GIS-based Liver Fluke Risk Forecasting System

"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."