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Host density increases parasite recruitment but decreases host risk in a snail-trematode system

机译:宿主密度增加寄生虫招募,但减少了蜗牛-METRATODE系统中的主机风险

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摘要

Most species aggregate in local patches. High host density in patches increases contact rate between hosts and parasites, increasing parasite transmission success. At the same time, for environmentally transmitted parasites, high host density can decrease infection risk to individual hosts, because infective stages are divided among all hosts in a patch, leading to safety in numbers. We tested these predictions using the California horn snail, Cerithideopsis californica (=Cerithidea californica), which is the first intermediate host for at least 19 digenean trematode species in California estuaries. Snails become infected by ingesting trematode eggs or through penetration by free-swimming miracidia that hatch from trematode eggs deposited with final-host (bird or mammal) feces. This complex life cycle decouples infective-stage production from transmission, raising the possibility of an inverse relationship between host density and infection risk at local scales. In a field survey, higher snail density was associated with increased trematode (infected snail) density, but decreased trematode prevalence, consistent with either safety in numbers, parasitic castration, or both. To determine the extent to which safety in numbers drove the negative snail-density-trematode-prevalence association, we manipulated uninfected snail density in 83 cages at eight sites within Carpinteria Salt Marsh (California, USA). At each site, we quantified snail density and used data on final-host (bird and raccoon) distributions to control for between-site variation in infective-stage supply. After three months, overall trematode infections per cage increased with snail biomass density. For egg-transmitted trematodes, per-snail infection risk decreased with snail biomass density in the cage and surrounding area, whereas per-snail infection risk did not decrease for miracidium-transmitted trematodes. Furthermore, both trematode recruitment and infection risk increased with infective-stage input, but this was significant only for miracidium-transmitted species. A model parameterized with our experimental results and snail densities from 524 field transects estimated that safety in numbers, when combined with patchy host density, halved per capita infection risk in this snail population. We conclude that, depending on transmission mode, host density can enhance parasite recruitment and reduce per capita infection risk.
机译:大多数物种在本地补丁中聚合。斑块中的高宿主密度增加了宿主和寄生虫之间的接触率,增加了寄生虫传输成功。同时,对于环境传播的寄生虫,高宿主密度可以降低个体宿主的感染风险,因为感染阶段在贴片中的所有主机中划分,导致数量的安全性。我们使用加州喇叭蜗牛,Cerithideopsis Californica(= Cerithidea Californica)测试了这些预测,这是加州河口中至少有19位Digenean震颤物种的第一个中间宿主。蜗牛被摄入震颤鸡蛋或通过自由游泳的麦芽菊患者渗透到沉积在沉积的最终宿主(鸟类或哺乳动物)粪便中的自由游泳髓鞘感染。这种复杂的生命周期使感染性阶段生产免于传输,提高当地尺度的宿主密度和感染风险之间的反比关系的可能性。在田间调查中,较高的蜗牛密度与震颤(感染的蜗牛)密度增加有关,但普及的发育率降低,与数字,寄生阉割或两者均一致。为了确定数字安全的程度推动了负蜗牛密度 - 潮流量 - 流行关联,我们在83个笼子中操纵了未感染的蜗牛密度,在鲤鱼盐沼(加利福尼亚州,美国)的八个地点。在每个站点,我们量化了蜗牛密度和使用关于最终主机(鸟类和浣熊)分布的数据,以控制感染阶段供应的场地之间的变化。三个月后,每个笼子的总体震颤感染随着蜗牛生物质密度而增加。对于鸡蛋传播的虫斑,每蜗牛感染风险随着笼子生物量密度和周边地区的蜗牛生物量密度降低,而每蜗牛感染风险不会降低miracidium传播的震颤。此外,发抖招生和感染风险都随感染性阶段的输入而增加,但这仅为Miracidium传播物种而显着。通过我们的实验结果和蜗牛密度参数化的模型估计数量的安全性,当与斑点的宿主密度相结合时,本蜗牛人口中的每人均感染风险减半。我们得出结论,根据传输模式,宿主密度可以增强寄生虫募集并减少人均感染风险。

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