The probability of cysticercus bovis detection in livestock from exposure to recycled water in non-endemic countries

Abstract

The probability of cysticercus bovis (CB) infection of cattle (cysticerci from Taenia saginata) in a country where T. saginata is not endemic (i.e. Australia) was assessed using a Quantitative Microbial Risk Assessment (QMRA) approach. Two important features of the QMRA were (i) a dose-response curve to describe ingestion of eggs of the helminth T. saginata (HE) by cattle and the development of cysticerci due to the infection, and (ii) characterisation of HE concentrations. Data limitations relating to HE quantification are described, and several other key variables provided the basis for a probabilistic QMRA model.Data from over 554 sewage samples from 11 wastewater treatment plants (WWTPs) in Southern Australia indicated the background concentration of T. saginata eggs was low (<0.1 HE L-1 measured, 0.003 HE L-1 as an estimated baseline modelled on a ratio of Taenia:Ascaris determined from the literature). Such a low sewage concentration was estimated to require only a 2.2 log10 reduction value (LRV) via sewage treatment to maintain the baseline risk of CB equivalent to background levels in Australia. However, to protect against potential future detectable outbreaks of Taeniasis in the human population and all potential exposure scenarios considered, a 3.5 LRV for WWTP was considered appropriate with confirmation by appropriate sewage monitoring. In addition, analysis of several specific exposure scenarios using the QMRA indicated that LRV credits (0.5 to 2.0 LRV) could decrease the required LRV for wastewater treatment based on the size of the WWTP and on-site management strategies (e.g. restriction of recycled water use for livestock drinking water, the years of exposure for cattle to sites irrigated with recycled water, and the use of fodder off-site). Without such measures, a HE LRV of 4.0 is recommend for WWTPs to ensure adequate protection of systems with no on-site controls.