Schistosomiasis mekongi: from discovery to control

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Abstract

In the Mekong River basin, the first case of schistosomiasis was reported in 1957. In the 1960s, endemic areas of the infection, of which profiles were similar to those of schistosomiasis japonica, were discovered in Khong Island, Laos, to Kratie province, Cambodia. A new intermediate snail host; Neotricula aperta was identified and the Mekong strain of schistosome was elevated to a new species: Schistosoma mekongi in 1978. Baseline epidemiological surveillance was performed and schistosomiasis mekongi was described as a public health implication in the middle Mekong River basin. Because of political and economical confusion, endemic situation had become worse, and no control program had been implemented until mass treatment program with praziquantel on Khong Island in 1983. Since then, the prevalence of S. mekongi infection has rapidly decreased in each endemic area. Serological diagnosis has been useful to detect new but low endemic foci. Clinical manifestations of S. mekongi infection are similar to those of S. mansoni and S. japonicum infections. As the reduction of prevalence and intensity of S. mekongi infection, morbidity due to the disease has changed, and ultrasonographic examination is now useful to evaluate morbidity due to schistosomiasis mekongi. Transmission of the disease occurs in a couple of months during low water season. Control of N. aperta is difficult and long-lasting effective control measurements have, so far, not been available. In the next step for controling S. mekongi infection, mass treatment should be continued, and it is needed to combine other appropriate control activities.

Section snippets

Introduction and historical background

The first case of Schistosoma mekongi infection was reported as ‘S. japonicum’ infection in a Laotian immigrant from Khong Island in 1957 [1]. Since then, some human cases have been reported from Thailand in 1959 and from Kratie province, Cambodia in 1968 [2], [3]. In order to find a new endemic focus of schistosomiasis along the Mekong River, some researchers performed biological and epidemiological surveillance in Thailand, Laos and Cambodia [4], [5], [6], [7], [8], [9], [10], [11]. A broad

Epidemiological situation before the 1980s

At the end of the 1960s, the first epidemiological surveillance was performed among the inhabitants who lived along the middle Mekong River in Laos and Cambodia (Table 1) [9], [10], [11]. Heavily endemic areas were found in Khong Island, Laos, and Kratie province, Cambodia. Schistosomiasis mekongi was, thus, described as a public health implication in the middle Mekong River basin, with population of more than 150 000. However, no control programme had been performed because of social and

Intermediate snail hosts

The intermediate snail host of S. mekongi is an aquatic snail, N. aperta consisting of three strains (alpha, beta and gamma). Both the alpha and gamma strains are commonly found in the main stream of the Mekong from Khong Island to Kratie province [24]. However, the beta strain is found only in the Mun River, a tributary of the Mekong River in Northeast Thailand (Fig. 1).

Although all the three strains show varied degree of capability of transmitting S. mekongi, the gamma strain alone is known

Pathology and clinical manifestations

The clinical manifestations of S. mekongi infection are similar to those of S. japonicum and S. mansoni infections [30], [31]. Acute schistosomiasis, or Katayama fever, follows initial infection with S. mansoni or S. japonicum. However, acute symptoms like Katayama fever has never been described in S. mekongi infections.

The primary lesion in S. mekongi infection is a granulomatous reaction to the deposited ova. In murine schistosomiasis mekongi, the reaction with foamy change surrounding liver

Reservoir hosts

Animal reservoirs for S. mekongi have been studied among domestic, wild and laboratory mammals. In 1968, 7 out of 24 dogs in Khong Island, were infected with S. mekongi by autopsy [10]. After mass treatment program, 12 out of 98 pigs (12.2%) were found to have infection of S. mekongi by stool or tissue examination [44]. Among 28 canine feces in Kratie Province, Cambodia, one (3.6%, egg density; 100/g stool) was positive for S. mekongi [45]

Before and after mass treatment program, S. mekongi

Achievement of mass treatment

In 1989, universal mass treatment with praziqauntel (PZQ) was performed for all the inhabitants on Khong Island [17]. In Cambodia, the cooperative program with Cambodian Government, WHO and Medicins sans Frontieres (MSF) started in 1995 [46], [47]. A Japanese NGO, Sasakawa Memorial Health Foundation joined this program in 1997. In schistosomiasis endemic areas of Kratie and Stung Treng provinces, the inhabitants have had universal mass treatment with PZQ every year. Efficacy of mass treatment

Acknowledgements

We express sincere thanks for financial support by Sasakawa Memorial Health Foundation and technical assistance by WHO. We thank Drs Cheam Saem and Choung Seng Ly, Kratie Provincial Health Office and Dr Heng Nhoeu, Stung Treng Provincial Health Office for cooperation in epidemiological survey. And we thank Prof. Kazuo Yasuraoka for his malacological contributions, and Drs Mam Bunheng and Moriyasu Tsuji for coordination of joint programme between Japan and Cambodia.

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