Epidemiology and clinical aspects of Strongyloides stercoralis infection in Cambodia

Background: The threadworm Strongyloides stercoralis is endemic in settings where sanitary conditions are poor and where the climate is warm and humid. More than 70 tropical countries in Southeast Asia, Sub-Saharan Africa, West Indies and Latin America are considered as high endemic settings. However, S. stercoralis is also prevalent in subtropical and temperate regions including Australia, Japan, Canada, United States and Europe. The global prevalence of S. stercoralis is heterogeneous. It is believed that about 30 - 100 million people worldwide are infected with S. stercoralis. But the true number and the global burden of infection remain unknown and most probably are today underestimating in many areas of the tropical resource poor countries. The low sensitivity of the currently available diagnostic tools and a scarcity of specialized survey are most important factors for that. Moreover, many epidemiological aspects of S. stercoralis infection are poorly understood or unknown. It is not known in detail where S. stercoralis is endemic, which infection rates and intensities can typically be expected in different settings and populations, and when an individual was infected at first-time and how quickly the re-infection can occur after successful treatment. Epidemiological information on S. stercoralis such as large-scale prevalence, re-infection, risk factors, clinical features and treatment efficacy are unknown in Cambodia and many parts of Southeast Asia.
Aim and objectives: This PhD thesis aimed to understand the importance of S. stercoralis infection in Cambodia by pursuing four main objectives: (i) assess S. stercoralis infection and risk factors, validate diagnostic methods and determine treatment efficacy among schoolchildren, (ii) determine large-scale prevalence and risk factors in two socioeconomic and ecological distinctly different settings, (iii) determine re-infection rates among schoolchildren, and (iv) document clinical aspects of patients with high intensity of S. stercoralis infection in rural communities.
Methods: School- and community-based studies were carried out in four primary schools and 120 villages of three provinces (Kandal, Preah Vihear and Takeo) in Cambodia, from 2009 to 2011. After obtaining the written informed consent from participants, an individual questionnaire was administrated to obtain demographic, risk-perception and behavioral data. The head of household was interviewed with a household questionnaire on socioeconomic indicators of the household such as house type, household assets, latrine and livestock. After the interview, each participant was given a pre-labeled plastic container (ID code, name, sex, age and date) for stool sample collection. In case a multiple stool samples analysis, another stool container was distributed upon collection of the first or second sample. The fecal materials were analyzed by Baermann method and Koga-agar plate (KAP) culture for diagnosing S. stercoralis and Kato-Katz method for helminth co-infections.
Two school-based studies were performed in four primary schools in Kandal province. In 2009, a cross-sectional study was carried out among 458 children, examining three fecal samples per child, to assess risk factors, diagnostic methods and treatment efficacy after three weeks of ivermectin treatment (100?g/kg/day for two days). A two-year cohort study was conducted among 302 schoolchildren from 2009 to 2011, analyzing two stool samples per child, to determine re-infection and risk factors of S. stercoralis.
Two large-scale cross-sectional community-based studies were conducted in 2010 and 2011 to assess infection prevalence and risk factors in two provinces (2396 participants from 60 villages of Preah Vihear province, analyzed two stool samples per participant; and 2861 participants from 60 villages of Takeo province, examined one stool sample per participant). Bayesian kriging was used to predict risk at non-surveyed locations in Preah Vihear province. A case-series study, nested in the survey in Preah Vihear province in 2010, was carried out to document the clinical features of 21 S. stercoralis cases, with high numbers of S. stercoralis larvae in their fecal specimen detected by Baermann technique.
Principal findings: A cross-sectional school-based survey in 2009 found that 24.4% of 458 schoolchildren were infected with S. stercoralis. The prevalence of S. stercoralis infection increased considerably (from 18.6% to 24.4%) when three stool samples were examined. The sensitivity of KAP culture and Baermann technique was 88.4% and 75.0%, respectively. Clinical features such as itchy skin and diarrheal episodes were significantly associated with S. stercoralis infection. Children who reported defecating in latrines were significantly less infected with S. stercoralis than those who did not use latrines (OR: 0.4; 95% CI: 0.2 – 0.6; P<0.001). Almost three-quarters of the infections could have been reduced by proper sanitation (PAR: 0.7; 95% CI: 0.5– 0.9). Ivermectin (200 µg/kg BW, PO, over 2 days) was highly efficacious against S. stercoralis infection, with a cure rate of 98.3% three weeks after treatment.
In Preah Vihear and Takeo provinces, S. stercoralis infection prevalence among general population was 44.7% and 21.0%, respectively. In both areas found that the male participants were significantly more infected than females (P<0.001) in all age classes. In Preah Vihear province, northern Cambodia, S. stercoralis infection statistically increased with age, starting at 31.4% in children less than 6 years to a peak of at 51.2% in participants older than 50 years. Participants defecating in latrines were significantly less infected with S. stercoralis than those who did not use latrine (OR: 0.5; 95% CI: 0.4 - 0.8; P<0.001). S. stercoralis infection exhibited almost no tendency to spatial clustering in this province. Infection risk significantly decreased with increasing rainfall and soil organic carbon content and to increase in lands occupied by rice fields. In Takeo province, southern Cambodia, S. stercoralis infection prevalence reached 14.5% in children under or equal to 5 years and 28.0% in participants aged between 56 and 60 years. Participants who reported having a latrine were statistically less infected with S. stercoralis infection than those who did not possess latrine at home (OR: 0.7; 95% CI: 04 - 0.8; P: 0.003). Muscle pain and urticaria were significantly associated with S. stercoralis infection.
A two-year cohort study among 302 schoolchildren revealed a prevalence rate of 24.2% and 22.5% at baseline (2009) and follow-up (2011), respectively. Almost one-third (31.5%) of 73 treated S. stercoralis cases at baseline were re-infected at follow-up. But, almost 70% of children infected at baseline and treated remained free of re-infection for the period of two years. Children reported having shoes and defecating in toilet were statistically less infected with S. stercoralis than those who did not possess shoes (OR: 0.3; 95% CI: 0.1 – 0.5; P: 0.031) and use latrine (OR: 0.4; 95% CI: 0.2 – 0.9; P<0.001) at follow-up. None of the reported clinical symptom was significantly associated with S. stercoralis infection at follow-up.
Clinical symptoms of 21 S. stercoralis patients with high intensity infection (more than 250 larvae in Baermann test) from Preah Vihear province were documented in 2010. The median age of the patients was 11 years (range: 5 - 67); 23.8% were females. Eleven patients (52.4%) were younger than 16 years. Out of 21 patients, 20 (95.2%), 18 (85.7%) and 14 (66.7%) reported frequent abdominal pain, diarrhea and periods of sensation of itching, respectively, during the previous six months. Five patients (23.8%) reported having experienced urticaria the week preceding the examination. One patient suffered from extended urticaria. Three weeks after ivermectin treatment (200µg/kg BW, single oral dose), most symptoms (diarrhea, abdominal pain and urticaria) almost entirely resolved.
Conclusions: S. stercoralis infection is highly prevalent in rural communities of Cambodia where appropriate diagnosis and treatment do not exist. The re-infection rate of S. stercoralis among schoolchildren after two years of ivermectin treatment is considerable, but more than two-third remains free of infection for at least 2 years. Preschool- and school-aged children are highly affected. Personal hygiene and sanitation including wearing shoes, possession and use of latrines, are significant predictors of S. stercoralis infection. Gastrointestinal and cutaneous symptoms are associated with S. stercoralis infection and resolve almost entirely after ivermectin treatment. Thus, S. stercoralis infection should no longer be neglected in Cambodia and elsewhere in tropical resource poor countries. Access to adequate diagnosis and treatment of S. stercoralis infection is an urgent need in Cambodia