B115 - BOVINE SPONGIFORM ENCEPHALOPATHY

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B115 - BOVINE SPONGIFORM ENCEPHALOPATHY

Nature of the disease
Bovine spongiform encephalopathy (BSE) - Mad Cow disease - is a transmissible, progressive, fatal central nervous system disease of cattle caused by an unconventional transmissible agent closely similar to that causing scrapie of sheep and goats. It was first described in Britain in 1986.

The causative agent belongs to an incompletely characterized group called prion to denote an infectious protein, because a partially protease resistant isoform of a normal host protein, PrP, is the only detectable macromolecule associated with infectivity. These agents are responsible of Creutzfeldt-Jakob disease in man. The Nobel price Stanley Prusiner demonstrated the link between the BSE prion and the new variant form of Creuzfeldt-Jacob disease (vCJD) that occurs in BSE infected countries.

Classification
OIE List B disease
Susceptible species
The natural hosts are cattle.

Some cases have also occurred in antelopes and feline (cats, tiger, puma, ocelot). A range of other species including mice, pigs, and sheep has been infected experimentally.

Humans are thought to be susceptible (vCJD).

Distribution
BSE was first recognised in the UK which has the highest yearly incidence of the disease, from its discovery to 2002 more than 180,000 cases were reported.

The disease is considered to have spread from Great Britain via export of contaminated Meat-and-Bone-Meal (MBM) in Europe (19 countries reported at least one case of non imported cattle) plus Israel and Japan.

Sporadic cases in imported cattle have been reported in Canada, Falklands and Oman.

Clinical signs 
BSE has a long incubation period of at least 4 to 5 years.

Onset is insidious but progresses to inevitable death in weeks or months.

There are variable central nervous system signs:

  • Behavioural changes — apprehension, nervousness and frenzy when confronted with obstacles, depression
  • Abnormalities in posture and gait with ataxia and hypermetria
  • Adventitial movements: muscle fasciculations, tremor and myoclonus
  • Autonomic dysfunction: reduced rumination, bradycardia and altered heart rhythm
  • Hyperaesthesia or hyperreflexia

There is increasing debility, loss of weight, recumbency, coma and death.

 

Post-mortem findings 

There are no characteristic gross lesions. A characteristic spongiform encephalopathy (bilateral and symetral vacuolisation of gray matter neuropil) is present most of the time.

 

Differential diagnosis
  • Metabolic diseases e.g. nervous ketosis and hypomagnesaemia
  • Rabies
  • Listeriosis
  • Brain tumours
  • Polioencephalomalacia or cerebro-cortical necrosis.
  • Lead poisoning
Specimens required for diagnosis
If BSE is suspected, the animal should be euthanised and the brain removed — half of it should be sent fresh, the other half fixed in formalin.

Fresh cervical spinal cord or caudal medulla (3 g), frozen as soon as possible after death can be submitted for scrapie-associated fibrils (SAF) and PrP detection.

There is a commercial rapid test, Prionics check which is used for systematic detection of PrPs.

Transmission
BSE is principally transmitted by infected meat and bone meal (MBM), however there is some evidence of possible vertical transmission but the biological mechanisms involved are unknown. There is no evidence of horizontal transmission.

The following tissues demonstrated infectivity: brain, eyes (retina), trigeminal ganglia, the spinal cord, the dorsal root ganglia and the distal ileum. But tissues that might be infected (such as tonsils and the entire intestine from duodenum to rectum) or that could be contaminated by specified risk materials (such as might occur during the slaughter process under certain circumstances) should also be considered at risk.

Risk of introduction
As there is no diagnostic test available for live animals, live animal imports and embryos represent a potential risk. Unless there is feeding of ruminant by-products back to cattle, the disease is unlikely to spread.

The highest risk of introduction is with contaminated feed, although Ruminants should never receive MBMs.

The risk of human exposure due to contaminated beef products should be assess with a proper risk analysis.

Control / vaccines
There are no treatment for BSE.

The main preventive measure is the interdiction of feeding MBMs to cattle.

Control of imports through proper risk analysis is essential for free countries.

In non free countries, control is done by screening and slaughtering of infected herds. Tracing and identification of cattle and control of treatment of animal proteins.

References
  • BSE, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 897-898
  • GEERING WA, FORMAN AJ, NUNN MJ, Exotic Diseases of Animals, Aust Gov Publishing Service, Canberra, 1995, 440p
  • Joint WHO/FAO/WHO Technical Consultation on BSE: public health, animal health and trade, Paris 2001
  • Office International des Epizooties, 2002
  • Risk analysis for BSE in Switzerland