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. Creutzfeldt-Jakob Disease 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 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 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 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 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. Immunity 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. Histopathogical changes Characteristic lesions under the light microscope consist of spongy changes
in the neuropil nerve cells and astrocytes with nerve cell degeneration and
astrocytosis. These changes generally take place in the grey matter of the
cerebrum and cerebellum. The distribution of the histopathology may vary
according to the strain of the disease, as has been shown with inbred strains
of mice. It may also vary with the site of inoculation, for example, if the infection
reaches the brain through the optic nerve, then the spongy degeneration is
clustered around the occipital lobe. Amyloid plaques may be seen between cells
in some TSEs (e.g. kuru, hamster scrapie) and strained with Congo red. These
contain PrP and have been shown to be infective. Electron microscocopy shows
twig-like structures 12 to 16 nanometers in width and 100 to 500 nanometers
long, which are found only in TSE and are now called scrapie associated fibrils
(SAF). Nature of the infective agent for TSEs PrP: A cellular protein found altered in cases of TSEs PrP is a sialoglycoprotein produced from a gene normally found in the genome
of the infected animal. This gene on chromosome 2 in mice (or chromosome 20 in
humans), may be conserved between strains but altered between species. PrP in a
normal brain has glycoside modifications and is held onto the membrain of the
cell by covalent attatchement to phospholipid. In scrapie, however, the PrP is
present much more within the membrane and has a different structural form
(PrPsc). This modification, whatever it may be, renders the protein resistant
to heat and to most proteases and when treated with proteinase K (a very
powerful enzyme) it is split into particles fo 27-30 kDa the structures of
which are uncertain. PrPsc structure has been shown to vary with the strain of
TME and for the altered form of PrP to induce such a change in normal PrP (this
is, however, very inefficient, and the infectivity of the PrPsc that is formed
has not been shown). The PrP of patients with GSS is mutated so that proline is
replaced by leucine at position 102. The PrP of CJD in the Libyan Jew and
Slovak clusters and some cases of familial CJD have a different alteration from
that in GSS and various changes have been reported. Patients with CJD that is
not familial do not generally have PrP mutations. When the gene for PrP in a
patient who suffers from GSS (PrPgss) is inserted into a hamster genome, about
one in twenty animals spontaneously develop a TSE. PrP is therefore greatly
involved in the infectivity and pathogenesis of the disease, but how it could
be the infective agent (vide infra) is not clear. Inbred mouse strains with
long and short incubation periods (known as sinc genes) for scrapie have been
found to have differences in their PrP codons, which are claimed by some to be
the sinc genes themselves. Mice have been developed that do not carry a PrP
gene at all. These mice appear to grow in an asymptomatic way and cannot be
infected with scrapie (nor do they become infected and remain asymptomatic). Structure of the transmissible spongiform infective agent Conjecture is currently complex and hence, there are many hypotheses: Virus: No specific particle (although many have been claimed to have been
seen under the elecron microscrope) has been isolated but the transmissibility
is similar to this form of agent. Prion: A protein infectious agent.Felt to be the PrP altered form (PrPsc),
which is resistant to destruction, would not cause antibody production in the
infected animal, and would vary according to the species infected. Purified
PrPsc has been claimed to transmit scrapie but experimental methods have been
challenged as being fully vaid. How a prion would transmit strains of disease
without an independent genome is currently unclear. The recent transmission of
GSS to hamsters by an artificial PrP GSS gene has made this hypothesis more
likely. Virino: Small fragment of DNA/RNA closely wrapped in protein. The SAF that
are found only in TSEs and that have PrP involved in their structure could be
virinos. They are sugested to contain some genetic material (although none has
been found), which could explain the different strains of disease. However, the
agent is resistant to chemicals and ionising radiation, which would be expected
to destroy the nucleic acid. Tubulofilamentous particles: these are the items seen under the electron
microscope in all cases of TSE. Their association with infection has not been
demonstrated. Crystalloids: These would be the growing crystals of prion protein that
would cause the PrP to change into the PrPsc form with the release of energy.
This would explain the strain types as being the form of crystalloid depending
on other factors apart from the PrP protein structure (possibly depending on
the glycan chains to which are attatched). Spiroplasmas. These are infective agents that can be cultured in vitro and
are well known in microbiology. Althogh no spiroplasma has been found
specifically, the appearance of the structure of the EM particles suggest it. Future prospects The incidence of CJD is apparently unchanged over any years, although poor
statistics to confirm this. The epidemic of BSE in the UK is expected to
continue there for up to 10 years but the incidence to decrease after 1993. It
is, however likely to spread to Europe and other countries that imported
infected British meat and bone meal. The precise risk that this may be followed
after 5-25 yrs by a wave of CJD, FSE and Zoological SE, is unknown but the
possibility should be considered.