There are two important species of liver fluke in Kenya - Fasciola gigantica and fasciola hepatica.

The former is the more important, being found throughout the lower warmer parts of the country. As the name suggests it is very large, being more than twice the size of Fasciola hepatica, which is found in the cooler highland areas of the country, and also in the temperate zones of the world such as Europe, North America, Australia and New Zealand. Liver flukes belong to a class of worms called trematodes.

The life cycle of liver flukes involves a snail, which acts as an intermediate host. If there is no snail there will be no flukes. The snail invoved requires still, stagnant water to survive Swift running water does not suit it. So swamps, ponds, lakes with a marshy edge and pools of water edged by vegetation are danger zones for grazing animals.

Cattle, sheep and goats are mainly affected, although other animals such as donkeys, horses, and many species of wildlife such as buffalo can also be infested. Humans can also be infested with liver flukes.

As the name suggests, the liver fluke attacks the liver.
Three forms forms of illness can occur.

A knowledge of the life cycle of the liver fluke is an aid in understanding how to control the disease.

The life cycle of Fasciola gigantica and hepatica both follow the same pattern. The intermediate host snail may differ but in other respects the life cycle is the same. This begins with the eggs of the fluke which mature in the bile ducts in the liver, pass down the ducts, into the gut and are excreted with the faeces. Once outside in the environment, which must contain water, the eggs hatch, releasing an active stage, called miracidia. These either actively invade a host snail or are eaten by a host snail. They then hatch in the snail's gut and a stage called a sporocyst develops in the tissues of the snail. 5 - 8 weeks later another stage called cercariae emerge from the snail and encyst on herbage or grass, where they are eaten by the final host - cattle, sheep, wildlife etc.

Temperature and time are critical in the early stages for the development of the miracidia- above 5-6 C, and best between 25 - 24C. They must find a suitable snail within 24 - 30 hours or they will die. Once ingested by the sheep or cow the cercariae develop into another stage called metacercariae. These invade the gut wall and travel to the liver where the now immature flukes wander through the body of the liver until they reach the bile ducts. Here they mature until they start to lay eggs and the life cycle begins again. It takes 10 - 12 weeks from infestation until eggs start to be laid. Mature flukes are long lived and sheep and cattle may be carriers for years.

The intermediate host snail employed by Fasciola hepatica is called Lymnea truncatula.That by Fasciola gigantica, Lymnea natalensis,but sometimes also truncatula.

The severity of clinical signs depends on the number of parasites ingested by the host animal over a short period of time.

Cattle appear to be able to develop an immune reaction to fluke infestation, but sheep do not. The acute form of the disease is more common in sheep than in cattle. Acute fascioliasis occurs 5 - 6 weeks after the ingestion of large numbers of metacercariae. There is a sudden invasion of the liver by masses of young liver flukes. Death takes place due to haemorrhage or liver failure, or both. The acute form of the disease is manifested by sudden death, or dullness, weakness, anaemia, lack of appetite, pain over the region of the liver and death in 48 hours. Because this is caused by migrating flukes which have not yet begun to lay eggs, no eggs are found in the faeces and diagnosis must rest on a post mortem examination. This will reveal a badly damaged, swollen liver. The capsule will have many small perforations and haemorrhages below the capsule. The body of the liver will contain tracts formed by the migrating flukes and the liver will be damaged and friable. Close examination will reveal the small immature flukes. This form of the disease occurs in sheep, rarely in cattle.

Cattle exhibit the sub-acute or chronic forms of the disease, as also do sheep and these can cause death in both.

These forms of the disease are due to the activity of the adult flukes in the bile ducts, causing anaemia and protein leakage. Body growth is reduced, there is loss of weight, submandibular oedema ("bottle jaw") and anaemia. Diagnosis is made by the finding of fluke eggs in faecal samples. These must be differentiated from those of Paramphistome rumen flukes.A post mortem examination will reveal large mature flukes in the bile ducts,which are usually thickened and which may be calcified in cattle, but not in sheep. There will be oedema, anaemia and often emaciation. Black Disease, caused by the multiplication of Clostridium Novyi in the tracts caused by migrating immature flukes is a disease of sheep aged sheep between 2 - 4 years. Acute fluke infestation usually kills younger animals.This age difference is helpful in making a diagnosis.

Diagnosis therefore rests on the clinical signs, grazing history, season of year- whether wet or dry-, post mortem findings, the finding of immature or mature flukes and the demonstration of fluke eggs in faecal samples.

Ideally control measures include the removal of flukes from affected animals, the reduction of the intermediate host snail populations and the prevention of livestock access to snail-infested pastures. In practice only the first of these is used in most cases.

Fencing off snail sites is feasible when the areas are small. Clearing away acquatic vegatation will help. As will the provision of adequately maintained water troughs with a good concreted area around them to prevent the area from becoming muddied. Drainage is the best permanent solution, but may be costly. Many chemicals used to control snails are poisonous to livestock and in many cases are inadequate to reduce snail populatons to levels sufficient to reduce snail infestations to acceptable levels.

Grazing livestock in Africa are more likely to contract liver fluke infestations during the dry season than in the wet, as during the dry season they enter snail infested marshes which they avoid when there is sufficient grazing elsewhere.In such situations is is advisable to keep animals out of such swamps for as long as possible so that fewer viable metacercariae are present on the herbage .When entry to the swamp is unavoidable then the older cattle should go in first as they are much more resistant than the younger cattle, followed, last of all, by the most susceptible sheep and goats.

By the use of these measures, together with the administration of fasciolicidal drugs administered at strategic intervals to those animals most at risk the disease can be considerably reduced.