Several species of the cestode genus Diphyllobothrium are parasites of man. By far, the most important and the species usually associated with diphyllobothriasis or fish tapeworm disease is D. latum (Linnaeus 1758; Lyhe 1910). The main distribution area of this parasite is the north temperate and subarctic regions of Eurasia and North America. It is important to recognize, however, that several other Diphyllobothrium species are frequently responsible for human infections. Diphyllobothrium dendriticum appears to be the species responsible for most human infections throughout the circumpolar region at high latitudes, beyond the range of D. latum (Curtis and Bylund 1991). Several species are implicated in human infections in communities bordering the Northern Pacific Ocean (D. ursi, D. dalliae, D. nihonkaiense, D. klebanovski, D. yonagoence, etc.). However, these species are recorded only from sporadic infections in man and apparently they are of minor importance from an epidemiologic point of view.

In 1973, Carneri and Vita (ref. von Bonsdorff 1977) estimated the number of human Diphyllobothrium carriers at 9 million. As routine examination for tapeworm ova is no more performed in hospitals in the

Nordic countries, there is no reliable estimate of the parasite prevalence in this region today, but apparently the prevalence figures have gone down during the last decades. A well-known focus for human diphyllobothriasis has been the eastern and southern Baltic region and adjacent areas of Russia. In Finland, for example, 20-25%

of the population harboured the fish tapeworm in 1952; 20 years later, the infection prevalence was 1.8% (Wikström 1972). There are also very sparse data available on the prevalence of Diphyllobothrium infection in northern native communities of the Eurasian continent and North America, that is, in the areas where human infections apparently has to be referred to other species than D. latum.

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