GIS-based Liver Fluke Risk Forecasting System

“常见的肝吸虫--肝片吸虫是一种影响羊和牛的吸虫寄生虫。肝吸虫的生命周期涉及一个中间钉螺宿主；在英国，主要物种是矮小池塘蜗牛Galba truncatula。卵子通过受感染动物的粪便传播。一旦从粪便中冲走，虫卵就会发育，释放出短暂的毛虫。它们需要水才能游过，一旦它们找到蜗牛，就会通过脚挖洞进入体腔。吸虫在钉螺体内繁殖，在大约6周时，尾蚴被释放，包裹在植被上形成感染性囊虫。然后，放牧的牲畜吃掉受污染的草本植物，囊虫孵化出来，新长出的幼虫穿过肠壁，迁移到肝脏。在牛身上，大约需要10-12周的时间，吸虫才能到达胆管，成熟并开始产生卵子，然后这些卵子会沉积在粪便中。吸虫的生命周期需要有G truncatula_存在(尽管其他种类的蜗牛，如_Radix spp_可以支持寄生虫的发展)。这些蜗牛以泥表面的藻类为食，排水不良、pH中性或略酸性的缓慢移动的水体是有利的条件。在泥泞的田野中，蜗牛经常出现在池塘、溪流、蹄印和拖拉机车辙的边缘。在冬天，蜗牛进入冬眠，蜗牛体内任何阶段的寄生虫的发展都会被阻止。在春天，蜗牛从冬眠中走出来，寄生虫的生命周期重新开始。在温和的冬季，死亡的蜗牛较少，因此在春季死亡的蜗牛更多。卵的发育速度取决于温度；在10℃以上，发育需要2-4周，天气越热，发育越快。在英国，片吸虫病是一种季节性疾病，尽管近年来模式一直在发生变化。寄生虫的自由生活阶段和软体内阶段的发育通常发生在5月至10月之间；夏季天气条件理想，在8月至10月期间，大量囊虫从蜗牛释放到牧场。感染包囊的数量取决于气候条件，通常在夏末和初秋达到顶峰，尽管囊虫病可能全年都在牧场上存在。因此，在危险牧场放牧的牲畜在一年中的任何时候都可能受到感染。此外，几乎没有证据表明牛对吸虫感染产生了免疫力或抵抗力，因此动物可能会反复感染。

"The common liver fluke, _Fasciola hepatica_, is a trematode parasite affecting sheep and cattle. The lifecycle of _F hepatica_ involves an intermediate snail host; in the UK the principal species is _Galba truncatula_, the dwarf pond snail. Eggs are passed in the faeces of infected animals. Once washed out of the faeces, the eggs develop, releasing short-lived miracidia. These require water to swim through, and once they find the snail, burrow through the foot into the body cavity. The flukes multiply in the snail, and at about 6 weeks, cercariae are released, which encyst on vegetation to form infective metacercariae. Grazing livestock then eat the contaminated herbage, metacercariae hatch, and newly excysted juveniles burrow through the gut wall and migrate into the liver. In cattle it takes about 10-12 weeks for the flukes to reach the bile ducts, mature, and begin producing eggs that are subsequently deposited in faeces. The fluke lifecycle requires _G truncatula_ to be present (although other species of snail such as _Radix spp_ can support parasite development). These snails feed on algae on the surface of mud, and slow-moving bodies of water with poor drainage and a neutral or slightly acidic pH are favourable conditions. Snails are commonly found around the edges of ponds, streams, hoof-prints and tractor ruts in muddy fields. During winter, the snails go into hibernation, and development of any stages of the parasite in the snail is arrested. In spring, snails come out of hibernation and the parasite lifecycle resumes. During mild winters, fewer snails perish, so more are present in spring. The rate of egg development is dependent on temperature; above 10ºC, development takes 2--4 weeks, the warmer the weather, the faster development. Fasciolosis is a seasonal disease in the UK, although patterns have been changing in recent years. Development of the free-living and intramolluscan stages of the parasite typically occurs between May and October; with ideal weather conditions over the summer, large numbers of metacercariae are released from snails onto pastures in August to October. The number of infective cysts is dependent the climatic conditions, typically peaking in late summer and early autumn, although metacercariae can be present on pasture all year round. Thus livestock grazing at-risk pastures can become infected at any time of the year. Moreover, there is little evidence that cattle develop immunity or resistance to fluke infection, and so animals can be repeatedly infected."