Contamination of Lettuces with Nematode Eggs by Spray Irrigation with Treated and Untreated Wastewater Available to Purchase

Field and glasshouse grown lettuces were spray irrigated with raw and treated wastewaters containing known numbers of human intestinal nematode eggs, and with clean water artificially seeded with Ascaridia galli eggs. Plants were examined for egg contamination at weekly intervals until harvest. Eggs recovered were tested for viability and the use of clean water irrigation to remove eggs was investigated. No contamination was detected on crops receiving less than 1 egg per litre. Eggs recovered from plants irrigated with raw sewage, effluent from an anaerobic waste stabilisation pond or seeded clean water were found to be undeveloped and therefore un-infective at harvest, although some were still viable. Clean water irrigation was found to remove eggs from the plants. A relaxation of the helminthological guideline for restricted wastewater reuse is discussed in the light of these findings.