Seasonal cercarial emergence patterns of Opisthorchis viverrini infecting Bithynia siamensis goniomphalos from Vientiane Province, Lao PDR

Abstract

Background: Snail intermediate hosts play a pivotal role in maintaining the life cycles of trematodes, including Opisthorchis viverrini. We investigated the emergence patterns of O. viverrini cercariae infecting Bithynia siamensis goniomphalos at foci in an endemic area in Vientiane Province, Lao PDR. Findings: Samples of B. s. goniomphalos collected during the hot-dry, rainy and cool-dry seasons were examined for O. viverrini infection by cercarial shedding. Emergence of cercariae from O. viverrini-positive snails was monitored daily from 06:00-18:00 h for seven consecutive days at 2 hourly intervals. Snail infections varied seasonally, being highest in the cool-dry season. Peak cercarial emergence was not consistent in different seasons, occurring between 08.00-10.00 h during the hot-dry season and between 12.00-14.00 h during the rainy and cool-dry seasons. The cercarial output was highest in the hot-dry season. The prevalence of infection and the emergence of cercariae were strongly dependent on snail size. Conclusions: This study shows that size of snails and environmental factors (such as season) may affect the emergence patterns of cercariae of O. viverrini in snails. These results have both fundamental and applied implications for opisthorchiasis epidemiology and control.

Seasonal and Spatial Environmental Influence on Opisthorchis viverrini Intermediate Hosts, Abundance, and Distribution: Insights on Transmission Dynamics and Sustainable Control

“…We believe that the low prevalence usually documented in the literature and the near absence of snail infection reported in our study reflect either 1) the biological processes underlying Ov development in the snails and/or 2) the limitation in our ability to detect Ov infection in snails through shedding procedures. For instance, infected snails can shed hundreds of cercaria daily [78] and accordingly it is assumed that one infected snail is enough to trigger infection prevalence in fish. It is thus relatively easy to imagine how unlikely finding infected snails can be even in highly endemic areas.…”

The role of evolutionary biology in research and control of liver flukes in Southeast Asia

“…Parasite genotypes that allow a good match between the timing of cercarial release and the presence as well as susceptibility of the target host should be favored. In the case of liver Fluke both the size of the snails and environmental factors are known to influence cercarial emergence patterns (Kaewkes et al, 2012; Kiatsopit et al, 2014). In a recent study, cercarial output was highest during the hot-dry season and during day time suggesting that peak cercarial release may be driven by temperature and light intensity (Kiatsopit et al, 2014).…”

“…In the case of liver Fluke both the size of the snails and environmental factors are known to influence cercarial emergence patterns (Kaewkes et al, 2012; Kiatsopit et al, 2014). In a recent study, cercarial output was highest during the hot-dry season and during day time suggesting that peak cercarial release may be driven by temperature and light intensity (Kiatsopit et al, 2014). While the adaptive significance of diurnal emergence, which may increase the cercariae chances to encounter fish mostly active during the day, appears plausible, the adaptive significance of peak cercariae release during the hot-dry season is more difficult to understand as fish abundance may be reduced during this drought-prone period.…”

“…Several other mechanisms, adaptive or not, may thus contribute to the patterns observed. For example, higher temperature and brighter conditions contribute to enhance snail metabolism, which may provide more energy resources to the parasites (Kiatsopit et al, 2014). In some digenean species cercarial production declines with the age of infection, a strategy assumed to maintain the host alive longer which inadvertently prolongs the period of cercarial production.…”

Prospects and Challenges towards Sustainable Liver Fluke Control

“…Subsequent simulations incorporating these experimental data, as well as regional temperature and precipitation records, snail abundance and egg availability in the environment, provided a useful modeling framework to assess and predict seasonal transmission risk [24]. The model outputs suggested that transmission risk varies greatly over the year (see also [25,26]), with very low transmission risk during the dry season and increasing transmission risk at the onset of the rainy season (i.e. June–July) with a peak in September.…”