Observations on Neotricula aperta (Gastropoda: Pomatiopsidae) population densities in Thailand and central Laos: implications for the spread of Mekong schistosomiasis

Abstract

BackgroundThe snail Neotricula aperta transmits Mekong schistosomiasis in southern Laos and Cambodia, with about 1.5 million people at risk of infection. Plans are under consideration for at least 12 hydroelectric power dams on the lower Mekong river and much controversy surrounds predictions of their environmental impacts. Unfortunately, there are almost no ecological data (such as long term population trend studies) available for N. aperta which could be used in impact assessment. Predictions currently assume that the impacts will be the same as those observed in Africa (i.e., a worsening of the schistosomiasis problem); however, marked ecological differences between the snails involved suggest that region specific models are required. The present study was performed as an initial step in providing data, which could be useful in the planning of water resource development in the Mekong. Snail population density records were analyzed for populations close to, and far downstream of, the Nam Theun 2 (NT2) project in Laos in order to detect any changes that might be attributable to impoundment.ResultsThe population immediately downstream of NT2 and that sampled 400 km downstream in Thailand both showed a long term trend of slow growth from 1992 to 2005; however, both populations showed a marked decline in density between 2005 and 2011. The decline in Thailand was to a value significantly lower than that predicted by a linear mixed model for the data, whilst the population density close to NT2 fell to undetectable levels in 2011 from densities of over 5000 m-2 in 2005. The NT2 dam began operation in 2010.ConclusionsThe impact of the NT2 dam on N. aperta population density could be more complex than first thought and may reflect the strict ecological requirements of this snail. There was no indication that responses of N. aperta populations to dam construction are similar to those observed with Bulinus and Schistosoma haematobium in Africa, for example. In view of the present findings, more ecological data (in particular population density monitoring and surveillance for new habitats) are urgently required in order to understand properly the likely impacts of water resource development on Mekong schistosomiasis.

Malacological and parasitological surveys along the Xe Bangfai and its tributaries in Khammouane Province, Lao PDR

“…This observation is in agreement with earlier studies, which prior to 2006 reported very high population densities (> 5000 ind.m -2 ) of the snail Neotricula aperta (Triculinae) at Mahaxai (Attwood & Upatham, 1999), but large decline of this population by 2011, which was attributed to elimination of low water conditions by the release of the NT2 Power House (Attwood & Upatham, 2012). For instance, calcium concentration changes could be an explanation of the decline of the population.…”

“…2), showed a high biodiversity and was dominated by triculine snails of the Pomatiopsidae; however, by April 2010 there were no molluscs found at the same site. In the Lao PDR, April has been cited as the time of year when population densities of adult Triculinae can be expected to reach their peak (Attwood, 1995), with population densities of Neotricula aperta alone reaching , on average across years, at XBF2 before the project (Attwood & Upatham, 2012). Consequently, the normal expectation would be for population densities and biodiversity to have remained high or to increase as more snails grew in size and became detectable.…”

Spatial and temporal variation of benthic macroinvertebrates in the Nam Gnom Basin receiving discharged waters from the Nam Theun 2 Reservoir (Lao PDR)

“…Interannual variation appeared to be a variable playing a significant role on the macroinvertebrates population structure and composition. Pomatiopsidae showed the highest correlation with the temporal gradient suggesting a turnover of populations in accordance to Attwood & Upatham (2012) who identified natural growth-decline cycles in Thailand and central Lao PDR.…”

A New Population and Habitat for Neotricula aperta in the Mekong River of Northeastern Thailand: A DNA Sequence-Based Phylogenetic Assessment Confirms Identifications and Interpopulation Relationships

“…1 However, the small size of triculine snails leads to problems in assessing character states and technical difficulties in their identification. These problems together with convergent evolution in the group and a relatively high degree of intraspecific variation in anatomical characters suggest that nucleotide sequence data will be useful in resolving relationships in cases where anatomy is ambiguous.…”