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Water Science and Technology Vol 33 No 10-11 pp 277–283 © IWA Publishing 1996 Evaluation of the WHO nematode egg guidelines for restricted and unrestricted irrigationUrsula J. Blumenthal*, D. Duncan Mara**, Rachel M. Ayres***, Enrique Cifuentes****, Anne Peasey*, Rebecca Stott**, Donald L. Lee*** and Guillermo Ruiz-Palacios*****London School of Hygiene and Tropical Medicine London WC1E 7HT UK ABSTRACT Epidemiological studies were conducted in Mexico to assess the health risks associated with restricted crop irrigation using raw and treated wastewaters. Experimental studies were done in northeast Brazil and Leeds, in which Ascaris lumbricoides contamination of lettuces spray-irrigated with raw wastewater and waste stabilization pond effluents containing different helminthological qualities was determined (Brazil); and Ascaridia galli infection was monitored in immunosuppressed chickens fed lettuces spray-irrigated with water containing different A. galli egg numbers (Leeds). The results indicate that the WHO guideline quality of  1 egg per litre protects crop consumers, but not necessarily fieldworkers and their families especially children. This is particularly the case when wastewater treatment systems are not stable, when recontamination of the partially treated wastewater with small quantities of raw wastewater may occur, and when wild vegetables are harvested and consumed. In such cases a stricter guideline quality of 0.5 egg per litre may be required.
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