Liver fluke disease (fasciolosis) is caused by Fasciola hepatica and is economically important in cattle and sheep in temperate areas.
Animals are infected by ingesting larval stages (metacercariae) with herbage on contaminated pasture. In the gut, larvae are freed from the cysts, penetrate the intestinal wall and migrate to the liver. Here, they migrate for 6-10 weeks causing damage. Juvenile flukes reach the biliary system where they may live for years feeding from blood and releasing 1000s of eggs that are released with the bile into the small intestine and then reach pasture via faeces. If humidity is high, a small larval stage (miracidium) develops and after leaving the egg, penetrates the intermediate host (fresh-water snail). In the snail, the larva grows via sporocyst and redia stages and multiplies asexually. Finally 500-1000 larvae (cercariae) leave the snail and settle on grass as metacercariae.
Economic losses caused by liver fluke are due to reduced meat and milk production. Depending on the level of infection, the parasite may cause meat production losses of up to 20% in cattle and 30% in sheep, and losses of milk production of up to 8% in cattle. Severe infections cause death in young animals, particularly in lambs and kids. In addition, fasciolosis causes reduced fertility and wool productio and increases susceptibility to other diseases such as salmonellosis and bovine Tuberculosis (TB), as well as serious interference with diagnostic testing for TB.
Fasciolosis is a neglected tropical disease in humans, where it is found in all 5 continents, especially in areas of Latin America, Asia and Africa where sheep or cattle are reared. People usually become infected by eating raw watercress or other water plants contaminated with immature parasite larvae.
Control in livestock is mainly based on the use of drugs (anthelmintics), prophylactically/routinely, as well as the treatment of individual clinical cases. Overuse of anthelmintics such as triclabendazole has induced resistant strains of F. hepatica, and this has been reported worldwide. Anthelmintic resistance, together with climate change, have been proposed as causes of the increased prevalence of liver fluke disease during the last years in many regions.
Liver damage due to chronic fluke infection
Vaccines are a promising tool to help control fasciolosis since protection is more durable than that conferred by anthelmintics. Vaccines are clean and compatible with organic farming. Since F. hepatica modulates the host immune response, developing protective vaccines for cattle and sheep is a challenge. In PARAGONE, the use of cocktail vaccines including several parasitic molecules relevant to parasite biology or immune modulation may engender consistent protection.