Scrapie

Also known as: TSE – Transmissible spongiform encephalopathy, Paraplexia enzootica ovium

Introduction

Scrapie is a progressive, fatal and non-febrile neurological disorder affecting sheep and goats. It belongs to a group of diseases called transmissible spongiform encephalopathy (TSE) and other TSE’s include Creutzfeldt-Jakob disease in humans, BSE, chronic wasting disease (CWD) in elk and deer, transmissible mink encephalopathy and feline spongiform encephalopathy has been found within cats in the UK. The disease is believed to be caused by a conformational change in the prion (PrP). A prion is a protein that occurs normally in the nervous and lymphoreticular tissues. It is only when the prion changes conformation into a protease-resistant protein PrP^sc that it causes degeneration of neurological tissue. The disease causes astrocyte proliferation and then vacuolization of neurons but demyelination does not occur . The abnormal protein is thought to act as a catalyst to convert more of the host’s protein into this abnormal form. The disease has been notifiable in the EU since 1993 but unlike BSE there is no evidence to suggest that scrapie is a risk to human health , ,, . Studies have suggested that after ingestion, PrP^sc first accumulates in Peyer’s patches of the small intestine, gut-associated lymphoid tissues (GALT) and ganglia of the enteric nervous system ,, , , , it then spreads throughout the lymph nodes, tonsils, spleen, and into the peripheral nervous tissue. It finally found in the brain several months later. The disease is thought to have come from imported Merino sheep from Spain and has since spread through the movement of infected sheep. Only Australia and New Zealand are recognized as being currently free of scrapie.

Signalment

Scrapie affects the majority of sheep between 3 and 5 years of age and has a long incubation period of two to five years. It is extremely durable and is able to withstand high temepratures and concentrations of formaldehyde. Unlike BSE, scrapie is influenced by breed and genetic variation of the PrP gene within sheep populations, which can affect the infectivity and incubation period of the scrapie. The disease has been shown to be effectively transmitted during lambing , , and experimental studies have shown that the ingestion of infected placenta can spread the disease in sheep and goats .

Clinical Signs

Picture from the rear shows bare patches from rubbing (Sourced from Wiki Commons)

Early clinical signs may include subtle behavioural and neuorological changes. Sheep often have a fixed gaze, and suffer from bruxism, fine tremors, and hyperaesthesia to sound or sudden movements. Affected animals may later become exercise intolerant and develop progressive ataxia. Sheep often find difficulty in turning, sway on their hind hindquarters and have gait abnormalities such as a high stepping gait in the forelimbs or a bunny hopping gait in the hindlimbs. Some sheep have intense pruritis that leads to compulsive rubbing, nibbling at the skin, or scraping against fixed objects. Wool loss is typically seen over the hindquarters and lateral thorax. Lip smacking or nibbling reflex can often be elicited by scratching over the lumbar region, which is characteristic og scrapie. Significant weight loss with or without a decrease in appetite, weakness, recumbency, and death are all seen within the later stages of the disease.

Diagnosis

File:Example.800px-Scrapie lymph node immunoglobulin labeling.png

Immunoglobulin in normal(a) and scrapie-affected (b)follicles -with light microscopy.

A pre-emptive diagnosis of scrapie may be made from the above clinical signs and history. There are no serological test available for scrapie, as is does not evoke an immune or inflammatory response. Diagnosis is confirmed on postmortem and PrP^Sc can be isolated from brainstem or lymphoid tissues by Western immunoblot, immunohistochemistry (IHC) and elisa tests. Immunohistochemistry usually shows vacuolation and an accumulation of prion proteins in various parts of the CNS (medulla, pons, midbrain, and spinal cord). However vacuolation is not completely diagnostic since it may also be present to a lesser extent in the brains of healthy sheep , . In most instances the abnormal prion is resistant to protein kinase digestion, a feature used in diagnostic techniques. IHC staining of tonsil and lymphoid biopsies have been used for preclinical scrapie testing.

Differential diagnosis: Viral encephalomyelitides (pseudorabies or Aujeszky’s disease, rabies, maedi visna), Bacterial meningoencephalomyelitides (listeriosis), Pregnancy toxemia (ketosis), Hypocalcemis-hypomagnesemia, Toxins (mercury, lead, organophosphates, plant toxins) and Mange, lice, bacterial dermatitis .

Treatment

Scrapie is a fatal condition and no effective treatment is currently available

Control

Good husbandry and hygiene around lambing can greatly reduce the infectious load. It is recommended that individual straw bale pens are used which can be destroyed after each lambing and that contaminated bedding and placenta should be removed immediately. Infection can be minimised by maintaining a closed flock and only obtaining replacement ewes or breeding rams from scrapie-free flocks. Animals of resistant genotypes should be used for breeding to further minimize the risk of scrapie infection in a flock , , , . Genetic resistance to scrapie depends on the prion genotype of the sheep and on the strain of scrapie present. Genotypes of sheep resistant to one strain of scrapie may be susceptible to another strain but on the whole the ARR allele confers resistance in all breeds. In 2001 the UK government set up the National Scrapie Plan NSP) which aims to increase the frequency of the ARR allele within UK sheep population. Since 1988 it has been illegal for ruminant derived meat and bone meal to be fed to ruminants.