HUMAN FASCOLIASIS : EPIDEMIOLOGICAL PATTERNS IN HUMAN ENDEMIC AREAS OF SOUTH AMERICA , AFRICA AND ASIA

Fascioliasis is an old disease found in prehistoric human populations of the Stone Age, the Mesolithic and the Neolithic periods, and caused by Fasciola hepatica, present in Europe, Africa, Asia, the Americas and Oceania, and F. gigantica, mainly distributed in Africa and Asia. Considered a secondary disease until the mid90s, recent research has furnished numerous hitherto-unknown aspects and new information which have given rise to a complete new general picture of this disease, explaining why human fascioliasis has recently been included within the list of important human parasitic diseases by WHO. Liver fluke development is very dependent of the environmental characteristcs according to the nature of free living life cycle phases, which take place fully in the external freshwater milieu, and the vector phase, which develops completely within a freshwater lymnaeid snail, in its turn also very dependent from the environment. Despite of these restrictions, fascioliasis has become the vector-borne disease presenting the widest latitudinal, longitudinal and altitudinal distribution known. Throughout its large geographical distribution, fascioliasis is a well-known veterinary problem. Moreover, studies carried out in recent years have shown it to be an important public health problem as well, human cases having been increasing in 51 countries of the five continents. However, a global analysis of the geographical distribution of human cases shows that the expected correlation between animal and human fascioliasis only appears at a basic level. High prevalences in humans do not seem to be necessarily related to areas where fascioliasis is a great veterinary problem. Three types of human endemic situations in areas presenting human fascioliasis have been established: hypoendemic, mesoendemic, and hyperendemic. The epidemiological classification also includes situations of imported cases and authochthonous, isolated, non-constant cases, as well as epidemic situations, with epidemics in animal endemic areas and epidemics in human hypo-, mesoand hyperendemic areas. Surveys in human endemic areas have proved that, although more prevalent and intense in children, adult subjects are also infected. The gender effect in fascioliasis is worth mentioning, prevalences and/or intensities in human hyperendemic areas appearing to be significantly higher in females. Several human hypoto hyperendemic areas present a very wide spectrum of epidemiological characteristics related to the very wide diversity of environments. According to the results obtained in the research activities carried out in the different human endemic areas of Europe, the Americas, Africa and Asia, it may be concluded that (i) fascioliasis shows a great adaptation power to new environmental conditions which is the consequence of its own capacities together with the adaptation and colonization abilities of the lymnaeid snail hosts, (ii) well known epidemiological patterns of fascioliasis may not always explain the transmission characteristics in a given area, (iii) when dealing with an endemic zone not previously studied, known epidemiological patterns of fascioliasis must always be taken into account merely as the starting base, (iv) control measures must consider the results of the ecoepidemiological studies undertaken in the zone concerned, as adaptation to new environmental conditions may change transmission characteristics. of European origin, with Galba truncatula as the original intermediate host species. In Europe it has been even found in prehistoric human populations of the Stone Age, living at the end of the Mesolithic period, 5000-5100 years ago and the Neolithic (Bouchet, 1997; Aspöck et al, 1999; Dittmar and Teegen, 2003), a period marked by the domestication of animals and the development of agriculture, among other characteristics. Human fascioliasis was considered a secondary disease until the mid-90s. This old disease has a great expansion power thanks to the large colonization capacities of its causal agents and vector species, and is at present emerging or re-emerging in many countries, including both prevalence and intensity increases and geographical expansion. WHO (PVC, Correspondence: Prof Dr Santiago Mas-Coma, Departamento de Parasitología, Facultad de Farmacia, Universidad de Valencia, Av. Vicent Andrés Estellés s/n, 46100 Burjassot, Valencia, Spain. Tel: 34-96-354-42-98; Fax: 34-96-354-47-69 E-mail: S.Mas.Coma@uv.es INTRODUCTION Fascioliasis is an important disease caused by two digenetic trematode species: Fasciola hepatica in Europe, Africa, Asia, the Americas and Oceania, and F. gigantica in Africa and Asia and with less important endemic areas in the southern parts of Europe, Turkey, the Near East, some southern states of the old USSR, particularly Armenia, and only sporadically mentioned in North America. Fasciola hepatica is believed to be SOUTHEAST ASIAN J TROP MED PUBLIC HEALTH 2 Vol 35 (Suppl 1) 2004 CPE, Headquarters Geneva) decided to launch a worldwide initiative against human fascioliasis including two main axes: A) transmission and epidemiology studies; B) control activities by mainly treatments with triclabendazole (Egaten®), a singledose highly effective drug. Results obtained in the research activities included in this WHO initiative during the last years have furnished numerous hithertounknown aspects and new information which have given rise to a complete new general picture of this disease, explaining why human fascioliasis has recently been included within the list of important human parasitic diseases by WHO.