Bovine Spongiform Encephalopathy (BSE)

 

Etiology
Mad cow disease is the popular term for Bovine Spongiform Encephalopathy (BSE) , one of a group of diseases in animals and humans called transmissible spongiform encephalopathies (TSE), or prion diseases. BSE is a fatal degenerative disease affecting the central nervous system of adult cattle. Pathologic changes are confined to the central nervous system and consist of sponge-like microscopic changes.

The average incubation period is about five years, but it may range from two to eight years. Clinical signs include changes in temperament such as nervousness or aggression; abnormal posture; lack of coordination, decreased milk production; loss of body condition despite continued appetite and eventually, difficulty in rising. Following the onset of clinical signs, the animal's condition deteriorates - usually for two weeks to six months - until it dies or is killed.

Epidemiology of BSE
Epidemiological evidence indicates that the primary cause of BSE in British cattle was probably the use of commercial cattle feed concentrates which contained meat and bone meal derived from sheep (and possibly cattle) presumed to have been infected with scrapie. Current data are consistent with 90% of cases occurring from infected feed. The feeding of animal protein specifically derived from ruminants ceased in the Britain in July 1988.
BSE was first observed in Great Britain in April, 1985, and was specifically diagnosed in 1986. By June, 1990, there were some 14,000 confirmed cases out of an estimated population of 10 million cattle in Great Britain. Since 1986, almost 200,000 cases of BSE in cattle have been identified in Britain. The epidemic peaked in 1992-93 at almost 1,000 cases per week. Currently, less than 100 cases are occurring per week. In the United Kingdom, 54% of the dairy herds and 15% of the beef herds have had at least one case of BSE.
When BSE was first observed in 1985, 200-300 infections had probably occurred already. Based on British epidemiology, age at infection is 1.3 ± 1.8 years and the mean incubation period (time between infection and clinical symptoms) is 5 years. The United Kingdom Spongiform Encephalopathy Advisory Committee (July 20, 1996) stated that cows can transmit BSE to their calves, but the rate of transmission is so low that it is not expected to perpetuate the disease. In a preliminary study, the rate of maternal transmission was approximately 10%, however the committee estimated that under UK field conditions the maternal transmission rate would be approximately 1%. A statistically significant correlation existed between BSE infected dams and BSE occurrence in calves, however, no causal relationship was established. Incidence is much higher in dairy herds, probably because cows are fed more protein and remain on feed a longer period of time. There is no evidence to date of genetic susceptibility (breed differences).
Experienced cattle practitioners claim to have seen BSE cases prior to the epidemic; these were (and in many countries, still are) often categorized as unresponsive "downer cows" or cows with "staggers". This led to the theory that BSE existed in the British cattle herd at some low level prior to the epidemic. The alterations in rendering practices (primarily loss of solvent extraction) may have led to recycling (and concentrating) of BSE infective agent through the cattle population via ruminant to ruminant feeding of offal until the titre was sufficient to result in an outbreak of sufficient magnitude that it was readily identified.

Clinical signs

• abnormally stilted gait
• high stepping
• heightened sensory perception
• itching
• anorexia
• excessive licking
• death.

Differential Diagnosis

• Hypomagnesaemia.
• Nervous ketosis.
• Encephalic listeriosis and other encephalitides
• Polioencephalomalacia or cerebro-cortical necrosis.
• Intra-cranial tumours.

Identification and isolation of the agent

• There is no available diagnostic test for the BSE agent.
• Bioassay of brain tissue of terminally affected cattle or other species by parenteral inoculation of mice is the only method currently available for detection of infectivity. This is impractical because of minimum incubation periods approaching 300 days.

Serological tests
The absence of detectable immune responses in BSE or other transmissible spongiform encephalopathies precludes serological tests.
Other tests

• Histopathological examination of the brain from clinically affected cases for characteristic bilaterally symmetrical spongiform changes of grey matter and subsequent immunohistochemical demonstration of accumulations of disease specific PrP.
• Examination for fibrils, homologous with scrapie-associated fibrils (SAF) by electron microscopy or electrophoretic separation and immunoblotting for detection of the disease specific isoform of PrP in extracts of unfixed, fresh or frozen brain.

Control / vaccines 
Animals generally recover quickly without treatment, however the disease is responsive to anti-inflammatory drugs. These drugs must be given for the expected course of the clinical disease. During fever, the paresis or paralysis responds to injected calcium borogluconate in the same manner as parturient paresis (milk fever).
An effective vaccine is available, and can be used to protect animals in the face of an advancing outbreak, or as a routine in endemic areas.