Scrapie

• It is highly fatal, non-febrile, chronic neurologic disease of sheep and goat
• It is characterized by pruritis, abnormal gait, emaciation, paralysis and death.
• Disease has got long incubation period ranging from several months upto 4 years
• Disease is always fatal causing death in 1-6 months from time of 1^st appearance of signs.

Etiology

• Prion or a virion
• These are small, proteinous infectious particle lacking nucleic acid and donot produce any inflammatory and immune reaction in host.
• The agent retains its infectivity in inanimate object for 3 years.
• It can survive decontamination process that are effective against conventional viruses.
• Boiling, rapid freezing, thawing or exposure to ether or 20% formalin is notable to destroy it.

Epidemiology:

• Most breeds of sheep are highly susceptible to disease.
• Occasionally, goats are also affected.
• It occurs in sheep in UK, Europe and North America enzootically.
• Sheep imported from enzootic areas may also show the disease occurrence.
• Outbreaks have also been recorded from Japan, Australia, New Zealand, Middle East, Scandinavia and India.
• Case fatality rate is 100%.

Transmission:

• Transmission occur through oral route by contagion of affected animal.
• Congenital transmission; from dam to offspring may be possible.
• AI and embryo transfer
• Through abrasive lesion in body or contact with infected animals.
• Through grazing in infected pasture

Pathogenesis:

• Ingested prions may be absorbed across gut wall at Payer’s patches which is part of MALT.
• Lymphoid cell phagocytose particles and travels to other lymphatic tissues, i.e. spleen, tonsils, etc.
• Prion then replicates in lympho-reticular system; spleen and L.N before invading NS
• After reaching CNS, they replicate and degenerate targeted neuron and produce nervous signs.

Clinical Signs:

• Disease has got very long incubation period; several months-4 years
• Infected sheep and goat show itching, rubbing themselves to trees, feeders.
• Aimless movement of animal, incoordination, paralysis
• Tremor in head and neck, inability to control leg movement, lip smacking, weight loss
• Biting of feet and legs due to itching
• Frequently stamp their head down.
• Affected animals urinates very frequently at scanty volume of urine. Sheep drinks small quantity of water.
• Other nervous signs: Nystagmus, Intermediate nodding of head, loss of vision

PM Findings:

• Hole in brain tissue, cytoplasm and nucleus of neuron on examination of brain tissue
• Enlargement of neuron, hypertrophy and hyperplasia of astrocytes.

Diagnosis:

• On the basis of clinical findings
• On the basis of PM findings
• Immunoblotting test
• Electron microscopy is used to detect scrapie associated fibril (SAP) which is protease resistant.

Differential Diagnosis:

1. Louping ill:

• It is tick borne viral disease
• Sudden onset of high fever for 2-3 days

2. Rabies:

• Dumb and paralytic form is seen
• Tendency to bite anything that passes nearby to him
• Howling sound

3. Pregnancy toxemia:

• Symptoms appear 1-3 weeks before parturition, muscle twitching
• Opisthotonus
• Grinding of teeth

Treatment and Control:

• There is no treatment for the disease.
• To control this disease, following steps should be undertaken:
  • Eliminating source of infection.
  • Infected materials along with aborted material should be properly disposed off.
  • Depopulation of infected farms and areas are to be made adopting slaughter eradication method.
  • In enzootic area, all clinically affected animals should be burnt as soon as possible.
  • Special care should be taken to avoid contact between susceptible animals and the placenta of a possibly infected one.