INFORMATION ON THE SCRAPIE
(Tremblante du mouton, Rida)
What is Scrapie?
What Causes
Scrapie?
How does Scrapie spread?
Symptoms
Diagnosis
Treatment, Incidence, Control
Scrapie
is a degenerative neurologic disease of sheep; it is the prototype of the “slow virus”
infections that produce subacute spongiform encephalopathies in animals and
man. Each of transmissible mink
encephalopathy (TME), chronic wasting disease of captive mule deer and elk,
kuru, Creutzfeldt-Jakob disease, and Bovine
Spongiform Encephalopathy is defined by the clinicopathologic features in
their natural hosts; all are transmissible to other species in which they are
virtually indistinguishable.
What causes Scrapie?
The
causal neuropathogens have properties so unusual that they are called
“unconventional viruses”. Although
viral or subviral in size, the lack of identification of any agent-specific
nucleic acids or proteins has severely limited progress in their
characterization and lead to speculation that they represent new forms of
“infectious” entities capable of producing death and tissue
degeneration.
Scrapie
is naturally transmitted to susceptible sheep by contact with infected animals
or pastures. There is no clear
evidence that it is vertically transmitted in the genome or by embryo transfer,
despite its common familial occurrence.
During the first year after natural exposure, presumably orally, the
agent slowly replicates in lymphoreticular tissues before spreading to the
Central Nervous System, either by intermittent hematogenous seeding of the
blood-brain barrier or by centripetal passage along nerve pathways. Once in the Central Nervous System, the
Scrapie agent replicates.
Degeneration
of targeted neurons after 2 years of age results in onset of clinical
signs. Susceptibility to infection
by natural exposure is controlled genetically. In the USA, it is almost exclusively a
disease of purebred sheep, notably Suffolks.
Symptoms:
The onset is insidious, i.e., the effects spread gradually. Affected sheep become more excitable, and fine tremors of the head and neck may be observed. The most characteristic feature is intense pruritus (itchiness), which often begins over the rump. In some cases, the pruritus makes it difficult for the animal to feed and rest normally. Nervous signs may be elicited from a quiet but affected sheep by sudden noise or movement. The wool is dry, separable, and brittle, which results in loss of fleece over large areas. Other areas may be rubbed raw. Occasionally, convulsions occur. When made to trot, there is often a peculiar hypermetria (irregularity of pacing) of the forelegs, sometimes with galloping movements of the hindlegs. After 2-6 months of progressive neurologic (nerve) deterioration characterized by emaciation, weakness, and ataxia (loss of coordination), the affected sheep becomes totally debilitated and soon dies.
Lesions
are microscopic and confined to the Central Nervous System; they include
microvacuolation (sponginess) of the grey matter and neuronal degeneration and
astrocytic hypertrophy (increase cell growth). There are no inflammatory cell
infiltrates, and no agent-specific immune reactions have been
demonstrated.
Examinations
of infected brain material for virus-like particles have been unsuccessful, but
amyloid-like fibrils have been identified and found to be specific. These
structures, called Scrapie-associated fibrils (SAF), are composed of a single
sialoglycoprotein (prion protein), which is coded for by a host gene. Regulation
of this gene is thought to be important in determining the length of the
incubation period in experimentally infected mice, and may also be important in
sheep susceptibility.
Diagnosis:
This
is based on clinical signs, flock history and microscopical examination of the
Central Nervous System. Studies are
in progress to determine if isolation of SAF and detection of prion protein by
Western blot analysis can be used to supplement clinicopathologic
findings.
Treatment, Incidence, Control:
There
is no treatment.