Human fascioliasis: review provides fresh perspectives on infection and control
27 July 2018 | Geneva –– An extensive review that addresses various aspects of transmission and epidemiology of human fascioliasis provides new approaches for the control of this neglected tropical disease. Fascioliasis – an infection mainly of the liver – is estimated to affect 2.4 million people in more than 70 countries, with several millions at risk.
The lead author of the review paper is Professor Santiago Mas-Coma et al. of the University of Valencia, Spain. Human fascioliasis infection sources, their diversity, incidence factors, analytical methods and prevention measures analyses infection sources and the ingestion of freshwater plants as the main source.
“Fascioliasis is spreading as a result of climate and global changes1 which are impacting our environment,” said Professor Mas-Coma. “We note that prevalence of infection is high among children in some localities and we need to be able to design the appropriate control measures for given endemic areas”.
Watercress, a freshwater plant involved in Fasciola transmission.
The paper discusses issues such as vegetables sold in uncontrolled urban markets and the distinction between infection sources by freshwater cultivated plants, terrestrial wild plants and terrestrial cultivated plants. It also addresses the risks posed by consuming traditional local dishes made from sylvatic plants and ingestion of raw liver, including drinking contaminated water, beverages and juices, and washing vegetables, fruits and kitchen utensils with contaminated water.
Other features of the review include:
- an analysis of the different transmission patterns, often linked to geographical and ecological variables, which indicate the high adaptability of the causative parasite to different conditions;
- risk factors related to community, familial and social variables;
- the different sources of infection and the methods used to assess their role in transmission; and
- a detailed overview of the viability and infectivity of the metacercariae.
The paper also outlines how to prevent transmission of Fasciola spp. This includes documented effects of physical and chemical agents on infectivity of metacercariae; and measures to be implemented to prevent or control infection at individual and population levels. Currently, WHO recommends the large-scale treatment of affected populations with triclabendazole2 in highly endemic areas.
Control also involves disease awareness, dissemination of health education and the cultivation of vegetables in water which is free of faecal pollution. Thorough cooking of vegetables before consumption is also important.
Although veterinary public health approaches combined with environmental measures can support long-term control, the paper considers a range of other public-health measures to strengthen control.
Fascioliasis is caused by parasitic flatworms (trematodes or leaf-shaped worms) that mainly affect the liver. It is acquired when people ingest aquatic vegetables to which the larval forms of the parasite are attached. Two species of trematodes cause fascioliasis: Fasciola hepatica and F. gigantica.
The life cycle starts when infected animals defecate in fresh-water sources. Since the worm lives in the bile ducts of such animals, its eggs are evacuated in their faeces. These hatch into larvae that lodge in a particular type of water snail (the intermediate host).
Once in the snail, the larvae reproduce and eventually release more larvae into the water. These larvae swim to nearby aquatic or semi-aquatic plants, where they attach to the leaves and stems and form small cysts.
When the plants with the small cysts attached are ingested, they act as carriers of the infection. Watercress (Nasturtium officinale) and water-mint are effective plants for transmitting fascioliasis.
1Global change in this context refers to artificial modifications of the environment, for example irrigation of vegetable plantations or import/export of livestock.
2Novartis has donated triclabendazole to WHO since 2005. The agreement was renewed in 2018 to extend the donation of 600 000 tablets annually until 2022, thereby allowing endemic countries to continue receiving the medicine for treatment of the disease.