Cryptosporidium's burden of disease attributable to consumption of wastewater-irrigated raw vegetables

Conventional wastewater treatment plants do not effectively remove parasitic protozoa resulting in their presence in wastewater effluent. Therefore, agricultural reuse of wastewater effluent could be a route of Cryptosporidium transmission into the food chain. Samples of secondary wastewater, wastewater-irrigated soil and vegetable samples were collected from an experimental field and analyzed using real-time PCR for quantification of Cryptosporidium oocysts. Quantitative microbial risk analysis (QMRA) was performed to determine the annual disease burden from Cryptosporidium associated with consumption of wastewater-irrigated vegetables. Detection of Clostridium perfringens spores as a potential indicator for the presence of Cryptosporidium was also performed. Cryptosporidium was observed in 64% of the effluent samples ranging from 16 to 162 oocysts L⁻¹, whereas no oocysts were detected in wastewater-irrigated soil and the vegetable samples. The mean annual disease burden for consumption of lettuce (6.6 × 10⁻⁵ DALY per person per year (pppy)) was higher than spring onion (3.4 × 10⁻⁶ DALY pppy) which both exceeding the WHO guideline of 10⁻⁶ DALYs pppy. However, the disease burden was within the acceptable range considering the less stringent level of 10⁻⁴ DALY pppy. Clostridium spores were detected in 92% of the effluent samples with no relationship with the presence of Cryptosporidium. The results of this study suggest that agricultural reuse of treated wastewater may not be a major health concern of cryptosporidiosis in a semi-arid region. However, because of the higher risk from lettuce, the choice of an appropriate crop would be needed to completely meet the WHO recommendations for safe reuse of wastewater. The accuracy of the QMRA model could be improved by further investigating the decay rate of oocysts in the same region.