GIS-based Liver Fluke Risk Forecasting System

“常见的肝吸虫，肝片吸虫，是一种影响绵羊和牛的吸虫寄生虫。_F hepatica_的生命周期涉及中间蜗牛宿主;在英国，主要物种是_Galba truncatula_，矮池塘蜗牛。虫卵通过受感染动物的粪便传播。一旦从粪便中冲洗出来，卵就会发育，释放出寿命短的毛蚴。这些需要水来游过，一旦它们找到蜗牛，就通过脚钻入体腔。吸虫在螺体内繁殖，约6周后释放出尾蚴，在植被上包囊形成感染性囊蚴。放牧牲畜吃了被污染的牧草后，囊蚴孵化，新孵化的幼蚴钻过肠壁，进入肝脏。在牛中，吸虫到达胆管、成熟并开始产卵需要大约10-12周，随后产卵沉积在粪便中。吸虫的生命周期需要_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育）。这些蜗牛以泥表面的藻类为食，排水不畅、pH值为中性或微酸性的缓慢流动水体是有利条件。蜗牛通常出现在池塘、溪流、蹄印和泥泞田地中的拖拉机车辙的边缘。在冬季，蜗牛进入冬眠，蜗牛体内寄生虫的任何阶段的发育都被阻止。在春天，蜗牛从冬眠中醒来，寄生虫的生命周期重新开始。在温和的冬天，蜗牛死亡较少，所以春天出现更多。卵的发育速度取决于温度;超过10ºC，发育需要2- 4周，天气越温暖，发育越快。肝片吸虫病是英国的一种季节性疾病，尽管近年来模式一直在变化。寄生虫的自由生活和软体动物内阶段的发展通常发生在5月至10月之间;在夏季理想的天气条件下，大量的囊蚴在8月至10月从蜗牛释放到牧场上。感染性包囊的数量取决于气候条件，通常在夏末和初秋达到高峰，尽管囊蚴可以全年存在于牧场上。因此，在风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免疫力或抵抗力，因此动物可以反复感染。"

"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."