Bovine spongiform encephalopathy (BSE) is a progressive, fatal, neurologic food-borne viral disease of cattle. It was first diagnosed in Britain in 1986 and has since been diagnosed in the Middle East, Japan, USA, and Canada.
Typical and atypical forms of this disease have been reported. The atypical (amyloidotic) form of bovine spongiform encephalopathy (BSE) termed 'BASE' is caused by a prion strain whose biological properties differ from those of typical BSE, resulting in a clinically and pathologically distinct phenotype.
'Typical' BSE is believed to be caused by a prion found in mammalian-derived meat and bone meal fed to cattle. The abnormal protein induces a structural change in normal host proteins that confers protease resistance. Thereafter, altered proteins, which are now infectious, accumulate in host tissues. BSE develops as a result of foodborne exposure to infectious proteins via contaminated animal-source proteins in cattle rations. Horizontal transmission is not a significant source of new BSE infections. Calves born to infected cows are at greater risk for acquiring BSE than calves born to noninfected cows; however, this mode of transmission is of minor importance relative to infections acquired through contaminated feed sources. There is no sex or breed predisposition.
Normal infection is via the oral route. However, the impacts of anaerobic fermentation processes in cattle on the stability of BSE-associated prion protein are still unresolved since these prions survive gastrointestinal digestion processes in cattle and might be excreted via faeces.
The incubation period following exposure is between 2 – 8 years, though cattle under 2 years have been diagnosed. The details of pathogenesis are unknown. Recent studies have contradicted this, suggesting that haematogenous spread is unlikely to be the route of CNS infection. Recent reports suggest that in some instances, spontaneous cases of BSE may occur in cattle not exposed to the infectious agent. Mutations in the cattle genome may alter sequences of the normal host prion protein.
The majority of cases are diagnosed in cattle 3 – 6 yr old, with signs developing 3 - 10 months post-infection.
Common signs include hyperesthesia, nervousness, difficulty negotiating obstacles, reluctance to be milked, aggression toward either farm personnel or other animals, low head carriage, hypermetria, ataxia, and tremors. Weight loss and decreased milk production are common.
Diagnosis requires histopathological determination of the presence of abnormal prion protein. Histologic changes are confined to the CNS and include bilateral, symmetric vacuolation of gray matter neuropil (spongiosis) and neurons and the accumulation of protein fibrils characteristic of prion-associated diseases. Contemporary diagnosis is more reliant upon ELISA and Western immunoblot methods for more accurate surveillance of cattle populations.
Differential diagnoses include nervous ketosis, hypomagnesemia, polioencephalomalacia, lead poisoning, rabies, and ingestion of plant or fungal tremoragens. In contrast to these other differential diagnoses BSE typically has a slow onset of clinical signs, with an extended and progressive clinical course. Veterinarians considering BSE as a likely differential diagnosis should contact regulatory personnel and ensure that definitive post-mortem diagnostic tests are performed.
There is no effective treatment for BSE. Euthanasia is advisable as soon as there is some certainty of the clinical diagnosis because animals become unmanageable and their welfare is at risk.
The most effective control measure is prohibition of the feeding of mammalian-derived protein sources to cattle. Most mammalian-source protein supplements for cattle have been banned in the USA since 1997. Control has been effected in Britain and other European countries by similar statutory prohibitions of the use of mammalian-derived protein in all farm animal diets.
The occasional isolated diagnoses of BSE in the USA and Canada raise questions that deserve serious consideration. Perhaps the ban on feeding animal-source proteins to cattle is less stringent or effective than desired. In some instances, cattle may have ingested animal-source protein prior to the ban in 1997. Finally, some background rate of spongiform encephalopathy resulting from spontaneous host mutations is likely in most mammalian species. Recent reports have suggested that spontaneous BSE is likely. Ongoing rigorous application of feed bans should prevent transmission to other cattle.
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