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Studies on Treatment of Fresh Vegetables Contaminated with Cysts of Endamoeba Histolytica. I. Acetic Acid

The cysticidal efficacy of acetic acid against cysts of the NRS strain of Endamoeba histolytica has been studied at temperatures between 25°C. and 50°C. with acid concentrations of 1 per cent, 2.5 per cent, and 5 per cent. The importance of the temperature factor has been demonstrated.

Evaluation of the acetic acid in tests against cysts in suspensions indicated the potential value as a cysticidal procedure of 15 minutes exposure to 5 per cent acid at 30°C. and of 5 minutes exposure to 2.5 per cent acid at 45°C. No cyst survival was evident under these conditions. One brand of vinegar tested gave similar results at the higher temperatures.

The importance of checking the survival of cysts actually remaining on vegetables after such treatment has been demonstrated. By a procedure of blending cyst-contaminated, treated lettuce in a Waring blender, estimates were made of numbers of cysts remaining on lettuce after treatment, and their viability was tested by inoculating into medium the filtrates and "mash" of the blended lettuce. Such experiments indicated that a few viable cysts remained on lettuce in 2 of 3 tests after 15 minutes exposure to 5 per cent acid at 30°C. and in 1 of 5 tests after 5 minutes exposure to 2.5 per cent acid at 45°C. In the latter case especially, the percentage of cysts surviving was apparently very low.

The treatment removed the greater part of the original cyst contamination from lettuce. The usefulness of the procedure is limited by the fact that some vegetables, especially leafy greens, deteriorate after a few hours storage and should be used as soon as possible after treatment. It is concluded that the use of acetic acid on frese vegetables would provide considerable, but not necessarily complete, protection against E. histolytica cysts present on the foods.

¹ Laboratory of Tropical Diseases.