The consumption of raw vegetables plays an important role in the transmission of parasites to humans. The recovery of parasites from vegetables helped us better understand the potential source of pathogenic parasite acquisition in this study area. This study demonstrated that the prevalence of parasitic contamination in vegetable samples from Nakhon Si Thammarat province was 35.1%, which was comparable to the prevalence of 32.6% and 34.7% from previous reports in Shahrekord, Iran  and Poland , respectively. However, higher contamination rates, ranging from 45% to 58%, were detected in Libya, Iran, Egypt, Brazil, and the Philippines [8, 13, 18,19,20,21,22]. In contrast, the rate of contamination was lower in some studies, such as 5.9% in Ankara, Turkey , 14.89% in Mazandaran province, northern Iran , 19.4% in Alexandria, Egypt , and 26% in Hanoi, Vietnam . According to the influence of season on the prevalence of parasites, previous studies reported that the rate of STH contamination was higher in warm seasons versus cold seasons [11, 19, 26]. This finding suggested that the transmission and prevalence rate of parasites were associated with climate and temperature. Differences between this study and others might be attributed to variations in climatic conditions, types of soil, types of water used for agriculture, and poor hygienic practices during the transportation and marketing of vegetables.
Celery was found to be the most frequently contaminated sample (63.3%) followed by peppermint (60.0%), gotu kola (57.1%), coriander (44.8%), leek (43.3%), culantro (36.7%), Chinese cabbage (23.3%), lettuce (20%), and Thai basil (20%), whereas Chinese morning glory (6.7%) was the least contaminated. These results were different from previous studies, which reported that lettuce possessed the highest parasitic contamination [9, 10, 21]. In Thailand, celery is commonly used for seasoning and garnishing in many Thai dishes. The highest contamination in celery might be because celery consists of a number of stalks that are connected at the base with its leaves near the top of stalks. The structure of its stalk is roughly U-shaped in a slit pattern, which allows the parasites to attach more easily to the surface of these vegetables and makes them more difficult to remove. Normally, celery samples obtained from the three markets were frequently sold with roots and stalks, which might increase the chance of soil contamination from roots to stalks. In contrast, the lowest parasitic contamination was observed in Chinese morning glory, which could be because the smooth surface of its stalks that may reduce the probability of parasitic attachment.
In this present study, hookworm was detected in 16.60% (44/265) of vegetables examined and was the most predominant pathogenic parasite. This finding is in agreement with other studies conducted in Ghana (13%) , Sudan (5.7%) , and northern Iran (4.40%) . In contrast, no ova of hookworm species were recovered in some previous studies [8, 10, 16, 18,19,20, 22,23,24,25, 27]. Our previous report in 2017 demonstrated that hookworm infection was the most prevalent parasitic infection in Nopphitam district, Nakhon Si Thammarat province, southern Thailand . The high prevalence of hookworm egg contamination of vegetables in this study could be attributed to poor sanitation and the use of human waste-contaminated water for irrigation in the region. Lack of proper footwear and exposure of skin to contaminated soil might be responsible for hookworm infection in the study area. In addition, the high prevalence of hookworm might be due to differences in geographical location, climate conditions, and the types of soil [28, 29]. The second most prevalent contamination found in this study was the larvae of Strongyloides stercoralis. Our finding was consistent with previous studies in Accra, Ghana and in Jimma Town, Ethiopia, which reported a 43% and 21.9% prevalence of Strongyloides stercoralis contamination, respectively [4, 22]. A high rate of Strongyloides contamination might be because Strongyloides spp. has a complex life cycle with a free-living stage in the environment that does not require a host for its proliferation .
Aside from the contamination of hookworm eggs, T. trichiura and Ascaris lumbricoides eggs were also detected in the vegetable samples collected from this area. Trichuriasis occurs by ingestion of contaminated food and water with embryonated eggs of T. trichiura . In the present study, eggs of T. trichiura were detected in 2.64% (7/265) of vegetable samples. This finding was consistent with previous reports from Khartoum state, Sudan , Mazandaran province, northern Iran , Accra, Ghana  and in villages of Qazvin province, Iran , where the contamination rates were 2.9, 2.2, 2 and 0.9%, respectively. In addition, eggs of Ascaris lumbricoides were detected in 2.64% (7/265) of vegetable samples. The rate of contamination with A. lumbricoides eggs was 68% in Tripoli, Libya , 20.83% in Arba Minch town, southern Ethiopia  and 8.17% in Shahrekord, Iran . Contamination by these STHs in vegetable samples might occur at any point along the chain; during planting, harvesting, transportation, or the marketing of vegetables. The differences in the degree of contamination might be attributed to the types of soil, the quality of water used for planting and irrigation, and hygienic practices during the process of marketing.
Beside STH contamination, this study also detected Toxocara spp. eggs in 2.64% (7/265) of fresh vegetable samples. In other locales, the rate of contamination with Toxocara spp. eggs in vegetables was 37% in Libya , 15.83% in Arba Minch town, southern Ethiopia , 3.3% in Shahrekord, Iran , 3% in Hanoi, Vietnam , and 1.5% in Ankara, Turkey . In contrast, no ova of Toxocara spp. in vegetables were reported in Ardabil, Khorramabad, and Qasvin, Iran, Accra, Ghana, Benha and Alexandria, Egypt, Minas Gerais, Brazil, or Khartoum state, Sudan [4, 9, 10, 19,20,21, 24, 27]. Human toxocariasis is a helminthic zoonosis caused by larval stages of Toxocara canis and less frequently by Toxocara cati . The long term survival of Toxocara spp. outside their hosts coupled with their high fecundity is responsible for significant contamination of soil with infective eggs . This study indicates that domestic animals (dogs and cats), which are the source of Toxocara eggs, may at some point shed contaminated feces onto cultivation areas.
The parasitic contamination rates were significantly different for samples collected from different markets; samples collected from Muang district showed the highest rate of contamination. The differences between the three markets might be due to the different sources of vegetables as well as the hygienic practices in handling and washing by different sellers.
The results of this study emphasized that raw vegetables from the markets in the study area could be possible vehicles of parasitic transmission to humans. The previous studies revealed that the standard washing procedure was an effective method to prevent the contaminations of helminths in raw vegetables [16, 23, 33]. Furthermore, to emphasize on the proper washing procedure, the previous study in Iran demonstrated that the pre-washing procedure using tap water or underground water could not completely get rid of parasites from vegetables . Hence, the health authorities should provide the knowledge of proper washing method for local people in order to prevent parasitic transmission.
However, it is important to note that our study has several limitations. This study did not demonstrate the effect of seasonal variation on parasitic contamination. We did not address the intensity of vegetable washing before display for sale or the source of water used by each seller. These factors might affect the rate of vegetable contamination in our study.