recognising the condition may merely slaughtere the infected animal and fail to report it to an agricultural officer.
In 1936 reserarchers showed that scrapie could be transmitted to a healthy sheep by the intra-ocular inoculation of a homogenate of scrapie infected brain. This experiment led to large amounts of resrarch conernig the mode of transmission of TSE.
Sheep inoculated with scrapie infected tissue intracerebrally will have a short (possibly as low as 2 months) incubation period, but on farms it is older sheep, usually more than 4 years old that show signs of disease. The sheep irritablility, excitability, and restlessness at the onset, giving rise to scratching, biting, rubbing of the skin (hence the name scrapei) patchy loss of wool, tremour (hence the French name 'tremblante'), loss of weight, weakness of the hindquarters, and in some animals, impaired vision. The disease is always fatal. Only a small number of animals in a herd suffer from the clinical signs of scrapie, and experts have rarely seen 10 cases in a single herd. The natural mode of transmission between sheep is unclear. Experiments in which sheep with scrapie and those without it have been kept together on a farm have given rise to differing results, but goats have been shown to catch the disease from sheep in this form of experiment. Lambs of scrapie infected sheep are more likely to develop the disease later in life, but the reason for this is unclear. The infective agent is present in the membranes of the embryo but it has been demonstrated neither in the colostrum and milk of the mother nor in the tissues of the newborn animal. Many cases of scrapie appeared following the accidental contamination of a louping-ill vaccine; however, the mode of infection in most cases of scrapie seen on farms is unknown.
Transmissible spongiform encephalopathies of humans
TSEs of humans are divided into specific clinical types, which may appear similar histopathologically but are either spread differently or have different patterns of distribution and prevalence.
CJD was first described in 1920/21 when it was known as 'spastic pseudosclerosis' or 'subacute spongiform encephalopathy'. The illness exists throughout the world and is claimed to have a similar prevalence in each ofthe countries tested with an annual incidence of approximately 1 case per million of the population. This is almost certainly an underestimate because histopathologists dislike carrying out necroscopies on cases that may have died of CJD and many older people dying of a dementing illness do not have necroscopies performed. There is an increased incidence among Libyan jews (26 cases per million) and spacial or temperal clusters in areas of Slovakia, Hungary, England, USA and Chile. Cases are clustered in urban areas (except in Slovakia) but this can be accounted for by the increased population density. The average age in typical CJD is 56yrs, and only 7 cases between 18 and 29 years have been reported. Between 4 and 15 % of cases have a familial connection with other cases. There is a slight excess of CJD in women.
Clinical prodromal symptoms start with changes in sleeping and eating patterns and progress over a few weeks to a clearly neurological syndrome. A rapid onset of neurological symptoms appears in 20% of cases, most commonly vibrating muscular spasms, dementia, loss of higher brain function and behavioural abnormalities. The disease progresses with deterioration in cerebral and cerebellar function to a condition which most neurological activity is decreased, sensory and visual function decays, and the patient dies, possibly after a decrease in lowere motor neurological function and seizures. 90% of the cases end in death within 1 year of onset, and the further 5% die within the following year. However, for 5% of the cases fatality may take up to 10 years, and in these cases neurological decay is relatively slow.
Diagnosis is by clinical assessment of patients with pre-senile dementia and by examination of electroencephalogram patterns, which characteristically show triphasic one cycle per second activity or slow wave bursts with intermittent suppression (also found in animals with TSE). Enlargement of the lateral ventricals and an increase in a IgG may be found but these factors are of limited diagnostic value. Post mortem diagnosis is currently carried out by histological examination under the light microscope of cerebral tissue, although this is not always reliably diagnostic. Research techniques have been used to demonstrate CJD (and other TSEs). These may involve the electron microscope examination of brain tissue for scrapie associated fibrils (SAF), the staining of the tissue for prion protein antigens, or the intracerebral injection of tissue into animals, which will go on to die of the disease.
In some patients, the source of CJD has been claimed to be an infection transferred from other patients with the condition. For example, in one case, cerebral electrodes that had been sterilised with only alcahol and formalin vapour after use in a patient with CJD, were both used in the brains of two young epileptic patients, both of whom contracted CJD after a short incubation. The transfer of CJD by corneal transplant in one patient, by cadaveric dura mater graphs in two patients and by concentrated human growth hormone injections in over 40 more have been reported. Some cases in the literature seem too improbably for the low incidence in a community. For instance the report of CJD in a neurosurgeon, a mortury attendant, in two men living 200 metres apart in sharing a general practitioner, in a patient who had visited the Eastern Highlands of New Guinea (the kuru area) ten months previously, in 3 patients who had been operated on in the same neurosurgical unit within a period of 8 months, in 2 people living together but not genetically linked and in an individual marrying into an infected family (although the spouse did not suffer from CJD). Two husband and wife couples have died of CJD as well as a life long vegetarian. Four farmers that had cases of BSE on their farms have died in the UK suffering from CJD since 1993, as have 2 teenagers. The mode of disease transmission may be by personal contact, but only medical procedures have been described as to how this takes place. If the disease is transmitted from animal sources, many routes have been suggested but insubstantial evidence has been available to prove them. The distribution of CJD in the world does not seem to be the same as that of scrapie in sheep, and human exposure to sheep is poorly associated with CJD. None of the animals that suffer from CJD except cows appear to be present in large numbers in all the countries where CJD is prevalent. Although their tissues are unliekly to be infective, pigs, which are generally slaughtered, are not consumed by Muslims and Jews, who also suffer from CJD.
Kuru is a condition of the Fore tribe of the Okapa district of the Eastern Highland in Papua New Guinea, in which a practice of ritual cannibalism of fellow tribesmen took place until around 1956. The disease affected mainly adult woment and children of both sexes to give an annual disease specific mortality of approximately 3%. Most deaths of women occured through this disease and some men who died from this disease were thought to have contracted it when young.
Kuru is caught by eating infected tissue. The brain of the dead tribal member was eaten by women and children and the muscle tissue by men. The possibility that has also transmitted the disease to men but with a lower dose of infective agent and, hence, a longer incubation period has not been ruled out. The cohort of children born since 1956 has not suffered greatly from kuru.
Clinically the disease is of a progressive cerebellar ataxia leading to uncoodinated movements, neurological weakness, palsies, and decay in brain cortical function. Most patients dying of kuru are not demented, and this is a major clinical difference between kuru and CJD. Patients with a longer incubation period appear to have a slower progression of symptions, but generally death from inter current infection or medullary involvement takes place with an average clinical period of 12 months.
Gerstmann-Straussler-Scheinker disease (GSS) is an autosomally dominant condition rarely present in families. The disease is similar to CJD except that it has a more extended onset and duration, a tendency towards cerebellar ataxia as the initial predominant neurological sign, and a large number of amyloid plaques present among the spongiform encephaloapthic changes of the brain. It has been transmitted to monkeys and rodents by intracerebral inoculation and to hamsters merely by the insertion of the human abnormal PrP gene from chromosome 20 into the hamster genome.
Alpers disease represents a group of very rare chronic progressive degenerative disorders of the central nervous system of infants and children. Histologically this condition is similar to CJD and can be transmitted, like CJD, to hamsters easily but not to guinea pigs by intracerebral inoculation. Unlike CJD, however, there is also a fatty degeneration of the liver.
Transmissible mink encephalopathy
TME is an uncommon fatal disease that occurs as outbreaks in ranch mink (mustella vision). The condition was first reported in 1947 in Wisconsin and has also been reported in Canada and Finland with a similar pattern. Most of the mink on the farm die rapidly after a short encephalopathic period. The incubation in experimental situations is considered to be approximately 6 months. Because mink are generally separate from each of the other on farms (except when less than 3 months old) and because little contact is made between them and external animals, the disease is thought to be derived from their food, which is contaminated with a TSE of an other animal. Fighting and canibalism among young mink is difficult to prevent and this may be reason why most animals on the ranch become infected. One outbreak in Stetsonville, Wisconsin, USA, followed the feeding of the mink with the meat and bone meal of a cow that had died of a disease similar clinically simialr to BSE. No sheep were included in their diet. The experimental feeding of food that contained scrapie or BSE infective agent to mink has given rise to the disease but not of apparently the same clinical type.
Chronic wasting disease of deer
CWDD is a TSE seen in 1978 in a mule deer herd and in an adjacent herd of elk at Ford Collins, Colorado, USA. Both herds were captive. The disease shows typical spongiform change in the cerebral grey matter and can be transmitted to deer and ferrets by inoculation.
Bovine spongiform encephalopathy
BSE, a condition seen generally in adult cattle of either sex was first recognised in 1986 in the UK, where it now infects greater than 55% of milking herds. The numbers are highest in Southern England where more than 60 cases have been reported in a single herd but are generally spread throughout the British Isles, often as less than 3 cases per herd of 100 cattle per annum. It has been reported now in Oman, Switzerland, France, Germany, Canada, Denmark, Portugal, and Italy but these cases are probably associated with the export of either infected animals or infected meat and bone meal for bovine feed from the UK. It is difficult to explain the cases in Portugal in that many are the offspring of cattle exported from the UK, whereas their mothers are apparently not clinically infected. The disease is thought to have been derived either from the change in the manufacture procedure of meat and bone meal (for bovine consumption) or from the inclusion of an uncomon bovine case of spontaneous BSE in bovine food in approximately 1978-1980. Claims have been made that this is not a new disease; in the past, although not histologically diagnosed, it has been seen in approximately 1 cow in 20,000 to 30,000. The rapid increase of the disease (850 cases reported per week in 1994) is probably due to the inclusion of undiagnosed cases of BSE in the meat and bone meal used for bovine food. This was stopped in the UK in July 1988, but the meal was simply exported to other countries by its manufacturers (this has now been stopped). BSE has now been transmitted to cattle, mice, sheep, and goats both orally and by inoculation, and to pigs, marmoset monkeys but not hamsters merely by inoculation. Over 18,000 cases have been developed BSE although they were born after the ban of oral infectious material being present in their food. It is still unclear whether the cattle become infected directly from the food that they eat or from asymptomatic mothers that have done this.The possibility that an environmental factor other than the BSE infective agent may be involved with the transmission of BSE has been suggested due to the relatively low incidence of disease on 'organic' farms and organosphorus insecticide use has been suggested as being involved.
The long incubation period (presumed to be more than 2 years and most commonly 5 years) means that case numbers have appeared to decrease accoreding to MAFF statistics since 1994. There is argument about the validity of this data. Clinically, the cow appears alert but agitated, anxious, and apprehensive. As the disease progresses, however, the animal starts to take a wide base stance, the abdomen is drawn up and the gait becomes abnormal and exaggerated and it gives rise to tumbling and skin wounds. Fine muscle contractions are seen involving small muscle groups over the surface of the neck and body with occaisional larger muscular jerks. The animal loses weight and is taken to frenzied movements including aimless headbutting.
The possibility that BSE may be infectious to humans was considered to minimal in the UK until November 1989, when the feeding of bovine tissue, lymphoid tissue, spleen, thymus or gut (from cattle over the age of 6 months) to humans was banned. All animals that show signs of BSE in the UK must now be slaughtered and disposed of by incineration or burial. Beef in the UK would be expected to carry a lower titre of the infectious agent at the present time, but the larger amounts eaten by humans and the long human lifespan make its safety unclear.
Feline spongiform encephaloapthy
FSE is a condition that was reported in May 1990 in a 5 year old male siamese cat and has then been reported in many other in the UK. The epidemiology of FSE is unclear at the present but attempts to find previous cases among demented or neurologically degenerate cats from the past have been uncussessful. We must therefore consider it to be a new disease. Histologically, it is similar to other TSEs. It is now felt to be due to BSE being present in feline food. The owner of the original cat with FSE denied feeding it tinned cat food and insisted that it was fed fresh meat.
Zoological spongiform encephalopathy
Zoological animal TSE has been reported since 1986 in an eland, a nyala, an Arabian oryx, a greater kudu, a gemsbok, a cheetah, a puma, and an ocelot in British zoos. Various offspring of the mother kudu have died of the disease and the possibility that it is passed vertically must be considered. They had not been given food thought to be infected. The animals became clinically unwell after the appearance of BSE on British farms, and they were probably infected either from the same source as the cattle or from BSE contaminated foodstuff. No TSE in similar animals has been reported before and, hence, these must be considered new diseases. Four ostriches in the zoos of northern Germany have developed simialr conditions but no evidence is currently available as to whether the diseases are transmissible.
Transmissible of spongiform encephalopathies
Infected nervous tissue from some animals was injected, often intracerebrally, into others to find the range of infectivity of the agent. From this it was found that only approximately 70% of animals developed disease.
Dose experiments were carried out to understand the nature of the infective agent. Scrapie infected brain was exposed to various agents (e.g. irradiation) and then injected in multiply diluted forms into the brain of an uninfected animal. In this way, the agent could be filtered. Multiple ten-fold dilutions were made of the infective material, and each of the dilutions was inoculated in a similar amount into an animal of the same species. The infective dose of the greatest dilution that caused the animal to die (often after years of incubation), was defined as infective unit (IU) if the animal receiving the inoculum was the same species as that of the animal that was donating it. The brain of an animal dying of TSE commonly contained between one million and ten thousand million IU per gram. The oral infective dose of scrapie for a mouse was 4 x 10,000 IU, which represents between one hundredth and one hundred thousandth of a gram of infected brain tissue. In these experiments, researchers noticed that the incubation period was inversely related to the dose given to the animal and that the animal may be infected with a small dose of TSE but die of old age before clinical signs appear.
Effect of host passage on the properties of the infectious agent
1. The infectous dose between species is usually higher than between animals of the same species (possibly a million fold), but it is some times the same (e.g. between scrapie doses for mink perhaps)
2. When a species has been infected with a TSE of a different species it can then go on to infect a range of animals that the original species could not, and with a different dose.
3. When a species has been infected, it can infect additional animals of the same species with much lower doses of agent.
4. The histopathology of the disease in an animal infected from another species is not the same as if it had been infected from one of the same species.
5. The incubation period of an animal infected from another species is much longer than that of an animal from one of the same species.
To demonstrate these factors, brain tissue of a sheep with scrapie would only need one 1IU to infect another sheep, but if mice were injected a much larger dose would be needed, the incubation period would be relatively long, and a low percentage of the mice would be affected. If brain tissue from these infected mice was inoculated into addicitonal mice, the dose would be one IU (a very small amount) the incubation period would be much lower, a high percentage of the mice would become infected, and the histology would be the same as in further passages of the disease in mice (but different from the histopathology of the mouse infected from the sheep). The mice would also be able to infect a distinct range of animals other than sheep.
These factors are known as the species barrier (SB) and behave as if the agent is altered by passage through a species to a form that is more likely to infect that species. The insertion of the hamster PrP gene (vide infra) in to the genes of a mouse removes the SB between the animals; i.e. when injected with scrapie from a hamster, such a mouse would develop scrapie as if it is a hamster. This has been explained by the possibility that the PrP protein is all or part of the infective agent and, as it is produced from the genes of the host animal, it has a different structure in different species.
Tissue infectivity of clinical cases of TSEs
TSE infectivity is present in most tissues tested (e.g. liver, kidney, muscle, brain, thymus, spleen, etc), and the distributions vary among species. The finding of infectivity in the buffy coat of blood has led to fears that CJD may be transferred by blood transfusions, but there has been no report of this at time. The finding that the scrapie agent was present in perpheral as well as central nervous tissue and in lymphoid tissue has given little surprise to the finding of TSE agent in muscles of goats, hamsters, mink and possibly humans.
Tissue infectivity during the incubation period
The animal is as asymptomatic for a long period before the disease becomes clinically apparent. During this time, many of the tissues of the body are infectious but at a relatively low titre compared to the nervous system during the symptomatic period. This titre is adequately high, however, to permit infection of other animals by intracerebral inoculation and possibly by parenteral or oral routes.
The mode of spread of TSEs inside the body
Research has shown that the agent will pass along peripheral nerves and hence will travel in this way form a site of absorbtion to the brain. Other resarch has shown it to be present in the buffy coat (probably the macrophages or lymphocytes) of the blood. The exact mode of spread of TSE inside the body is unclear.
Developed immunity against the infective agent has not been demonstrated. Apparently no antibodies that react with it are produced, even in chronically infected animals. The possibility that this may permit multiple inoculations of sublethal doses of the agent to be effectively additive in their effect has been considered and is presumed by some researchers but no specific proof of this has been shown. Rabbits may produced antibodies against PrP derived from sheep (vide infra).
Resistance of the agent to destruction
Chemical disinfectants (e.g. domestic bleach), weak acids, DNAase, RNAase, proteinases (including those found in the animal gut), ultraviolet light, ionising radiation, heat (cooking tempertures), and chemicals that react with DNA (psoralins/UV light, hydroxylamine, zinc ions), all have little effect on the infectivity of the agent. High temperature autoclaving (135 degrees centegrade for 18 minutes) decreases the infectivity dramatically, as does the use of 1M NaOH, but neither will fully destroy the agent, as it has been found to remain infective after 360 degrees C for 1 hour or even after incineration. Internment of infective tissue in the soil for three years did not destroy the agent. Some phenols and proteases will decrease the infectivity of the agent but not to an adequate degree to be of value in disinfection.
Prevention of TSEs
Nosocomial CJDs should be prevented by prohibiting CJD, GSS, or Alpers disease patients (or those with obscure neurological conditions) from becoming blood or tissue donors, by the incineration or high temperature autoclaving of all materials that came into contact with blood, or post mortem tissue from such a patient, and by the disposal of all surgical instruments used for brain surgery on such a patient. The body should not be used for teaching anatomy or surgery. Correct action to be taken concerning BSE infected herds is currently under intense discussion.
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