Foodborne trematode infections

Fascioliasis epidemiology

Both animals and humans contract fascioliasis infection in the same way.

Transmission of the infection in the environment is usually perpetuated by animals. Humans do not typically contribute to the parasite's life-cycle; they are only occasionally infected after failure to observe basic hygiene measures (consuming larvae-contaminated uncooked vegetables or drinking larvae-infected water). Moreover, Fasciola worms are not well adapted to humans and, in some cases, fail to develop into mature adult worms and produce eggs.

In some areas, transmission to humans is constant and intense, and a geographical aggregation of cases may be observed. This pattern is possibly explained by a human-to-snail-to-plant-to-human transmission cycle, without the involvement of any animal. Indigenous communities in the South American highlands represent well known "hot spots" for fascioliasis: here, highly prevalent infection has been reported and may be explained by such transmission pathway.

Where it occurs sporadically, fascioliasis affects people from all age-groups, and there is no specific risk group. Where the infection is highly endemic, the prevalence and intensity of infection tend to peak in school-age children. People living in rural areas are typically more likely to become infected; however, cases may occur anywhere and can follow the trade routes of the carrier plants, which are part of the usual diet in many countries.

Fascioliasis is a global disease, and human cases have been reported from more than 75 countries worldwide. Recognized areas of high transmission are the highlands of south America, the Nile valley, the Caspian sea basin, as well as east Asia and south-east Asia. No countries can be considered free from the risk of fascioliasis.

The epidemiological pattern of fascioliasis is quite varied: the infection usually has a hypo-endemic pattern, with low and stable levels of prevalence among a defined population. Sporadic outbreaks may occur among such populations: these are usually related to sudden changes in climatic conditions that boost the life-cycle of either the parasite or the snail, or both. Scientists have also found that the epidemiology of fascioliasis is strictly linked to the geographical and environmental characteristics of the area where transmission occurs, and different patterns can be distinguished: this suggests that fascioliasis may adapt to different ecological niches.

In Africa and Asia, where both F. hepatica and F. gigantica are present, mixed infections are possible. In Asia, hybridization among the two species occurring in co-infected humans or animals has been described. The offspring resulting from such hybridization is characterized by intermediate morphological characteristics between the two species as well as by different ploidies (diploid, triploid, and mixoploid); such worms are frequently non-fertile.

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