Widespread Soil-Transmitted Parasitic Contamination in Raw Vegetables at Fresh Markets in the Inner Zone of Bangkok: Findings from a Cross-Sectional Study

ABSTRACT.

Bangkok, a globally popular travel destination, hosts numerous fresh markets and street food vendors catering to locals and visitors. Although raw vegetables are integral to Thai cuisine, their consumption may pose risks of soil-transmitted parasitic infections. This cross-sectional study assessed the prevalence of parasitic contamination in raw vegetables from Bangkok’s inner zones, focusing on five districts: Huai Khwang, Khlong Toei, Pathum Wan, Chatuchak, and Phra Nakhon. A total of 200 samples across eight vegetable types were randomly collected from these areas and prepared for analysis using sedimentation techniques followed by microscopic examination. Observers conducting the microscopy were blinded and operated independently to minimize bias. The overall prevalence of parasitic contamination was 77% (n = 154/200). The highest contamination levels were in Huai Khwang and Chatuchak (90%, P <0.05). Identified contaminants included Strongyloides stercoralis larvae (57%), hookworm eggs (20%), Ascaris lumbricoides eggs (16%), Trichuris trichiura eggs (12%), Toxocara spp. eggs (10%), and others. Chinese cabbage was significantly more contaminated with hookworm eggs (P <0.0001) and T. trichiura eggs (P <0.001) than other vegetables, although less so with S. stercoralis (P <0.0001). These findings highlight raw vegetables as potential sources of parasitic infections. Consumers are advised to thoroughly wash vegetables before consumption, whereas vendors should wear gloves to minimize exposure to skin-penetrating parasites. Prophylactic antiparasitic treatment, particularly for children and immunocompromised individuals, is recommended annually.

INTRODUCTION

Bangkok, the capital of Thailand, is located in a tropical climate zone and is one of the world’s leading destinations for tourism and medical travel.¹ Approximately 40 million tourists visit the city annually.² A popular activity for visitors in culturally rich Bangkok is enjoying local dishes, which often feature a variety of raw vegetables as primary ingredients or side dishes, especially in traditional Thai cuisine and street food. However, raw vegetables may serve as a vehicle for soil-transmitted parasitic infections, affecting both children and adults.³

Global reports have identified common parasitic contaminants in vegetables, including hookworms, Strongyloides stercoralis, Ascaris lumbricoides, Trichuris trichiura, Enterobius vermicularis, Fasciola hepatica, Capillaria spp., and Toxocara spp.^4–10 Yet, no studies to date have specifically examined parasitic contamination in raw vegetables from the inner zones of Bangkok. This study, therefore, assessed the prevalence of parasitic contamination in eight commonly consumed raw vegetable types obtained from five fresh markets located in the inner zone of Bangkok, including Huai Khwang, Khlong Toei, Pathum Wan, Chatuchak, and Phra Nakhon districts. Notably, these fresh markets are key distribution centers, supplying diverse food types to residents and visitors throughout Bangkok.

The WHO recommends a daily intake of 400 g of vegetables and fruits to help prevent noncommunicable diseases.¹¹ Accordingly, a weight of 200 g per vegetable variety was selected to assess contamination, aligning with WHO’s recommended consumption amounts. Findings from this study, which observed a notable prevalence of parasitic contamination in raw vegetables in Bangkok, could help raise awareness among residents and tourists about the risks associated with consuming raw vegetables and potentially support recommendations for prophylactic antiparasitic measures, particularly for children and immunocompromised individuals.

MATERIALS AND METHODS

Study area.

A cross-sectional study was conducted in Bangkok, the capital city of Thailand, between May and September 2023. Bangkok is situated at the geographical coordinates 13°45′14.33″N latitude and 100°30′5.18″E longitude. The city lies approximately 2 m above sea level and experiences a tropical monsoon climate, with an average annual temperature of 28–30°C and annual precipitation levels ranging from 1,400 to 1,600 mm according to the Climate Center of the Thai Meteorological Department in 2023.

Administratively, Bangkok is divided into three main zones: the inner, middle, and outer zones. This study focused on Bangkok’s inner zone, which comprises 21 districts and is regarded as the central zone of the city, densely populated with economic, governmental, commercial, and tourism hubs. Five districts were selected for inclusion in the study: 1) Huai Khwang (13.7699°N, 100.5865°E), 2) Khlong Toei (13.7189°N, 100.5672°E), 3) Pathum Wan (13.7462°N, 100.5308°E), 4) Chatuchak (13.8167°N, 100.5493°E), and 5) Phra Nakhon (13.7573°N, 100.4951°E) (Figure 1).

Figure 1.

Study areas and their percentages of parasitic contamination. Eight types of raw vegetables were sampled from five districts of Bangkok: 1) Huai Khwang, 2) Khlong Toei, 3) Pathum Wan, 4) Chatuchak, and 5) Phra Nakhon. The choropleth map, created using Datawrapper (online version, Datawrapper GmbH, Berlin Germany), visualizes the overall percentage of parasitic contamination in the vegetables from each area, with scaling colors ranging from red to light pink.

The total samples comprising eight different types of vegetables were collected from five structured markets in the Bangkok inner zone. All selected markets met the definition of a structured market: permanent facilities with complete roof coverage and enclosed merchandise stalls constructed with permanent materials featuring smooth, sloped surfaces that were easy to clean. Each vendor panel area measured at least 2 m², with a minimum height of 60 cm from the floor. Vendors in these structured markets primarily sourced their vegetables from wholesale markets in the area.

Sample collection.

A total of 200 samples of commonly consumed raw vegetables comprising eight types—1) lettuce (Lactuca sativa), 2) coriander (Coriandrum sativum), 3) leek (Allium porrum), 4) celery (Apium graveolens), 5) Chinese cabbage (Brassica rapa ssp. pekinensis), 6) culantro (Eryngium foetidum), 7) water spinach (Ipomoea aquatica; also known as Chinese morning glory), and 8) Chinese chives (Allium tuberosum)—were randomly collected from five structured fresh markets. Each market contributed five samples of each vegetable type (5 samples × 8 vegetables = 40 samples per market), yielding a total of N = 200 for this epidemiological prevalence study. All samples were collected during the same period on weekdays. For each vegetable type, approximately 250 g were purchased from different vendors within the same market to ensure a representative sampling. The samples were collected in clean, labeled plastic bags and immediately transported to the Parasitology Laboratory, Faculty of Medicine, Srinakharinwirot University in a cooler box maintained at 4–8°C within 2 hours of collection for further examination.

Soil-transmitted parasite elution.

Each raw vegetable sample, weighing 200 g, was manually chopped into bite-sized pieces to represent typical consumption. To extract parasites, the vegetables were first washed in 1,000 mL of 0.85% sodium chloride solution (Merck Millipore, Germany) with 1% lauryl glucoside solution (ROSRIN, Thailand) and agitated at 200 revolutions per minute for 30 minutes. The resulting solution was then transferred to a funnel sedimentation cylinder (Kartell, Italy) and allowed to settle for 24 hours. After sedimentation, the supernatant was carefully removed, leaving approximately 10 mL of sediment at the bottom. This sediment was transferred to a 15-mL centrifuge tube (BD Biosciences, NJ) and concentrated by centrifugation (Thermo Fisher Scientific, Waltham, MA) at 400 × g for 10 minutes, after which the supernatant was discarded. Then, the remaining sediment (approximately 1.5 mL) was agitated gently and examined under a light microscope (Figure 2).

Figure 2.

Workflow of parasite elution and detection. Eight types of raw vegetables (N = 200) were included in this study. A 200-g portion of each sample was chopped, washed in a solution containing 0.85% NaCl and 1% lauryl glucoside, and then, shaken for 30 minutes. The mixture was allowed to sediment for 24 hours, after which the sediment pellet was concentrated by centrifugation (400 × g for 10 minutes). The concentrated sediment was subsequently processed and examined by two independent observers. If the two independent observers could not reach consensus regarding the presence of unidentified or artifact-like parasite eggs, a third observer was then included in the investigation. rpm = revolutions per minute.

Detection of soil-transmitted parasite.

A total of 160 µL (20 µL × 4 slides × 1 sample × 2 independent observers) of concentrated sediment per sample was examined under a light microscope using both low-power (10×) and high power (40×) fields. To improve detection sensitivity, wet mount slides were prepared by adding Lugol’s iodine (Ethos Biosciences, NJ) at a 5:1 dilution for parasitic staining. The slides were then analyzed by two independent blinded observers. If any unidentified or artifact-like parasites were observed, a third observer was included for coinvestigation. The final identification was determined based on a two-of-three agreement among the observers.

STATISTICAL ANALYSES

Statistical analysis was conducted using GraphPad Prism 9 for Windows (GraphPad Software, Boston, MA). The prevalence of parasitic contamination was expressed as both the number and percentage of contaminated samples. To compare the prevalence of soil-transmitted helminth (STH) eggs/larvae among different vegetable types and study areas, the Fisher exact test, which is appropriate for categorical data with small sample sizes, was performed. The Mann–Whitney U test was used to compare the levels of contamination between different groups as the data did not follow a normal distribution. A P-value of ≤0.05 was considered statistically significant.

RESULTS

The prevalence of overall parasitic contamination according to different Bangkok areas.

The overall prevalence of parasitic contamination in vegetables from five districts was 77% (n = 154/200). To compare the prevalence of contamination across these areas, the Fisher exact test was performed. For all vegetables combined (without considering individual vegetable varieties), the results indicated that both Huai Khwang and Chatuchak had the highest contamination levels compared with the other areas (P <0.05), whereas Phra Nakhon had the lowest contamination levels (P <0.05) as shown in the final row of Table 1 and Figure 1.

Table 1.

Number of parasite-contaminated vegetables in different Bangkok areas

Type of Vegetable (n = 5/vegetable/area)	Inner Zone of Bangkok (n = 40/area)					All Areas Based on Each Vegetable (n = 25/each type of vegetable)
	Huai Khwang	Khlong Toei	Pathum Wan	Chatuchak	Phra Nakhon
Lettuce	3 (60)	3 (60)	5 (100)	5 (100)	2 (40)	18 (72)
Coriander	5 (100)	3 (60)*	5 (100)	5 (100)	5 (100)	23 (92)*
Leek	5 (100)	4 (80)	4 (80)	4 (80)	3 (60)	20 (80)
Celery	5 (100)	5 (100)	5 (100)	4 (80)	2 (40)†	21 (84)†
Chinese cabbage	4 (80)	5 (100)	4 (80)	5 (100)	4 (80)	22 (88)
Culantro	5 (100)‡	5 (100)§	1 (20)‡§‖	3 (60)	2 (40)	16 (64)‖
Water spinach	5 (100)‡¶	4 (80)	1 (20)‡#	5 (100)¶#	2 (40)	17 (68)¶
Chinese chives	4 (80)	3 (60)	4 (80)	5 (100)**	1 (20)†**	17 (68)†
Total	36 of 40 (90)††	32 of 40 (80)	29 of 40 (73)	36 of 40 (90)¶	21 of 40 (53)†	154 of 200 (77)‡‡

The prevalence of parasitic contamination in vegetables from five districts of Bangkok (n = 40 per area or n = 25 per vegetable type for a total of N = 200 samples) is reported as the count (and percentage) of contaminated samples in each vegetable type and area of observation. The Fisher exact test was used to compare the proportional contamination prevalence.

^*P <0.05 for Khlong Toei versus all areas.

^† P <0.05 for Phra Nakhon versus all areas.

^‡ P <0.05 for Huai Khwang versus Pathum Wan.

^§ P <0.05 for Pathum Wan versus Khlong Toei.

^‖ P <0.05 for Pathum Wan versus all areas.

^¶ P <0.05 for Chatuchak versus all areas.

^# P <0.05 for Pathum Wan versus Chatuchak.

^**P <0.05 for Phra Nakhon versus Chatuchak.

^†† P <0.05 for Huai Khwang versus all areas.

^‡‡ Total contamination prevalence from all observed samples (N = 200).

To investigate the contamination levels for each vegetable variety in relation to the different areas, a subanalysis was conducted (Table 1). The data revealed that coriander purchased from Khlong Toei had a significantly lower contamination level (60%) compared with coriander from all areas (92%, P <0.05). For celery, Phra Nakhon exhibited significantly less contamination than all areas (40% versus 84%, P <0.05). In contrast, culantro purchased from Huai Khwang and Khlong Toei had significantly higher contamination levels (100%) compared with that from Pathum Wan, which had only 20% contamination (P <0.05). Not surprisingly, this vegetable purchased from Pathum Wan also had less contamination than all areas (P <0.05). For water spinach, the contamination level in Huai Khwang and Chatuchak was significantly higher than in all areas, particularly Pathum Wan (P <0.05). For Chinese chives, Chatuchak exhibited the highest contamination level (100%) compared with Phra Nakhon and all areas (20% and 68%, respectively, P <0.05).

Distribution of diverse parasitic contamination in relation to vegetable varieties.

Without separating by Bangkok area, the overall parasitic contamination rates in all vegetable samples were as follows: S. stercoralis larvae (57%), hookworm eggs (20%), A. lumbricoides eggs (16%), T. trichiura eggs (12%), Toxocara spp. eggs (10%), Fasciola/Fasciolopsis/Echinostoma spp. eggs (2.5%), and Taenia spp. eggs (0.5%) (Table 2). Examples of parasitic morphology in contaminated vegetables are shown in Figure 3.

Table 2.

Distribution of soil-transmitted parasites in relation to the type of vegetables

Type of Vegetables (n = 25/each)	Strogyloides stercoralis Larvae	Hookworm Eggs	Ascaris lumbricoides Eggs	Trichuris trichiura Eggs	Toxocara spp. Eggs	Fasiola/Fasiolopsis/ Ehiostoma spp. Eggs	Taenia spp. Eggs
Lettuce	17 (68)*	2 (8)*	2 (8)	3 (12)*	1 (4)	1 (4)	0
Coriander	19 (76)†‡	2 (8)†	6 (24)	4 (16)†	1 (4)	0	0
Leek	16 (64)§	6 (24)§	2 (8)	2 (8)§	3 (12)	3 (12)	0
Celery	18 (72)‖	3 (12)‖	4 (16)	2 (8)‖	3 (12)	0	1 (4)
Chinese cabbage	2 (8)*†§‖¶#**††	20 (80)*†§‖¶#**††	5 (20)	11 (44)*†§‖¶#**††	3 (12)	0	0
Culantro	11 (44)‡¶	3 (12)¶	7 (28)	1 (4)¶	5 (20)	1 (4)	0
Water spinach	16 (64)#	3 (12)#	3 (12)	0#‡‡	2 (8)	0	0
Chinese chives	14 (56)**	1 (4)**§§	4 (16)	1 (4)**	1 (4)	0	0
Total (N = 200)	113 (57)††	40 (20)††§§	31 (16)	24 (12)††‡‡	19 (10)	5 (2.5)	1 (0.5)

The parasitic contamination in each type of vegetable from five districts of Bangkok is represented by the number (and percentage) of contaminated vegetables categorized by various types of soil-transmitted parasites found in those vegetables. The Fisher exact test was conducted for comparing these prevalence, with P ≤0.05 considered statistically significant.

^*P <0.05 for lettuce versus Chinese cabbage.

^† P <0.05 for coriander versus Chinese cabbage.

^‡ P <0.05 for coriander versus culantro.

^§ P <0.01 for leek versus Chinese cabbage.

^‖ P <0.01 for celery versus Chinese cabbage.

^¶ P <0.01 for culantro versus Chinese cabbage.

^# P <0.01 for water spinach versus Chinese cabbage.

^**P <0.01 for Chinese chives versus Chinese cabbage.

^†† P <0.01 for Chinese cabbage versus all vegetables.

^‡‡ P <0.05 for water spinach versus all vegetables.

^§§ P <0.05 for Chinese chives versus all vegetables.

Figure 3.

Representative images of parasite eggs and larva in the observed vegetables. (A) Strongyloides stercoralis larva, (B) a hookworm egg, and (C) an Ascaris lumbricoides egg were stained with Lugol iodine solution before examination under a light microscope. The photographs were captured using the NIS Elements program v. 4.3 for Windows (Nikon, Japan). The black arrow indicates the genital primordium of S. stercoralis larva.

To further investigate the occurrence of parasitic contamination in specific vegetable types, data analysis was conducted based on the identified soil-transmitted parasites and the various vegetable varieties (Table 2). The results revealed that S. stercoralis larvae were detected in the majority of vegetables, particularly in coriander (76%) followed by celery (72%) and lettuce (68%). Interestingly, Chinese cabbage had only 8% contamination with S. stercoralis larvae, which was significantly lower than that in the other vegetables (P = 0.0001). However, hookworm eggs and T. trichiura eggs were particularly prevalent in Chinese cabbage compared with all vegetables, with contamination levels of 80% versus 20% (P = 0.0001) and 44% versus 12% (P = 0.0008), respectively. For A. lumbricoides eggs, the highest contamination levels were observed in culantro (28%) and coriander (24%). However, no statistically significant differences were found among the different vegetable types. For other parasites, no statistically significant differences in contamination levels were observed between the different vegetable types (Table 2).

The most common specific parasitic contamination in each area.

After analyzing specific parasitic contamination based on vegetable type and study areas, the five most commonly identified parasites were S. stercoralis larvae, hookworm eggs, A. lumbricoides eggs, T. trichiura eggs, and Toxocara spp. eggs. Sufficient samples (with acceptable statistical power) were available to perform further analysis on the number of each parasitic contaminant in each area using the Mann–Whitney U test (Figure 4). The results demonstrated that S. stercoralis larvae contamination was observed in all areas but that it was significantly less prevalent in Phra Nakhon compared with Khlong Toei (P <0.05). Additionally, the heat map analysis of the association between the area and type of vegetable revealed that Chinese cabbage was less likely to be contaminated with S. stercoralis larvae (Figure 4A), which is consistent with the prevalence analysis described earlier (Table 2).

Figure 4.

The most common parasitic contamination in each area of the inner zone of Bangkok. The most common parasitic larvae and eggs, including (A) Strongyloides stercoralis larvae, (B) hookworm eggs, (C) Ascaris lumbricoides eggs, (D) Trichuris trichiura eggs, and (E) Toxocara spp. eggs, were primarily detected in eight types of vegetables. The numbers of parasitically contaminated vegetables are illustrated by individual dot plots (with medians and interquartile ranges shown as bar charts) for each area alongside heat maps displaying the percentages of contamination for each vegetable type. To compare these data, the Mann–Whitney U test was performed. AG = celery; AP = leek; AT = Chinese chives; BR = Chinese cabbage; CH = Chatuchak; CS = coriander; EF = culantro; HK = Huai Khwang; IA = water spinach; KT = Khlong Toei; LS = lettuce; PN = Phra Nakhon; PT = Pathum Wan. *P <0.05 and **P <0.01.

For hookworm and T. trichiura egg contamination, no statistically significant differences were observed among the study areas. However, the heat map clearly showed that both parasites were commonly found in Chinese cabbage (Figure 4B and D). Regarding A. lumbricoides eggs, these were more commonly found in Huai Khwang compared with Pathum Wan (P <0.01), Chatuchak, and Phra Nakhon (P <0.05). Although no statistically significant differences were noted between Khlong Toei and Huai Khwang, the contamination in Phra Nakhon was notably absent (P <0.05) (Figure 4C). Toxocara spp. eggs were primarily found in Huai Khwang, with a statistically significant difference from all areas (P <0.01). Furthermore, Toxocara spp. eggs were not detected in any vegetables from Pathum Wan and Phra Nakhon (Figure 4E).

DISCUSSION

Globally, soil-transmitted parasitic infections are estimated to affect over 1 billion individuals, particularly those residing in tropical regions. Numerous parasitic infections have been documented, including A. lumbricoides, S. stercoralis, hookworms (Necator americanus or Ancylostoma duodenale), and T. trichiura.¹² In this study, the prevalence of parasitic contamination in raw vegetables consumed daily in the inner zone of Bangkok was found to be 77%, indicating a significant risk of infection from these parasites. High levels of parasitic contamination have also been reported in Libya (58%),¹³ Iran (53%),¹⁴ Brazil (51%),¹⁵ and the Philippines (45%),¹⁰ whereas studies in Poland, Vietnam, and Turkey have reported levels below 30%.^8,16,17 The contamination level found in our study is considerably higher than the 26–45% reported in comparable studies across Southeast Asia.^{10,16,18–20} It is important to consider methodological and environmental factors that may have influenced these differences. The methodology using sedimentation and light microscopy is a standard approach for detecting STH eggs and larvae, similar to previous studies.^14–19 However, variations in sample processing, local sanitation, and environmental factors likely contribute to the observed differences. For example, the use of a surfactant in the protocol likely enhanced parasite recovery,²¹ which may partly explain the higher detection levels compared with earlier reports. Additionally, sampling during the wet season (May to September) may have contributed to the increased contamination levels.^22,23

Analysis across the five districts of Bangkok revealed that Huai Khwang and Chatuchak exhibited the highest contamination levels (90%) followed by Khlong Toei (80%), Pathum Wan (73%), and Phra Nakhon (53%) (Figure 1). This geographical variation could be attributed to factors such as the differences in vegetable cultivation sources, market sanitation, and the types of vegetables examined. Therefore, it is essential to investigate contamination levels at a more localized scale as this could enhance awareness among the public health organizations about the varying contamination levels in different regions.

Analysis of contamination levels for specific types of vegetables across different areas revealed that overall contamination levels ranged from 64% to 92% for various vegetable types (Table 1). Coriander had the highest contamination level in this study (92%), with 100% contamination observed in most areas, except for Khlong Toei (60%). However, a study conducted in southern Thailand reported a lower contamination level of 44.8% for coriander.¹⁸ Lettuce samples showed a high contamination level (100%) in Pathum Wan and Chatuchak, whereas Phra Nakhon exhibited a lower contamination level (40%). Consequently, the overall contamination level for lettuce in this study was 72%. A study from Brazil also reported a high contamination level of 77.8% for lettuce purchased from open street markets.¹⁵ Leek, celery, and Chinese cabbage exhibited overall contamination levels ranging from 80% to 88% across all areas. However, celery purchased from Phra Nakhon showed a significantly lower contamination level (40%). Both water spinach and Chinese chives had overall contamination levels of 68%. Water spinach purchased from Pathum Wan and Chinese chives from Phra Nakhon had lower contamination levels (20%) compared with other areas. Interestingly, the least contaminated vegetable in our study was culantro (64%), particularly in Pathum Wan and Phra Nakhon, where contamination levels were 20% and 40%, respectively. These findings suggest that coriander has the highest contamination level among all vegetables. This is likely because of its morphological characteristics, including multiple stalks connected at the base and roots that are situated close to the ground surface, making it easier to trap soil particles and STH eggs/larvae. Additionally, in this study, coriander samples were typically sold with both the roots and stalks intact, which may further increase the potential for contamination. This result is consistent with the report from Saudi Arabia.²⁴

The study identified several contaminants, with S. stercoralis larvae being the most prevalent (57%) followed by hookworm eggs (20%), A. lumbricoides eggs (16%), and T. trichiura eggs (12%). Toxocara spp. eggs accounted for 10%, whereas Fasciola/Fasiolopsis/Echinostoma spp. eggs and Taenia spp. eggs were less common, representing 2.5% and 0.5%, respectively. These findings are consistent with a recent review on common parasitic contamination in raw vegetables.¹² Furthermore, the types of STH found in our study are similar to those reported in Bangkok and its surrounding areas.²⁰ To examine the relationship between vegetable types and the parasitic contaminants, we analyzed the distribution of parasites across specific vegetable varieties (Table 2). Strongyloides stercoralis was detected across multiple vegetables (44–76%), with the highest contamination in coriander (76%) and the lowest contamination in Chinese cabbage (8%). In contrast, hookworms were predominantly found in Chinese cabbage (80%), suggesting that this vegetable provides a more suitable environment for hookworms than for S. stercoralis. The lower contamination level of S. stercoralis in Chinese cabbage is consistent with a study conducted in southern Thailand.¹⁸ Additionally, T. trichiura was also detected in Chinese cabbage (44%). Both Ascaris and Toxocara eggs were distributed across all vegetable types, with overall contamination levels of 16% and 10%, respectively. Previous studies have also reported similar contamination levels of approximately 20% for both parasites.²⁵ For the less common parasitic contaminants, Fasciola/Fasiolopsis/Echinostoma spp. eggs and Taenia spp. eggs were infrequently observed across various vegetables. Fasciola/Fasiolopsis/Echinostoma spp. eggs were detected in lettuce (4%), leek (12%), and culantro (4%), whereas Taenia spp. eggs were found only in celery (4%). Prior studies also report lower occurrences of these parasites.^26,27

To further refine the relationship between the most common parasitic contaminants and their distribution in different areas, we visualized the five most common parasites (S. stercoralis larvae, hookworms, A. lumbricoides eggs, T. trichiura eggs, and Toxocara spp. eggs) across each area in the inner zone of Bangkok and for each type of vegetable (Figure 4). This analysis confirmed that S. stercoralis was specifically less localized in Chinese cabbage, whereas hookworms and T. trichiura appeared to be more prevalent in Chinese cabbage across most areas of Bangkok. Ascaris lumbricoides and Toxocara spp. were observed in all vegetable types but were absent in lettuce from Huai Khwang. In contrast, these parasites were not consistently present in all vegetable types in Phra Nakhon. Differences in parasitic contamination among vegetable types may be influenced by their structural characteristic, proximity to soil, and ability to retain moisture. Vegetables grown close to the ground or with textured surfaces are more likely to trap soil particles and parasitic eggs/larvae.^17,18,20 Chinese cabbage, which had higher contamination with hookworm and T. trichiura eggs, has broad, crinkled leaves with multiple layers, creating a favorable microenvironment for parasite survival. Additionally, its stem is close to the soil, further increasing the risk of contamination. In contrast, the smoother lower leaf surfaces of Chinese cabbage dry faster, potentially limiting S. stercoralis larvae survival and explaining their lower prevalence in this vegetable. Additionally, variations in agricultural practices across farms supplying different markets may contribute to differences in parasite prevalence among districts. Parasitic contamination of vegetables can occur at multiple points in the production and distribution chain. Infected human and animal feces serve as the primary sources of contamination, particularly in areas where untreated sewage or manure is used as fertilizer.^25,26 Water sources contaminated with helminth eggs may also contribute to contamination during irrigation.^6,28 Market environments, where vegetables are stored and handled, may further increase the risk.³ The presence of both human-specific (A. lumbricoides and T. trichiura) and animal-associated (Toxocara spp.) parasites in this study suggests that contamination in Bangkok is likely linked to both human and animal waste. Such contamination may result from exposure to contaminated soil, irrigation water, or splashing during rain.^29,30

The impact of this study on public health and travel medicine, particularly in Bangkok, is notable as the five observed areas are key attractions for visitors and important food distribution centers for local citizens. For instance, Chatuchak is a well-known open-air market that attracts over 200,000 visitors each week, many of whom likely consume Thai street food dishes containing various types of raw vegetables.³¹ Unfortunately, Chatuchak was one of the areas with the highest parasitic contamination in this study. Consequently, without adequate preventive measures, both citizens and visitors are at increased risk of parasitic infections from consuming raw vegetables. The most common contaminants in this study, such as S. stercoralis and hookworms, pose major public health concerns, particularly in low- and middle-income countries.³² Infections caused by S. stercoralis can result in significant gastrointestinal and dermatological morbidity, potentially leading to malnutrition in children.³³ Similarly, hookworm infections have the highest burden among STHs, negatively impacting both physical and mental development in children and reducing the productivity of agricultural workers. Chronic hookworm infections may also result in iron-deficiency anemia.³⁴ Furthermore, coinfection with both S. stercoralis and hookworms may occur, particularly in poor developing countries, where approximately 60% of such coinfections were detected.³³ Even though this study focuses on Bangkok, the findings have broader implications for urban food safety in environments with similar sanitation and market structures, particularly in developing countries. Many urban centers face challenges related to wastewater management, hygiene in food markets, and consumer awareness of vegetable washing.^3,13,28 Our results emphasize the need for improved water, sanitation, and hygiene interventions, which are critical to reduce foodborne parasitic infections.³⁵ The generalizability of these findings is further supported by similar contamination patterns reported in other tropical and subtropical urban markets.^6,10,14–17

The effective prevention of parasitic infections in raw vegetable consumers can be achieved through several measures. A meta-analysis has shown that sanitation access (the availability or use of latrines) can reduce infection risk (odds ratio [OR]: 0.66, 95% CI: 0.57–0.76). However, sanitation was found to be less effective for hookworm infections (OR: 0.80, 95% CI: 0.61–1.06). Handwashing with soap significantly decreased the rate of infection with soil-transmitted parasites (OR: 0.53, 95% CI: 0.29–0.98). Furthermore, for raw vegetable vendors and farmers, wearing gloves and shoes substantially reduced the risk of soil-transmitted parasitic infections, particularly hookworm infection (OR: 0.29, 95% CI: 0.18–0.47).³⁵ In terms of treatment, benzimidazole anthelmintics, such as mebendazole and albendazole, are commonly used to treat soil-transmitted parasitic infections, particularly ascariasis and hookworm infections.³⁶ The WHO recommends controlling these infections through preventive chemotherapy, with regular treatment as prophylaxis to prevent high-intensity infections in entire populations or at-risk groups.³⁷ Therefore, it is recommended that all consumers, especially children and immunocompromised individuals, take antiparasitic medications once or twice a year to prevent infection.

This study provides a localized perspective on food safety risks by examining soil-transmitted parasite contamination in fresh vegetables, specifically in the inner districts of Bangkok. Unlike previous studies that primarily report overall contamination levels, our analysis focuses on contamination patterns across different vegetable types and urban districts, revealing significant geographical variations. By identifying specific parasites associated with each vegetable type and location, our findings offer valuable insights that could guide targeted public health interventions. A limitation of this study is the absence of molecular confirmation for vegetable samples that showed no observable parasitic contamination. The reliance on light microscopy, although a widely used standard in clinical laboratories for diagnosing soil-transmitted parasitic infections, may have limited the precise identification of specific parasite species. Additionally, data on vegetable farming locations, agricultural practices, transportation methods, and market hygiene conditions were not collected, which may have contributed to residual confounding. Future studies should incorporate these factors and use molecular techniques to enhance the accuracy of parasite identification and provide a more comprehensive assessment of contamination sources.

CONCLUSION

The findings of this study reveal a high prevalence of pathogenic parasite contamination in raw vegetables within the central region of Bangkok, Thailand’s capital. This significant level of contamination suggests that these vegetables may serve as potential vectors for parasitic infections. To mitigate the associated risks, it is essential for consumers to rigorously wash raw vegetables prior to consumption. For vegetable vendors, the use of gloves is recommended as a primary preventive measure to reduce exposure to skin-penetrating parasites. Moreover, annual or biannual prophylactic administration of antiparasitic medications is advised, particularly for vulnerable populations, such as children and individuals with compromised immune systems.