A Population Growth Trend Analysis for <i>Neotricula aperta</i>, the Snail Intermediate Host of <i>Schistosoma mekongi</i>, after Construction of the Pak-Mun Dam

<div><p>Background</p><p>The Pak-Mun dam is a controversial hydro-power project on the Mun River in Northeast Thailand. The dam is sited in a habitat of the freshwater snail <i>Neotricula aperta</i>, which is the intermediate host for the parasitic blood-fluke <i>Schistosoma mekongi</i> causing Mekong schistosomiasis in humans in Cambodia and Laos. Few data are available which can be used to assess the effects of water resource development on <i>N. aperta</i>. The aim of this study was to obtain data and to analyze the possible impact of the dam on <i>N. aperta</i> population growth.</p><p>Methodology/Principal Findings</p><p>Estimated population densities were recorded for an <i>N. aperta</i> population in the Mun River 27 km upstream of Pak-Mun, from 1990 to 2011. The Pak-Mul dam began to operate in 1994. Population growth was modeled using a linear mixed model expression of a modified Gompertz stochastic state-space exponential growth model. The <i>N. aperta</i> population was found to be quite stable, with the estimated growth parameter not significantly different from zero. Nevertheless, some marked changes in snail population density were observed which were coincident with changes in dam operation policy.</p><p>Conclusions/Significance</p><p>The study found that there has been no marked increase in <i>N. aperta</i> population growth following operation of the Pak-Mun dam. The analysis did indicate a large and statistically significant increase in population density immediately after the dam came into operation; however, this increase was not persistent. The study has provided the first vital baseline data on <i>N. aperta</i> population behavior near to the Pak-Mun dam and suggests that the operation policy of the dam may have an impact on snail population density. Nevertheless, additional studies are required for other <i>N. aperta</i> populations in the Mun River and for an extended time series, to confirm or refine the findings of this work.</p></div

Fit of conventional linear models to the population density estimates for <i>Neotricula aperta</i>.

<p>For the Simple Linear Regression, t represents time in weeks from the first sample (t = 0). <i>P</i> values:. >0.2,</p>*<p><0.0001.</p><p>The equation for the negative binomial is exp(−0.0001189t+7.1245608), full data, and exp(−0.0004606t+7.382287) excluding outlier.</p

Summary statistics for the time series observations of <i>Neotricula aperta</i> population density in the Mun river at Ban Hin Laht.

<p>The sampling period was 1990–2011.</p

Confidence intervals (95%) for estimates of mu, with the “best-fit” GSS model, based on the original (empirical) data set, from bootstrapped data sets and from simulations (where mu = 0).

<p>Confidence intervals (95%) for estimates of mu, with the “best-fit” GSS model, based on the original (empirical) data set, from bootstrapped data sets and from simulations (where mu = 0).</p

Model-based predictions of the 1991, 1995 and 2002 population densities.

<p>Values are given ± a 95% confidence interval (two-tailed); those for SLR and GLM are based on the standard error of the regression and those of the GSS on bootstrap resampling. The observed densities were 300 m⁻², 2108 m⁻², and 979 m⁻² for 1991, 1995 and 2002 respectively. Predictions were made using the data set excluding the 1991 observation. The time series was also extrapolated to give the expected density in 2020 under each model.</p

Statistics relating to each data set used in the analyses.

<p>Length, total number of sites in alignment; Taxa, number of taxa; Length (no gaps), total number of sites excluding those with alignment gaps; H, haplotype diversity; D, Jukes-Cantor corrected nucleotide diversity based on the total number of mutations; PS, number of polymorphic sites (with parsimony informative sites in parentheses); PT, significance of Tajima's test for neutrality based on the total number of mutations. The COI+12S dataset included only the unique haplotypes.</p

Loci sequenced and sample sizes for the taxa collected during this study.

<p>Loci for which no corresponding sequence data had been previously published for the taxon concerned are emboldened. Populations (Code) previously unknown in phylogentic studies are also shown in bold. Sequence lengths are given in base pairs. For explanation of codes see <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0000200#pntd-0000200-t001" target="_blank">Table 1</a>.</p

DNA sequence data obtained from published sources and used in the present study.

<p>DNA sequence data obtained from published sources and used in the present study.</p

Taxa sampled, collecting sites (populations) and dates of field collection for samples used during the present study.

<p>Life cycle stage sampled: L, from a laboratory line (with year line established); M, sampled from a new infection using cercariae from naturally infected snails; N, natural infection, i.e. sampled from a natural infection in field trapped definitive hosts. Numbers of infected snails or field trapped rodents (as applicable) used are given in parentheses under ‘Isolate details’.</p

Results of a Bayesian estimation of divergence times (in millions of years) for nodes representing the most recent common ancestor (MRCA) of relevant clades.

<p>ESS, effective sample size; HPD, the 95% highest posterior density interval (equivalent to a confidence interval); Likelihood, posterior log likelihood (of the model, given the observed data); TMRCA, time to MRCA (/Ma). Explanation of clades: ingroup, MRCA of all taxa excluding the outgroup (<i>S. incognitum</i>); <i>japonicum</i>, MRCA of <i>S. japonicum</i> and all ingroup taxa excluding <i>S. sinensium</i>; <i>malayensis</i>, MRCA of <i>S. malayensis</i> and all <i>S. mekongi</i> taxa; <i>mekongi</i>, MRCA of all <i>S. mekongi</i> taxa.</p