Giardia duodenalis Contamination of Fresh Vegetables in a Wholesale Market

Abstract

Thai menus usually contain raw vegetables as the main or side dish. Currently, many outbreaks of intestinal parasitic infections that are related to eating raw vegetables have been revealed worldwide. Fresh vegetables can be contaminated by protozoan cysts, helminth eggs and larvae. The detection of helminth eggs and larvae by microscopy is easier than the detection of protozoan cysts which depends on expertise and techniques. The protozoan contamination in vegetables, which has been reported in the U.S. and other western countries, and Thailand, etc., is mostly from food- and water-borne protozoa. However, little is known about the prevalence or distribution of these waterborne protozoa due to difficulty related to detection methods. Giardia duodenalis is one of the waterborne protozoa that causes diarrhea in humans and many animals. Most infected adults are asymptomatic. They are less concerned about getting diagnosed, which results in a low estimated prevalence. However, infection has an adverse impact on children’s linear growth and psychomotor development. The conventional method of G. duodenalis detection is microscopy which has low sensitivity and specificity.

To estimate the possible occurrence of G. duodenalis in lettuce, white cabbage and cabbage from one large wholesale market, a sensitive molecular tool was used to detect protozoan DNA. These three vegetables are mostly eaten raw in many Thai food dishes. A total of 96 unwashed vegetables, of which 35 lettuces, 30 nappa cabbages and 31 cabbages were collected from September 2019 to June 2020. Samples were prepared as follows: 1) 200g of vegetable were chopped and washed with 0.85% saline, 2) sediment was concentrated by leaving it overnight, 3) DNA was extracted from the sediment. Nested polymerase chain reaction (Nested PCR) was used to detect G. duodenalis contamination targeting the glutamate dehydrogenase gene (gdh).

The results of this study showed 6 positives for G. duodenalis out of 96 (6.25%) samples. The protozoa were found in nappa cabbage (10.0%), cabbage (6.45%) and lettuce (2.86%). Unwashed vegetables from the largest wholesale vegetable market were contaminated with G. duodenalis. The Giardia cysts may be in the inner layers of these vegetable leaves due to the physical characteristics of the vegetables.  

 Although a small number of G. duodenalis contaminated vegetables were found, consistent and prolonged consumption of these vegetables may lead to Giardiasis. At present, many intestinal parasitic infections occur without knowing the sources of infection. One contaminated food is raw vegetables. Therefore, it is important to wash vegetables several times with running water or a washing solution, to reduce protozoal contamination from each leaf layer. The washing step is important to remove any pathogens from vegetables, before consuming them in their raw form.

The results of this study can be used as baseline data for G. duodenalis contamination in fresh vegetables, especially vegetables that are eaten raw. The benefit of a molecular detection method is that it can detect the minimum number of cysts, as one cyst can be detected by PCR. The development of easier, sensitive and faster detection methods could be valuable in the field of food sanitation, particularly pathogen surveillance systems for fresh vegetables and fruits.