Integrated Health Risk Management for urban and peri-urban wastewater Irrigation

Wastewater use in urban agriculture can serve as a common denominator for the transmission of diarrhoeal, helminth and malaria diseases if not undertaken in a safe manner. The aim of this project was to assess the contribution of informal urban wastewater irrigation to both pathogenic and mosquito vector related disease risks. The specific objectives were: 1. To assess if there is any relationship between irrigation water quality (including pathogens) and mosquito vector population density in wastewater irrigated fields; 2. To identify informal wastewater irrigation practices associated with increased risk of diarrhoeal, helminths and malaria disease risks; 3.To estimate the fraction of diarrhoeal, helminths and malaria risks attributable to urban wastewater irrigation. To address these objectives, a multidiciplinary study involving entomology, microbial and physico-chemical analysis, socio-demographic surveys, epidemiological surveys and quantitative microbial risk assessment was undertaken. All investigations were carried out in the Kumasi Metropolitan Area, Ghana. Following are some of the preliminary results: Anopheles (malaria mosquito larvae), Culex and Aedes were the main mosquito species across the study farm sites. The mosquito larvae were found in furrows, ponds and streams where farmers sourced their irrigation water. The total species of mosquitoes were generally high in the furrows except for Aedes, which was slightly more in ponds. In particular, Anopheles larvae were two times more likely to breed in furrows than in streams (IRR=2.2 (1.34-3.78). Anopheles larvae were also likely to breed in ponds compared to stream (IRR=1.19 (0.24-5.74)). All the mosquito species, particularly Anopheles was three times as likely to be found in the wet season than in the dry season (IRR=3.5 (1.44-8.70)). The breeding of Anopheles larvae was associated with a number of physico-chemical and microbial parameters of the irrigation water used by farmers. In this regard, dissolved oxygen, electrical conductivity, nitrite, phosphorus, and faecal coliforms in the irrigation water were all significantly associated (p<0.05) with the breeding of Anopheles larvae accross all the farm sites. Dissolved oxygen (DO) in irrigation water was negatively associated with the breeding of Anopheles larvae (IRR=0.851 (0.74-0.97). Nitrite also had a similar effect on Anopheles larvae (IRR=0.10 (0.01-0.93)). Also, the number of faecal coliforms was negatively associated with the level of Anopheles larvae in the irrigation water (IRR=0.98 (0.97-0.99)). In contrast, phosphorus in irrigation was positively associated with the level of Anopheles larvae (IRR=1.31 (0.98-1.75)). Similarly electrical conductivity increased the level of Anopheles larvae in the irrigation water (IRR=1.002 (1.001-1.004)). These findings suggest that wastewater irrigated farms contribute to the breeding of malaria mosquitoes and subsequently the transmission of malaria disease. In terms of microbial health risk (diarrhoea and helminth infection) the study revealed that the use of wastewater for vegetable irrigation in urban areas contributes significantly to the transmission of helminths and diarrhoeal diseases. The results showed high infection risk and disease burdens for Ascaris, Salmonella and E.coli O157:H7. In this regard, the annual Ascaris infection risk for farmers exposed to wastewater as well as consumers of wastewater irrigated lettuce or salad was within a magnitude of 10-1 per person per year. E.coli O157:H7 infection risk was particularly of major public concern. The study showed that consumers of wastewater irrigated lettuce salad at restaurants and street-food shops were are at a high risk of infection above the WHO tolerable infection risk of 10-4 per person per year. The E.coli O157:H7 infections among the consumers resulted in 367 diarrhoeal disease cases, 1.45 HUS cases, 0.14 ESRD cases and 3.1 x10-1 deaths per year. The annual infection risk for Salmonella associated with the consumption of wastewater irrigated lettuce salad was acceptable (i.e. 10-4 per person per year). The foregoing findings indicate that wastewater use in urban agriculture serves as a common denominator for the transmission of helminths, diarrhoeal and malaria diseases. An integrated health risk assessment tool (Helse-Risiko) with accompanying manual for rapid assessment of the risk associated with wastewater use in urban agriculture has been developed; and will be made available to national and international stakeholders. This is also being supplemented by the development of local guidelines for wastewater use in in the informal urban agricultural sector as part of the rolling revision of the WHO guidelines.

Worldwide, more than 20 million ha of agricultural fields in urban and peri-urban areas are irrigated with diluted, partly diluted and untreated wastewater (Future Harvest, 2001). The benefits of the practice are enormous. It sustains the livelihood of po or urban farmers, enhances urban food security, reduces pressure on potable water supply systems, provides an option for wastewater management and improves environmental quality by diverting wastewater to agricultural fields. However, the practice can als o be counter-productive on public health and lead to the erosion of all the aforementioned benefits if not undertaken in a safe manner. This is the case in most urban and peri-urban areas in developing countries where wastewater irrigation is largely info rmal and wastewater treatment facilities for safe irrigation are either inadequate or non-existent. In these countries, wastewater irrigation has been identified as a significant risk factor for diarrhoeal and helminth disease transmission. It is also inc reasingly associated with mosquito vector borne diseases.The challenge thus is how to optimize the benefits of wastewater irrigation without compromising public health. Confronting this challenge will require a full understanding of the contribution of ur ban wastewater irrigation to the disease burdens associated with both microbial pathogens and mosquito vector related diseases. The study will identify and assess the contribution of urban wastewater irrigation practices that increase the risks of these d iseases to enable the development of integrated interventions for their mitigation in a cost-effective way.