Risk assessment of abattoir effluent should BSE be found in cattle in Australia
The purpose of this report is to assess the risk of abattoir effluent transmitting bovine spongiform encephalopathy (BSE) in the cattle population should the disease ever occur in domestic cattle in Australia. Since some abattoir effluent is used for irrigation of crops and pastures to be consumed by cattle there is a potential that infected material can be recirculated within the cattle population.
Irrigated paddocks close to abattoirs are used mainly as holding areas before slaughter. Effluent propagation of BSE is not an issue under such circumstances because of the long incubation period for the disease. It is believed to be close to five years in cattle at low doses. However, there could still be a problem with the use of irrigated pastures for occasional general grazing and this was explored.
A semi-quantitative approach was used to express the risk level as the remaining mass of infectious nervous tissue in treated effluent. An average abattoir processing 425 cattle a day was used in the model and a maximum of 100 BSE infected cattle in total in Australia. It was further assumed that both primary and secondary effluent treatments were poorly maintained. Current knowledge suggests an ID50 for cattle of about 0.1g of infected nervous tissue via the oral route, that is this dose will establish infection with disease in 50% of the challenged animals.
It was estimated that one BSE-infected animal processed at the selected abattoir could in a worst case scenario yield three grams of infectious tissue distributed in at least 11 ML of treated effluent. Usually effluent would be spread several times during the year over a large area, some of which would be used for grazing. In the worst case scenario it was assumed that a maximum of 0.8 ML/ha of water was used for irrigation, resulting in an attachment to grass surfaces of 2.2x10^-2 g of infectious material per hectare. As a rough rule of thumb a hectare can feed 70 animals over one day with one animal grazing an area of about 140 m^2 . This would in a worst case scenario yield a daily intake of 3.1x10^-4 g per animal or around 320 times less than the ID50 dose. A safety factor of 100 is often considered adequate in risk analysis.
A sensitivity analysis showed that the assumed ID50 dose would not be exceeded even under extreme circumstances, although at this highly unlikely exposure level some cases of BSE can be expected given the definition of ID50 and a potential cumulative effect of successive doses. However, conditions used in the high exposure scenario are considered far from realistic and abattoir effluent deemed an unlikely route for propagating BSE in the Australian cattle population even when allowing general grazing of effluent-irrigated areas.
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Risk assessment of abattoir should BSE be found in cattle in Australia 
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