Schistosoma mekongi May Be More Widespread than Previously Thought

A new genetic analysis suggests that the parasitic worm Schistosoma mekongi is more widespread than previously thought. According to the study, the human population at risk of infection could be up to 10 times greater than estimated. Furthermore, the results of the analysis suggest that there is an increased possibility of the spread of the parasite across Laos and Vietnam.

Before this study, it was assumed that S. mekongi originating in Yunnan, China, migrated southwards across Laos and into Cambodia, and later became extinct in Laos due to conditions unsuitable for transmission. However, investigators Dr. Stephen W. Attwood of China's Sichuan University (Chengdu, China) and colleagues analyzed DNA sequences of sample organisms collected from the Mekong River and its tributaries in Cambodia, Laos, and Malaysia. They found that S. mekongi was confined to a small section of the lower Mekong River, as well as in its tributaries across Cambodia. In addition, the range of the snail intermediate host and the ecologic conditions for potential transmission were shown to be much broader than once thought. The analysis suggested a more recent, and ongoing, migration northwards from Vietnam, towards Cambodia and Laos.

Schistosomiasis, a parasitic disease that affects over 200 million people worldwide, is caused by several flatworm species of the genus Schistosoma. In the Mekong River basin in South-East Asia, the disease is transmitted by the species S. mekongi. A mass treatment program in the mid-1990s greatly reduced the prevalence of the disease.

According to the investigators, further work is required to investigate this problem because if there is no reason to assume that ecologic conditions in Laos are unsuitable for transmission, it is still possible that in the future this disease will spread northward, from Vietnam and Cambodia into Laos.

The study was published in the March 2008 issue of the journal PLoS Neglected Tropical Diseases.


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