Giardia intestinalis has a worldwide distribution and represents the most frequently detected intestinal parasite in humans. The reported prevalence varies between 0.5 and 30% depending on the area. In Sweden, during recent years Giardia has been identified and reported in approximately 1500 cases annually (Smittsamma sjukdomar 2001).
Seasonal trends in giardiasis and dependence on sex and age have also been noticed (Addis et al. 1992). Higher prevalence is usually observed in sub-tropical and tropical countries and in regions with poor sanitation where giardiasis may be endemic (Meloni et al. 1993). The disease is more frequent in children than in adults. Parasites are transmitted directly from person to person by faecal-oral contamination with cysts or by ingestion of cyst-contaminated water or food. Ingestion of 10-100 cysts is enough to result in infection (Rendtorff 1954). Infection occurs frequently in day-care centres and within families of
infected children (Pickering et al. 1984). Transmission among male homosexuals has been described as a result of anal-oral sexual behaviour (Schmerin et al. 1978).
Despite the endemic form of transmission in certain regions (Fraser and Cooke 1991; Isaac-Renton et al. 1996), giardiasis appears repeatedly as a water-borne epidemic (Neringer et al. 1987; Moore et al. 1993). Travellers, campers, and hikers may become infected after drinking contaminated, untreated surface or ground water. Community-wide outbreaks occur as a result of faecally contaminated municipal water supplies. Man is the main reservoir for human infections. An infected individual can produce 105-107 cysts/gram of stool (Feachem et al. 1983). One human stool deposited in a medium-sized water reservoir (9.5 x 106 l) can lead to a density of 6-7 cysts/l (DeRegnier et al. 1989). Cysts are well adapted to survival and transmission in the environment; in the cold and humid conditions they retain their viability for several weeks. Although the zoonotic nature of the parasite and its host specificity are still controversial, there are some evidences for inter-species transmission (Davies and Hibler 1979; Thompson et al. 1988; Majewska 1994). Wild and domestic animals infected by morphologically identical parasites may represent reservoirs for human infections and thus contribute to the environmental transmission of the parasite.
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