Health risks in wastewater irrigation: comparing estimates from quantitative microbial risk analyses and epidemiological studies.

The combination of standard quantitative microbial risk analysis (QMRA) techniques and 10,000-trial Monte Carlo risk simulations was used to estimate the human health risks associated with the use of wastewater for unrestricted and restricted crop irrigation. A risk of rotavirus infection of 10(-2) per person per year (pppy) was used as the reference level of acceptable risk. Using the model scenario of involuntary soil ingestion for restricted irrigation, the risk of rotavirus infection is approximately 10(-2) pppy when the wastewater contains < or =10(6) Escherichia coli per 100ml and when local agricultural practices are highly mechanised. For labour-intensive agriculture the risk of rotavirus infection is approximately 10(-2) pppy when the wastewater contains < or = 10(5) E. coli per 100ml; however, the wastewater quality should be < or = 10(4) E. coli per 100ml when children under 15 are exposed. With the model scenario of lettuce consumption for unrestricted irrigation, the use of wastewaters containing < or =10(4) E. coli per 100ml results in a rotavirus infection risk of approximately 10(-2) pppy; however, again based on epidemiological evidence from Mexico, the current WHO guideline level of < or =1,000 E. coli per 100ml should be retained for root crops eaten raw.

Guidelines for the safe use of wastewater in agriculture: revisiting WHO guidelines.

The use of wastewater in agriculture is occurring more frequently because of water scarcity and population growth. Often the poorest households rely on this resource for their livelihood and food security needs. However, there are negative health implications of this practice that need to be addressed. WHO developed Guidelines for the Safe Use of Wastewater in Agriculture in 1989. The Guidelines are currently being revised based on new data from epidemiological studies, quantitative microbial risk assessments and other relevant information. WHO guidelines must be practical and offer feasible risk management solutions that will minimize health threats and allow for the beneficial use of scarce resources. To achieve the greatest impact on health, guidelines should be implemented with other health measures such as: health education, hygiene promotion, provision of adequate drinking water and sanitation, and other health care measures.

The risk of enteric infections associated with wastewater reuse: the effect of season and degree of storage of wastewater.

The effect of season and wastewater storage on the risk of Ascaris lumbricoides infection and diarrhoeal disease associated with wastewater reuse was studied in Mexico in 1991. Data were collected from 10,489 individuals during a dry-season survey. Exposure was to untreated wastewater, or effluent from 1 reservoir (< or = 1 nematode egg/L), or no wastewater irrigation (control group). The results were compared with a previous rainy-season survey which included effluent from 2 reservoirs in series. Direct exposure to untreated wastewater was associated with an excess risk of A. lumbricoides infection in children aged < 5 years (OR = 18.0) and persons aged > 5 years (OR = 13.5) and an increased risk of diarrhoea, particularly to children aged < 5 years (OR = 1.75); effects were stronger in the dry than in the rainy season. There was also an excess risk associated with the 1-reservoir group for A. lumbricoides infection (OR = 21.2 and 9.4) and for diarrhoeal disease (OR = 1.1 and 1.5) but little excess associated with the 2-reservoirs group. Therefore, wastewater retention in 1 reservoir (quality 10(5) faecal coliforms/100 mL, < or = 1 egg/L) does not significantly reduce risks of Ascaris infection and diarrhoeal disease whereas retention in 2 reservoirs in series (quality 10(3) faecal coliforms/100 mL, no detectable eggs/L) does.

Guidelines for the microbiological quality of treated wastewater used in agriculture: recommendations for revising WHO guidelines.

Three different approaches for establishing guidelines for the microbiological quality of treated wastewater that is reused for agriculture are reviewed. These approaches have different objectives as their outcomes: the absence of faecal indicator organisms in the wastewater, the absence of a measurable excess of cases of enteric disease in the exposed population and a model-generated estimated risk below a defined acceptable risk. If the second approach (using empirical epidemiological studies supplemented by microbiological studies of the transmission of pathogens) is used in conjunction with the third approach (using a model-based quantitative risk assessment for selected pathogens) a powerful tool is produced that aids the development of regulations. This combined approach is more cost-effective than the first approach and adequately protects public health. The guideline limit for faecal coliform bacteria in unrestricted irrigation (< or = 1000 faecal coliform bacteria/ 100 ml) is valid, but for restricted irrigation < or = 10(5) faecal coliform bacteria/100 ml is recommended when adult farmworkers are exposed to spray irrigation. A limit of < or = 10(3) faecal coliform bacteria/100 ml is recommended if flood irrigation is used or children are exposed. The guideline limit for nematode eggs for both types of irrigation is adequate except when conditions favour the survival of nematode eggs and where children are exposed; in these cases it should be reduced from < or = 1 egg/l to < or = 0.1 egg/l.

Risk factors for Giardia intestinalis infection in agricultural villages practicing wastewater irrigation in Mexico.

This study assessed the risk factors for Giardia intestinalis infection in an agricultural population in Mexico. Exposure groups included 2,257 individuals from households exposed to untreated wastewater, 2,147 from a group using the effluent from a series of reservoirs, and 2,344 from rain-fed agricultural villages. Stool samples were collected from 6,748 individuals. Wastewater samples were tested for fecal coliforms/100 ml and Giardia sp. cysts/L. Untreated wastewater samples contained 10(8) fecal coliforms/100 ml and up to 300 Giardia sp. cysts/L. Hydraulic retention (3-7 months) in the reservoirs, however, provided an improved effluent quality (10(1)-10(4) fecal coloforms/100 ml and < or = 5 Giardia sp. cysts/L). Children 1-14 years of age had the highest prevalence of infection (20%). Data showed marginal associations between storing drinking water in unprotected containers and lack of facilities for feces disposal and the risk of infection (odds ratios [ORs] = 1.76 and 1.19, 95% confidence intervals [CIs] = 0.95-3.23, and 0.97-1.45, respectively). Individuals purchasing vegetables at the city market had higher rates of infection than those buying at the village shop (OR = 2.49, 95% CI = 1.00-6.17). No excess risk was found in individuals exposed to untreated wastewater compared with controls (OR = 1.07, 95% CI = 0.84-1.36); the group using reservoir water was not different from the controls (OR = 1.22, 95% CI = 0.94-1.58). No risk from agricultural activities was detected (OR = 0.83). This pattern of infection may be addressed by primary health care and wastewater treatment.

Health risk in agricultural villages practicing wastewater irrigation in central Mexico: perspectives for protection.

9,435 individuals participated in a cross-sectional survey in the irrigation districts of the Mezquital Valley (central Mexico). Exposure groups were: 848 households irrigating with untreated wastewater, 544 households irrigating with the effluent from a series of interconnected reservoirs, and 928 households farming with natural rainfall. The unit of analysis was the individual, and the health outcomes included diarrhoeal diseases and Ascaris lumbricoides infection. Water quality was assessed using faecal coliforms (FC) and nematode eggs, as suggested by (WHO, 1989) for the safe use of wastewater in agriculture. Children from households irrigating with untreated wastewater (10(8) FC/100 mL and 135 nematode eggs/L) had a 33% higher risk of diarrhoeal diseases and a fivefold increase in risk of A. lumbricoides infection (OR = 5.71) compared to children from the control group, farming with rainfall. The risk of A. lumbricoides infection in older individuals was even higher (OR = 13.18). The final analysis showed that drinking unboiled water and cultivating vegetables crops were both associated with a higher risk of diarrheal diseases (OR = 1.45 and 2.00); individuals infected with A. lumbricoides infection came mostly from landless households with poorer dwellings and low standards of sanitation (OR = 2.20, 2.23, 1.72 and 1.43, respectively). These results are discussed in the context of health protection measures and policy recommendations.

The public and domestic domains in the transmission of disease.

This paper discusses the distinction between the transmission of infectious diseases within the domestic domain (the area normally occupied by and under the control of a household) and that in the public domain, which includes public places of work, schooling, commerce and recreation as well as the streets and fields. Whereas transmission in the public domain can allow a single case to cause a large epidemic, transmission in the domestic domain is less dramatic and often ignored, although it may account for a substantial number of cases. Statistical methods are available to estimate the relative importance of the two. To control transmission in the public domain, intervention by public authorities is likely to be required. Two examples show how environmental interventions for disease control tend to address transmission in one or the other domain; interventions are needed in both domains in order to interrupt transmission.

Environmental health indicators and sanitation-related disease in developing countries: limitations to the use of routine data sources.

This article explores conceptual issues in the development and use of environmental health indicators for basic problems related to water and sanitation in developing countries. In this context, faecal contamination is the most important environmental health problem, responsible for the death of approximately 3 million children a year, and the infection of hundreds of millions. Good indicators would be invaluable in assessing the magnitude and source of such problems in different settings.

[Epidemiologic setting of the agricultural use of sewage: Valle del Mezquital, Mexico]. / Escenario epidemiológico del uso agrícola del agua residual: el Valle del Mezquital, México.

Wastewater from Mexico City is used to irrigate over 85,000 hectares mainly of fodder and cereal crops in the Mezquital Valley. A cross-sectional study method is being used to test the impact of exposure to raw wastewater and wastewater from storage reservoirs on diarrheal disease and parasitic infections in farmworkers and their families. The study population in the rainy season survey included 1,900 households: 680 households where the farmworker is exposed to untreated wastewater (exposed group), 520 households exposed to reservoir water (semiexposed group), and 700 households where the farmworker practices rain-fed agriculture (control group). Preliminary analysis of the data from the rainy season study (dry season study in progress) has been carried out. Current information indicates that the risk of Ascaris lumbricoides infection is much higher in the exposed group than in the control group (95% CL = 2.9-10.8). According to the procedures employed Entamoeba histolytica infection was more frequent among subjects aged 5 to 14 years from households exposed to raw wastewater, than among subjects of the same ages belonging to the control group (95% CL = 1.07-1.72). When diarrheal disease rates were analyzed, children under 5 years from exposed households had a significantly higher prevalence than controls (95% CL = 1.03-1.64). The final results of this study are expected to aid decisions within the National Wastewater Reuse Programme in Mexico.(ABSTRACT TRUNCATED AT 250 WORDS)

[The health problems associated with irrigation with wastewater in Mexico]. / Problemas de salud asociados al riego agrícola con agua residual en México.

Over 85,000 hectares in the Mezquital Valley of central Mexico are irrigated with wastewater. The main crops are fodder and cereal crops. A two cross-sectional survey was carried out to test the impact of exposure to raw wastewater and from storage of that wastewater in reservoirs. The main outcomes were diarrhoeal disease and parasitic infections in farmworkers and their families. The total study population in the dry season included 2,049 households. The study population was classified in households where the farmworker is exposed to untreated wastewater (exposed group), households exposed to reservoir water (semi-exposed group), and households where the farmworker practices rain-fed agriculture (control group). Preliminary analysis of the data has been carried out. Current information indicates that the risk of Ascaris lumbricoides infection is much higher in the exposed group than in the control group (95% CL = 4.0-67.3 and 4.7-78.8). According to the procedures employed, exposed children from exposed households were at higher risks of diarrhoeal disease than controls (95% CL = 1.03-2.03). The final results of this study are expected to aid decisions within the reuse programmes in developing countries.

Human IgE, IgG4 and resistance to reinfection with Schistosoma haematobium.

A well recognized feature of the immune response to parasitic helminth infections, including schistosomiasis, is the production of large amounts of specific and nonspecific IgE1,2. Immunological pathways involving IgE can lead to damage to the developing schistosomulum and it has been suggested that responses involving IgE could have evolved as protection against helminth infections. There has been no epidemiological evidence to support this idea and the only significant IgE responses known in man are those involved in the pathogenesis of allergic disease. Here we measure serological response during reinfection with S. haematobium and demonstrate that IgE antibodies in man can be beneficial. Our results support the hypothesis that the slow build-up of IgE to high levels and the early production of IgG4 antibodies, which may block IgE pathways are responsible for delaying the development of protective immunity to S. haematobium.

Recent epidemiological studies to test microbiological quality guidelines for wastewater use in agriculture and aquaculture.

BACKGROUND: New guidelines for wastewater reuse (WHO 1989) are controversial. Epidemiological studies are needed to test their validity. Cross-sectional studies of the impact of excreta use in aquaculture in Indonesia, and of wastewater use in irrigation in Mexico have been carried out. METHODS: Each study involved an exposed group using wastewater/excreta with no treatment, a control group with no wastes use, and an intermediate group, where wastewater/excreta was used but some health protection measure existed. In Mexico, the intermediate group used wastewater from a storage reservoir which met the new WHO guideline for restricted irrigation. In Indonesia, the intermediate group did not have domestic exposure to pondwater, whose quality was around forty times higher than the tentative WHO bacterial guideline for fishpond water. RESULTS: In Indonesia, the prevalence of diarrhoeal disease was low in adults, and unrelated to exposure, but high in children under 5 years. Multiple logistic regression analysis gave odds ratios of 1.4 (p = .06) for consumer exposure, 1.9 (p = .01) for recreational or occupational exposure and 1.6 (p = .01) for domestic exposure, when allowance was made for all other exposures and several confounding factors. In Mexico, preliminary analysis of the wet season data suggests that the increased risks of Ascaris infection and diarrhoeal disease from the use of raw wastewater are removed when water of WHO guideline quality from storage reservoirs is used. CONCLUSION: WHO (1989) guidelines can be tested using cross-sectional epidemiological studies which indicate that guidelines for restricted irrigation and for aquaculture may be around the right level.

Health impact evaluation of wastewater use in Mexico.

Wastewater from Mexico city is used to irrigate over 85 000 hectares, mainly of fodder and cereal crops in the Mezquital Valley. A cross-sectional study method is being used to test the impact of exposure to raw wastewater and wastewater from storage reservoirs on diarrhoeal disease and parasitic infections in farmworkers and their families. The study population in the rainy season survey included 1 900 households: 680 households where the farmworker is exposed to untreated wastewater (exposed group), 520 households exposed to reservoir water (semi exposed group), and 700 households where the farmworker practices rain-fed agriculture (control group). Preliminary analysis of the data from the rainy season study (dry season study in progress) has been carried out. Current information indicates that the risk of Ascaris lumbricoides infection is much higher in the exposed group than in the semi-exposed and the control groups. According to the procedures employed Entamoeba histolytica infection was more frequent in individuals aged 5 to 14 years exposed to wastewater, than in those in the control group. When diarrhoeal disease rates were analyzed, children under age 15 years from exposed households had a significantly higher prevalence than controls. The final results of this study are expected to aid decisions on low cost treatment options and other health protection measures within the National Wastewater Reuse Programme in Mexico. The implications for assessment of the WHO guidelines for the use of wastewater in agriculture are discussed.

Resistance to reinfection with Schistosoma haematobium in Gambian children: analysis of their immune responses.

The relationship between reinfection with Schistosoma haematobium and immunological parameters was studied in a group of Gambian children aged from 8 to 13 years. Each individual's exposure to infection was assessed from observations of water contact, cercarial densities and infected snail densities at water contact sites. Eosinophil counts were made and responses to egg antigen (SEA) and adult worm antigen (WWH) measured by ELISA. Low levels of reinfection were associated with a high eosinophil count, high levels of antibodies against WWH and SEA, increased age and low exposure. In a multiple regression analysis of the association of reinfection with eosinophil count, antibody levels, exposure, age and sex, the effects of eosinophil count and exposure were still very significant after allowing for all the other variables. The effects of the antibody levels were close to significance after allowance for exposure and eosinophil count (for WWH: P = 0.09; for SEA: P = 0.07), although the evidence was less clear after additional allowance was made for age and sex. The ability of sera from the children to recognize different parasite antigens was also examined by immunoprecipitation of labelled schistosomulum surface, WWH, SEA and S. haematobium adult worm mRNA in vitro translation products. Schistosomulum surface antigens were recognized by all the sera and there was little variation in this response. There was more variation in their responses to SEA and WWH and a marked heterogeneity in the response to in vitro translation products. However, the pattern of antigen recognition appeared unrelated to susceptibility to reinfection.

Resistance to reinfection after treatment of urinary schistosomiasis.

The process of reinfection after treatment was studied in a cohort of subjects in a focus of intense Schistosoma haematobium infection. Detailed observations were made at water contact sites of cercarial densities and of water contact by members of the cohort. Individual values of a cumulative index of exposure to infection were calculated using these observations and assumptions which were made about the effect of different water contact activities on the entry of cercariae into the skin. Among groups of subjects with an apparently similar intensity of exposure to infection, reinfection tended to be much heavier in children under 10 years of age than in 10 to 14-year-olds, while only light infections were found in the few adults who became reinfected. This trend for reinfection to decrease with increasing age, after an allowance for variation in exposure, was highly significant (p less than 0.001). These observations suggest that subjects in this area slowly acquire an increasing degree of immunity to the acquisition of S. haematobium infection which is effective in the absence of a mature egg laying infection.

Eosinophilia and resistance to Schistosoma haematobium in man.

We have measured the levels of infection with Schistosoma haematobium in children resident in an endemic area of The Gambia before and 3 months after successful chemotherapy and following reinfection. An exposure index was calculated from data collected on water contact, cercarial densities and infected snail densities at water contact sites. Peripheral blood eosinophil levels were recorded and the ability of serum (heat inactivated) from the children to allow killing of schistosomula of S. haematobium was examined. Of 50 children with a post-treatment egg count of less than 1 ovum/10 ml urine, 26 were classified as reinfected, acquiring greater than 1 ovum/10 ml urine over the transmission season. Twenty-four were classified as not reinfected, acquiring less than 1 ovum/10 ml of urine over the same period. These two groups did not differ with respect to their estimated age, weight or pretreatment egg counts. Children who were reinfected had significantly higher levels of exposure and significantly lower peripheral blood eosinophil counts than children who were not reinfected. At all levels of exposure children with high eosinophil counts were less likely to be reinfected than those with lower counts. But antibody-dependent, complement-independent killing of schistosomula of S. haematobium by eosinophils was barely detectable and did not differ between reinfected and non reinfected groups. These observations suggest that subjects with elevated counts are less susceptible to reinfection but the mechanisms involved are not apparent.