Prevalence of intestinal parasitic infection and associated risk factors among village health volunteers in rural communities of southern Thailand

This cross-sectional study was carried out from January to April 2016 in Nopphitam District, Nakhon Si Thammarat Province, southern Thailand. Nopphitam District is located at the geographical coordinates of 8°43′10″N latitude and 99°45′6″E longitude, which is about 780 km south of the Thai capital of Bangkok (Fig. 1). Average temperature is 27.1 °C, with a minimum 25.8 °C in January and maximum 29.3 °C in May. Annual rainfall is 1454.3 mm (Climatological Center, Thai Meteorological Department, Annual Report 2015). This district is divided into four sub-districts (tambon), namely Na Reng, Karo, Nopphitam, and Krungching, which are further subdivided into 38 villages (muban). Nopphitam District is considered remote and rural and there is only one community hospital. These four sub-districts share the same geographic landscape. The main access road is non-asphalt while the remaining roads are asphalt. The majority of land is used for farmland and rubber plantations. According to the Department of Provincial Administration, the estimated total population of these sub-districts in 2015 was 33,183. Agriculture is the main economic activity of people in this area.

The study was conducted among village health volunteers residing in four sub-districts of Nopphitam District, Nakhon Si Thammarat Province. The sample size was determined using the single population proportion formula. It was calculated from the prevalence rate (p) of 19.8% from a previous study [2], with a 95% confidence interval (z = 1.96), 5% margin of error (d = 0.05). The calculated sample size was 244. We assumed that the final sample size would be reduced by around 30% due to unable to pass stool on the study date and thus aimed for a sample size of 330. Simple random sampling method was used to selected village health volunteers from each sub-districts. We randomly selected 330 VHVs, then gave instructions and distributed plastic containers for stool collection. Finally, a total of 324 village health volunteers returned stool specimens, which consisted of 22.22% (72) from Na Reng, 24.38% (79) from Karo, 16.98% (55) from Nopphitam, and 36.42% (118) from Krungching. We decided to keep all the samples in the study for the sake of health surveillance regardless of the calculated sample size, hence, the final sample size was 324.

A structured questionnaire was developed and used to collect demographic and socioeconomic data (i.e. age, education level and income). Information on possible risk factors (i.e. personal hygiene behaviors such as hand washing and shoe wearing) was also collected via the questionnaire (Additional file 1). All completed questionnaires were checked for accuracy and completeness. After proper instruction, a clean plastic container labeled with the subject’s name and identification number was distributed to each participant and one day later they returned containers back with at least 10 g of stool. Each specimen was checked for correct labeling, quantity of sample, and procedure of collection and transported immediately to the parasitology laboratory of the School of Medicine, Walailak University.

A portion of each specimen was processed immediately using a direct microscopic technique with a saline and iodine solution to detect cysts, trophozoites, eggs, and larvae of intestinal parasites. The remainder of each specimen was analyzed using the formol-ether concentration technique. Briefly, 1 g of stool sample was suspended in 3 ml of 10% formalin, sieved with cotton gauze, and transferred to a 15-ml conical centrifuge tube. Then, 8 ml of 10% formalin and 3 ml of diethyl ether were added and the resulting solution was centrifuged for 5 min at 2000 rpm. Subsequently, the supernatant was discarded, and the remaining sediment was observed under light microscope at 10× and 40× objectives to detect eggs and larvae of helminths, cysts, and trophozoites of protozoan parasites. Iodine solution was used to detect and identify cysts of protozoan parasites. In addition, each stool sample was analyzed by the modified Kato-Katz technique [9] (kits were purchased from Department of Helminthology, Faculty of Tropical Medicine, Mahidol University). The intensity of soil-transmitted helminth (STH) infection was estimated indirectly by counting the number of eggs per gram (EPG) of stool and calculated by multiplying each slide count by 40. The results of intensity for STH were classified into low, moderate, and high according WHO recommendation; for Ascaris lumbricoides: low 1–4999 EPG, moderate 5000–49,999 EPG, and high 49,999 EPG; for Trichuris trichiura: low 1–999 EPG, moderate 1000–9999 EPG, and high 9999 EPG; and for hookworm; low 1–1999 EPG, moderate 2000–3999 EPG, and high 3999 EPG, respectively [10]. To eliminate observer bias, each stool sample was examined by two trained senior medical laboratory technologists who were not informed about the health status and other details of the study participants.

We used proportions with 95% confidence intervals (CIs) to describe the prevalence of intestinal parasites. Quantitative variables were described by mean and standard deviations (SD) and qualitative variables were described by frequency (percentage). Initially, the differences of the presence of intestinal parasites among sub-districts and were assessed by chi-square test. The prevalence ratios were computed by Poisson regression with robust estimation to measure the strength of the association between each independent variable and STH infection. To adjust for possible confounders, all variables with P-value less than 0.1 in the univariate analysis were analyzed using multivariate Poisson regression analysis. P-value less than 0.05 was considered statistically significant. Data were entered, cleaned, and analyzed using IBM SPSS Statistics for Windows, Version 23.0. Armonk, NY: IBM Corp.

A total of 324 study participants, comprising 38 males and 286 females, were enrolled in the study. The mean age of participants was 45 years (standard deviation 9.0), with a range of 23 to 71 years. The age-group distribution showed that a majority (39.5%) were in the 41–50-years age group. All participants were Buddhist. Most participants were married (89.5%), and a majority was farmers (80.2%). About half (53.1%) of participants indicated secondary education was the highest level received, and around one-third (32.4%) indicated primary education was the highest level (Table 1).

The overall prevalence of intestinal parasitic infection among participants was 45/324 (13.9%, 95% CI: 10.3–18.1). A majority of the infected participants 97.8% (44/45) had a single infection while one participant 2.2% (1/45) was found to have a double infection (hookworm and Trichuris trichiura). Among helminth infections, hookworms 27/324 (8.3%, 95% CI: 5.6–11.9) was the predominate parasite, followed by Strongyloides stercoralis 3/324 (0.9%, 95% CI: 0.2–2.7) and T. trichiura 1/324 (0.3%, 95% CI: 0–1.7). No cestodes or trematodes were detected in this study. Of 27 participants who were positive for hookworm infection, 85.2% (23/27), 11.5% (3/27), and 3.7% (1/27) had low (1–1999 EPG), moderate (2000–3999 EPG), and high (≥4000 EPG) intensity of infection, respectively. All participants infected with T. trichiura had low (1–999 EPG) intensity.

The prevalence of helminth infection ranged from 7.6% to 11.4% at the study sites. The highest prevalence of the helminth infection was observed in Karo sub-district, and protozoan infection was found frequently in Krungching sub-district. The overall difference in parasite prevalence among the study sites was not statistically significant (p > 0.05) (Table 2). The prevalence of helminth infection was twice the prevalence of protozoan infection. With regard to protozoa infection, the prevalence of protozoan infection ranged from 1.8% to 5.9%. The highest prevalence was due to Blastocystis hominis 13/324 (4.0%, 95% CI: 2.2–6.8) and followed by Giardia intestinalis 2/324 (0.6%, 95% CI: 0–2.2) (Table 2).

The distribution of infection among female and male participants is shown in Fig. 2. Male participants had the overall prevalence at 18.4% (7/38) whereas the infection rate in female participants was slightly lower at 13.3% (38/286). The difference in prevalence of intestinal parasites between female and male participants was not statistically significant.

Univariate analysis of factors potentially associated with STH, as shown in Table 3, revealed significant associations between STH infections with having dogs at home. VHVs who had dogs at home were two times more likely to be infected than those who did not (Crude prevalence ratio 2.3; 95% CI: 1.0–5.2, p = 0.047). STH infection was not significantly associated with gender, age group, or monthly family income (p > 0.05). All the participants with STH infection graduated the highest level of education at primary/secondary school. The prevalence ratio regarding level of education could not be calculated due to none of participants with bachelor’s degree or higher had STH infection. After adjusting for possible confounders by multivariate analysis, none of the risk factors were associated with STH infection (Table 3).