High prevalence of large trematode eggs in schoolchildren in Cambodia
a b s t r a c t
Large trematode eggs(LTE)resembling Fasciola spp. eggs were reportedly found in the stools of schoolchildren in Kandal province, Cambodia. This study aimed to assess the prevalence of LTE in the stools of children attending the affected school, identify potential risk factors for infection and ascertain the trematode species. We performed a cross-sectional study involving an in-depth questionnaire admin- istered to schoolchildren at the affected school, and examined cattle droppings in the surrounding area and the livers of slaughtered cattle. Three stool samples were examined per child, using Kato–Katz and formalin–ether concentration techniques .In addition, blood serum enzyme-linked immunosorbent assay (ELISA) and coprological polymerase chain reaction(PCR) was conducted for species clarification.Cattle droppings wereexaminedbycupsedimentationandcoprologicalELISA.LTEwereobservedinthestools of 106 schoolchildren(46.5%).Two blood serum samples from schoolchildren were positive for Fasciola hepatica in afirstELISAbutwerenegativeinaconfirmationimmunofluorescenceantibodytest.Out of 221 PCR samples, only one tested positive for Fasciola spp. and none for Fasciolopsis buski. The consumption of raw aquatic plants (oddsratio(OR)=3.3) and fermented fish sauce(OR=2.1) were significantly associ-ated with LTE in the stool. Fasciola spp. flukes were observed in 18.3% of 191 cattle livers. The prevalence of fascioliasis in cattle droppings was 88.8%.The low prevalence of schoolchildren that tested positive for Fasciola spp. with specific molecular diagnostics and who had no diagnostic evidence of F. buski strongly indicates that the majority of microscopically observed LTE are from Echinostoma spp. Fasciola spp. transmission from cattle to human is possible and public health services need to be alerted accordingly.
1. Introduction
In low-and middle-income countries, intestinal multipara-sitism is the rule rather than the exception(McKenzie, 2005; Petney and Andrews,1998;Steinmannetal.,2010). In South-
east Asia, intestinal helminth and protozoa multiparasitism is well known and has been documented(Lee etal.,2002;Parketal.,2004;Sayasoneetal.,2011,2009;Sinuonetal.,2003;Schäretal.,
2014). Several endemic species of food-borne trematodes (FBT) have been identified, such as liver flukes(Opisthorchis viverrini and Fasciola spp.) and intestinal flukes(Fasciolopsis buski and Echinos- toma spp.) (Hien etal.,2001;KeiserandUtzinger,2005;Quang etal.,2008;Sayasoneetal.,2011;Sohnetal.,2011a,b). Helminthic multiparasitism poses a serious challenge for parasitological diag-nosis, as eggs from different species may morphologically resemble one another. For example, the size, oval shape and presence of small and inconspicuous operculum of Fasciola spp. Eggs are indis-tinguishable from F. buski and some Echinostoma spp. eggs.The average egg size of all three parasite egenera is similar in terms
of length and width: Fasciola hepatica 106.5–171.5/63.9–95.4m, Fasciola gigantica 150.9–182.2/85.1–106.2m (Valero etal.,2009), Echinostoma revolutum 97–117/61–65m (Sohn etal.,2011a); Echinostoma ilocanum 89–99/52–58m (Sohn etal.,2011b) and
F. buski 130-140/80-85m (Garcia andBruckner,1988). They canbe characterized as Fasciola-like eggs or large trematode eggs(LTE). In 2009,LTE were observed in the stools of 27/150 schoolchil-
dren (18.0%) in the Damrei Chhlang village primary school, Kandal province, Cambodia, during a standard parasiteological survey using
the Kato–Katz technique(Khieu etal.,2013). Given the high preva-lence of Fasciola spp. Infection among cattle in southern Cambodia
(Tum etal.,2007), LTE could have been attributable to Fasciola spp. infections. However, other trematode species could also have been responsible. The aim of this study was to assess the prevalence of LTE in the stools of schoolchildren, identify risk factors for infection and ascertain the trematode species, all with a focus on Fasciola spp. We performed across-sectional study among the schoolchildren,examined cattle droppings in and around Damrei Chhlang village and inspected cattle livers in a local slaughterhouse.
2. Materials and methods
2.1. Ethical considerations
Ethical clearance for this study was obtained from the Ethics
Commission of Basel-Stadtand Basel-Land, Switzerland(EKBB;
reference no.159/11) and from the National Ethics Commit-tee for Health Research(NECHR), Ministry of Health(MOH)in
Phnom Penh, Cambodia(referenceno.30/NECHR). Before field work started, the provincial and district health authorities, village and school authorities and parents were informed of the study and, in turn, working permission was granted. Prior to enrolling each participant, written informed consent was obtained from the par-ents, legal guardian or the participant him/herself if they were of legal age. In cases of illiteracy, the informed consent was signed by fingerprint before the village chief, who signed as a witness.
2.2.Study design, population and area
The study was carried out in Central Kandal province, which surrounds the capital Phnom Penh. It is in habited by 1.2 mil- lion people and divided into 11 districts. Between May and June 2011, across-sectional study was carried out among schoolchil- dren at the Damrei Chhlang village primary school, S’ang district (11◦21_29.43__ N, 104◦59_18.05__ E), a district of 1831 people. Another cross-sectional study aimed to estimate the prevalence of Fasciola spp. Eggs in cattle droppings in the villages surrounding the Damrei Chhlang primary school(S’ang district). The sevil- lages included Damrei Chhlang and Preaek Khmer(11◦21_21.70__ N, 104◦58_59.33__ E), both located in S’ang Phnom commune; and Preaek Run(11◦21_16.30__ N, 104◦59_46.54__ E), located in Preaek Koy commune. Additionally, an abattoir study of a small facility that is slaughtering local cattle was conducted in Preaek Koy commune.
2.3. Procedures in the field
On day 1,enrolled study participants assembled at the commu- nity hall next to the school. A trained interviewer questioned each child about potential risk factors for infection and about experi- ences of ill-health. Subsequently, a physician assessed participants’ general health status and clinical symptoms by using a standard- ized assessment form. Additionally, a nurse drew a venous blood sample of 5 ml from each child. The blood samples were stored in a cooling box at about 5–10 ◦C until they reached the laboratory of the National Center for Parasitology, Entomology and Malaria Control (CNM), Ministry of Health in Phnom Penh.
A pre-labeled stool container was given to each child, along with instructions on how to fill it. Children were asked to collect morn- ing stools to ensure freshness of the stool samples. The following morning, the filled container was collected and a new pre-labeled container was provided. Three stool samples were collected from each child over 4 consecutive days. Each day after collection, con- tainers were directly transported to the laboratory of the CNM for analysis.
The stool containers for cattle droppings were distributed among randomly selected cattle owners, each of whom collected one fresh stool sample from every one of their cattle. Contain- ers were collected In the morning, the day after distribution, and directly transported to the laboratory of the CNM.
The butcher at the study abattoir examined the livers of slaugh- tered cattle on a continuous basis for meat quality assurance. For study purposes, he recorded in a diary the total number of slaugh- tered cattle and of Fasciola spp. Infected livers.
2.4. Laboratory procedures
For each of the three stool samples per child, one Kato–Katz thick smear (Katz etal.,1972) was prepared directly after the samples arrived in the laboratory. After a half an hour clearance time, the smears were examined under a light microscope (magnification 400×) for intestinal helminth eggs. The number of eggs per parasite species was counted and recorded.
From each of the three stool samples per child, approximately1 g was preserved in 15 ml sodium acetate–acetic acid–formalin (SAF) fixative for analysis by formalin–ether concentration tech-nique (FECT)(Marti andEscher,1990). Four weeks after collection, the fixed samples were processed and examined under a light
microscope (magnification1000×) for intestinal helminthes and protozoa.
From the first stool sample per child, approximately 0.5 g of fresh stool was preserved in 70% ethanol for later molecular analy-sis using real-time quantitative polymerase chain reaction(PCR) to diagnose Fasciola spp. (Alasaad etal.,2011) and F. buski infections (unpublished, developed by Dr. med. Stefanie Kramme, Swiss TPH).The preserved samples were sent to the Swiss TPH laboratory in
Basel, Switzerland, where molecular analysis was carried-out about 2 months later. First, from each preserved stool sample,200_g was
washed twice with phosphate buffered saline(PBS)to eliminate the ethanol. Subsequently, DNA was extracted with the QIAamp DNA Stool Mini Kit(QIAGEN,USA).The extracted DNA was frozen at−20 _C until PCR was performed,1week later. On 0.5 g of the first stool samples from each child, a commercially available F. hepatica coproantigen enzyme-linked immunosorbent assay(ELISA)(Bio-X, Belgium) was performed at the CNM laboratory.
Upon arrival, blood samples were stored at room tempera-ture to clot. Subsequently, the blood samples were centrifuged and the serum was frozen and stored at −20 _C at the CNM laboratory. The frozen samples were sent to the Diagnostic Center of the Swiss TPH, Switzerland, where an ELISA for F. hepatica-antibodies was performed 2 months after sample col-lection. Sera samples with a positive result were retested with an immunofluorescence antibody test(IFAT)for confirmation. All sera samples with a positive or critical(close to posit