Department of Biology, Radford University, Radford, VA 24142, USA.
Parasitol Res. 2013 Feb;112(2):799-805. doi: 10.1007/s00436-012-3203-4. Epub 2012 Dec 13.
Many trematodes infect a single mollusk species as their first intermediate host, and then infect a variety of second intermediate host species. Determining the factors that shape host specificity is an important step towards understanding trematode infection dynamics. Toward this end, we studied two pond snails (Physa gyrina and Helisoma trivolvis) that can be infected as second intermediate hosts by the trematode Echinostoma trivolvis lineage a (ETa). We performed laboratory preference trials with ETa cercariae in the presence of both snail species and also characterized host suitability by quantifying encystment and excystment success for each host species alone. We tested the prediction that trematodes might preferentially infect species other than their obligate first intermediate host (in this case, H. trivolvis) as second intermediate hosts to avoid potentially greater host mortality associated with residing in first intermediate hosts. In our experiments, ETa had roughly equivalent encystment success in Helisoma and Physa snails, but greater excystment success in Physa, when offered each species in isolation. Also, the presence of the symbiotic oligochaete Chaetogaster limnaei in a subset of Helisoma snails reduced encystment success in those individuals. When both hosts were present, we found dramatically reduced infection prevalence and intensity in Helisoma-ETa cercariae strongly preferred Physa. Thus, the presence of either an alternative host, or a predator of free-living parasites, offered protection for Helisoma snails from E. trivolvis lineage a infection.
许多吸虫类寄生虫以单一的软体动物物种作为第一中间宿主进行感染，然后再感染各种第二中间宿主物种。确定形成宿主特异性的因素是了解吸虫类寄生虫感染动态的重要步骤。为此，我们研究了两种圆田螺（Physa gyrina 和 Helisoma trivolvis），它们可以作为第二中间宿主被吸虫类寄生虫 Echinostoma trivolvis 谱系 a（ETa）感染。我们在存在这两种蜗牛物种的情况下进行了 ETa 尾蚴的实验室偏好试验，并且通过单独量化每种宿主物种的包囊和逸出成功率来表征宿主适宜性。我们测试了这样一种预测，即吸虫类寄生虫可能会优先感染与其强制性第一中间宿主（在这种情况下为 H. trivolvis）不同的物种作为第二中间宿主，以避免与寄居在第一中间宿主中相关的潜在更大的宿主死亡率。在我们的实验中，ETa 在 Helisoma 和 Physa 蜗牛中的包囊成功率大致相同，但在单独提供每种物种时，在 Physa 中逸出成功率更高。此外，在一组 Helisoma 蜗牛中，共生寡毛类 Chaetogaster limnaei 的存在降低了这些个体的包囊成功率。当两种宿主都存在时，我们发现 Helisoma-ETa 尾蚴的感染率和强度显著降低，Helisoma 蜗牛强烈偏好 Physa。因此，无论是替代宿主的存在，还是自由生活寄生虫的捕食者的存在，都为 Helisoma 蜗牛提供了免受 E. trivolvis 谱系 a 感染的保护。
