INTESTINAL PARASITES IN COMMUNITIES BORDERING TONLE SAP LAKE

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Correspondence: Peter Odermatt, Swiss TropicalInstitute, PO Box, 4002 Basel, Switzerland.Tel: + 41-61-284 82 14; Fax: + 41-61-284 81 05Email: peter.odermatt@unibas.ch

INTRODUCTION

Tonle Sap Lake, also known as “GreatLake”, is located in central Cambodia. It is thelargest freshwater lake in Southeast Asia. It issituated within the floodplain of the Mekongand represents a vast natural resource area.Fish, water plants, and the mineral-rich soil ofthe annually flooded shoreline attract humansettlements. Water resources are intensivelyused, therefore, promoting the spread of wa-ter-borne infections, potentially also Schisto-soma mekongi.

Schistosomiasis mekongi has been de-scribed in Kratie and Stung Treng Provincesin Cambodia, (Biays et al, 1999; Stich et al,1999) and in Champassack Province of south-ern Lao PDR (Urbani et al, 2002). Although

INTESTINAL PARASITES IN SCHOOL-AGED CHILDREN INVILLAGES BORDERING TONLE SAP LAKE, CAMBODIA

Tep Chhakda1, Sinuon Muth1, Duong Socheat1 and Peter Odermatt2

1National Malaria Center, Phnom Penh, Cambodia; 2Department of Public Health andEpidemiology, Swiss Tropical Institute, Basel, Switzerland

Abstract. The objective of this study was to study Schistosoma mekongi and other intestinalparasitic infections, and intestinal symptoms and related complaints among school-age chil-dren and adolescents living around Tonle Sap Lake. Villages were selected where there werepotential signs of schistosomiasis (hepatomegaly), and where subjects complained of intesti-nal symptoms. Stool samples were collected from 1,616 children and were examined by Kato-Katz, SAF concentration, and Baermann technique; short clinical examinations were also per-formed. No S. mekongi infection was detected, although a high level of intense human watercontacts was reported. Helminth infection such as Ascaris lumbricoides (27.7%) and hook-worms (29.7%) were common. Trichuris trichiura 4.4%), Hymenolepis nana (6.2%), Giardialamblia (4.2%), and Entamoeba spp (14.4%) were also recorded. Strongyloides stercoraliswas frequently diagnosed (20.2%). It was concluded that it is unlikely that S. mekongi is trans-mitted in Tonle Sap Lake. However, other intestinal parasitic infections are widespread. Inparticular, S. stercoralis should be considered an important etiologic agent in children andadolescents with abdominal complaints.

control efforts targeted against this severeparasitic infection have been undertaken inLao PDR since 1983 and since 1994 in Cam-bodia, transmission continues (Ohmae et al,2004) and patients with active infections of S.mekongi are still diagnosed in both countries.Neotricula aperta, the intermediate host snail,is found in the Mekong River but also in itstributaries, such as the Xe Kong River in south-ern Lao PDR and northern Cambodia (Attwoodet al, 2004).

There has been considerable discussionwhether the transmission of S. mekongi oc-curs in Tonle Sap Lake. On several occasions,patients with an S. mekongi infection werereported among refugees originating from theprovinces bordering Tonle Sap Lake, in par-ticular Battambang Province (Keittivuti et al,1982a, 1982b, 1983). Furthermore, for healthimpact assessments of large-scale water re-sources development projects of the Mekongriver system such as dams and lakes, an in-

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depth understanding of the suitable transmis-sion area of S. mekongi, and therefore its pres-ence in Tonle Sap Lake, is of great importance.

The objective of the present study wasto investigate S. mekongi and other intestinalparasitic infections, and intestinal symptomsand related complaints among school-agechildren and adolescents.

MATERIALS AND METHODS

Study area and population surveyed

The study was conducted between De-cember 1998 and June 1999. Districts of allprovinces around Tonle Sap Lake were vis-ited, except those of Siem Reap Province.School-age children were selected for thestudy according to the following procedure:first, health authorities at the district level wereasked about villages where schistosomiasis-related morbidity was suspected (reports ofhepatomegaly, or blood or mucus in stool).Second, in the identified villages, teachers andhealth staff were interviewed about childrenwho had suspected symptoms of schistoso-miasis. Third, in each village children with sus-pected symptoms were included; and/or asample of schoolchildren and adolescentswere randomly selected at the local school (50< n < 150, depending on the size of the school)and enrolled in the study.

Field and laboratory procedures

A single stool sample was obtained fromeach participant and examined for intestinalparasites using the Kato-Katz thick smeartechnique (Katz et al, 1972). One slide wasexamined per stool. A random sub-sample(50%) of the stools was fixed in sodium ac-etate-acetic acid-formalin (SAF) and examinedat the National Malaria Center, Phnom Penh,for the presence of helminthic and protozoalintestinal parasites using a concentration tech-nique (Yang and Scholten, 1977). A furtherrandom sub-sample (10%) was examined on

the survey day for the presence of S.stercoralis larvae using the modified Baermanntechnique (Watson and Al Hafidh, 1957).

Children were clinically examined for thepresence of hepatomegaly and splenomegaly,and screened for the presence of blood andmucus in stool, intestinal complaints, and riskfactors for intestinal parasitic infections (wa-ter contacts such as bathing, and fishing;wearing of shoes, and utilization of latrines).

Data management and statistical analysis

Data was entered in EpiInfo (version 6.0)and analyzed with STATA (Stata Cooperation,College Station, TX, USA). Rates and meanswere calculated for categorical and continu-ous data, respectively. Standard statisticalprocedures (95% confidence interval, χ2 test,Student’s t-test) were applied where appro-priate.

The study was approved by the Ministryof Health, and provincial and district healthdepartments. Oral consent was obtained fromhead of the villages and school teachers. Di-agnosed infections were treated appropriately.

RESULTS

We visited 23 villages in all districts of fourof the five provinces bordering the lake (ap-proximately equivalent to two-thirds of itsshoreline) (Table 1). A total of 1,616 childrenwere enrolled from whom stool samples wereobtained and examined using the Kato-Katztechnique. Forty-nine percent (789) and 12%(188) of the stools were examined by SAF con-centration and Baermann technique, respec-tively. Ninety-eight percent (1,584) were ex-amined clinically.

Fifty-four percent (876) of the enrolledchildren were male. The sex ratio of the sub-samples for SAF concentration technique(57% male, p = 0.2), Baermann technique(56% male, p = 0.6), and clinical examination(54% male, p = 0.9) did not differ significantly

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from the overall sex ratio. The overall meanage was 11.2 years (95% CI: 11.1-11.4). Themean ages for the sub-samples were 11.3(95% CI: 11.0-11.4), 11.5 years (95% CI: 11.2-11.8), 11.3 years (95% CI: 11.1-11.4) for theSAF concentration technique, Baermann tech-nique, and clinical examination sub-samples,respectively. There were no significant differ-ences between the mean ages of the overallsample and the sub-samples (SAF concen-tration technique: p = 0.93; Baermann tech-nique: p = 0.06; clinical examination p = 0.97).

Table 2 summarizes the results of theparasitological examinations. We found nochild with a S. mekongi infection. Furthermore,none of the health staff at provincial, district

or village levels reported a child or adult withthe typical signs and symptoms of advancedschistosomiasis.

The clinical examination of the childrenrevealed only very low prevalence rates ofhepatomegaly (4.0%) and splenomegaly(0.1%) (Table 3). Enlargement of the liver andspleen, when present, was modest.

Behavior risk factors for potential trans-mission of schistosomiasis were reported inhigh frequencies (Table 3). For example, 68.1%of the children reported that they do not haveaccess to any latrine or toilet at home, and34.5% reported regular recreational swimmingand bathing in the lake. In addition, 7.0%mentioned regular fishing activities.

Province Village n / Village n / Province

Kampong Thom Kumrou 50Kompong Chertoul 65Tang Krasav 71Koung Meac 65Tang Kroung 60Kompong Chean 61Pol Hounl 64Pobakor 57Preak Sbouv 63Techomarc 63 619

Kampong Chnang Chnok Trou 108Koh Ressey 84Koh Thom 107Koh Tkuv 75Stung Chrouv 104Ta Koh 79 557

Pursat Kan Chor 103Koh Chom 57Kompong Loung 48Snar Ansar 49 257

Battambang Bak Prea 54Koh Chivanh 69Prey Chas 60 183

Total 1,616

Table 1Sample distribution of the survey.

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frequencies. Others identified were Enterobiusvermicularis, Taenia spp and Giardia lamblia.Twenty point two percent of the Baermanntechnique analysis yielded a positive result forS. stercoralis infection.

As expected, a high level of intestinalsymptoms was reported. Within the previousthree days, 44.3, 55.1, 20.0, and 15.6% ofthe children reported having had diarrhea,abdominal pain, and blood and mucus in theirstool, respectively.

The risk factors for intestinal helminthswere highly prevalent, thus assuring the trans-mission of soil-transmitted helminths. Morethan two-thirds of the children (68.1%) re-ported not having a latrine or toilet in thehousehold. Only one-third of the children(32.6%) were wearing shoes on the survey day,and only two-thirds of them (62.5%) reportedwearing shoes when defecating.

DISCUSSION

No S. mekongi infection has been de-tected in any of the more than one thousandpersons examined by the two diagnostic tech-

n=1,616 (%) n=789 (%) n=188 (%)

Schistosoma mekongi 0 0 NAAscaris lumbricoides 448 (27.7) 215 (27.1) NATrichuris trichiura 71 (4.4) 0 NAHookworm 480 (29.7) 187 (23.6) NAStrongyloides stercoralis NA 21 (2.7)a 38 (20.2)Hymenolepis nana 81 (5.0) 49 (6.2) NAEnterobius vermicularis 12 (0.7) 2 (0.3) NATaenia sp 1 (0.1) 1 (0.1) NAEntamoeba sp NA 114 (14.4) NAGiardia lamblia NA 33 (4.2) NA

NA = not applicable, alarvae observed

Table 2Parasitological results.

Kato Katz technique SAF concentration Baermann technique

Number (%)

Clinical signs Hepatomegaly 63 (4.0) Splenomegaly 2 (0.1)Reported symptoms Diarrhea 703 (44.3) Abdominal pain 872 (55.1) Blood in stool 317 (20.0) Mucus in stool 247 (15.6)Risk factor No toilet/latrine at home 1,078 (68.1) Recreational swimming 561 (35.4) Fishing 125 (8.0) Treatment with anthelminths 52 (3.3) Wearing shoes (observed) 515 (32.6) Shoes during defecation 988 (62.5)

Table 3Results of the clinical examination, reported

symptoms and risk factors (n = 1,584).

However, other intestinal parasites werefrequently encountered, such as Ascarislumbricoides (Kato-Katz: 27.7%, SAF: 27.1%)and hookworms (Kato-Katz: 29.7%, SAF:23.6%). Hymenolepis nana and Entamoebacysts were also diagnosed with considerable

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niques. Neither the Kato-Katz technique (thestandard diagnostic field approach) nor theSAF concentration technique (used to screenlight infections) obtained any infection with thisparasite. Furthermore, typical clinical signs,such as hepatosplenomegaly, were rare andof low degree only. This was not comparableto the clinical burden detected in the schisto-somiasis endemic province of Kratie where,in 1995, more than half of the population hadhepatomegaly (Biays et al, 1999).

The Mekong River contributes heavily tothe annual flooding of Tonle Sap Lake via theTonle Sap River. During this event the inter-mediate snail host, N. aperta, can easily betransported into the lake. However, the mainecological features of the shoreline of theGrant Lake are distinctly different from thoseof the Mekong river shores. The lake’s wateris stagnant; its’bed is mainly clay, which pro-duces turbid waters, and this is likely less fa-vorable for sustaining N. aperta snails, whichrequire predominantly clear waters, and sandyor rocky substrates (Urbani et al, 2002). Snailsurveys should be conducted in the future toconfirm these observations. Finally, additionalstudies carried out in the province of KampongChnang confirmed the absence of S. mekongiin schoolchildren (Sinuon et al, 2003).

The present study confirmed the highlevel of soil-transmitted helminths and asso-ciated risk-factors for transmission that havebeen largely documented in Southeast Asiancountries. However, more importantly, itshowed that S. stercoralis may reach substan-tial prevalence rates. This parasite is rarelydiagnosed in surveys because it requires aspecific concentration technique (Baermanntechnique). Clinically, this infection may leadto skin (larva currents) or to gastro-intestinaldiseases. In immune-depressed subjects, S.stercoralis may lead to systemic disease dueto its ability for autoinfection. Therefore, infec-tion with S. stercoralis requires more atten-tion.

Our study demonstrated that schistoso-miasis mekongi is not present in the studiedvillages along the Tonle Sap Lake, and that itis very unlikely that it is transmitted in TonleSap Lake. This conclusion is of importanceto water development projects that create ar-tificial lakes featuring similar ecological set-tings.

ACKNOWLEDGEMENTS

The authors highly appreciate the partici-pation of the health authorities at provincialand district levels as well as the interest of theparticipating head of villages, schoolteachers,and schoolchildren. Funding was granted byMédecins Sans Frontiéres.

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