Contamination of lettuces with nematode eggs by spray irrigation with treated and untreated wastewater

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 seeded with Ascaridia galli eggs. Contamination with eggs was ascertained weekly and recovered eggs tested for viability. No contamination was detected on crops receiving less than 1 egg per litre. Lettuces were contaminated at harvest if irrigated with water containing 14-500 eggs per litre. Egg density, expressed in terms of fresh lettuce weight, did not increase with time, suggesting that irrigation water washed eggs from plants in addition to depositing them. Eggs recovered from plants were underdeveloped, but some remained viable. Removal by clean water irrigation was demonstrated. WHO helminthological guidelines for restricted irrigation was an order of magnitude too strict. There was little risk to consumers from irrigating industrial, cereal or fodder crops, although care should be taken by agricultural workers.