The complicated life cycle of D. latum is completed and maintained if:
• the feeding habits of the human population include ingestion of raw or inadequately prepared fish products of the species harbouring plercoid larvae;
• sanitary habits and sewage disposal facilitate the spread of worm eggs to appropriate natural waters;
• the water biotope available is suitable for development of the larval stages and has the plankton and fish species appropriate for the larvae.
The infection is established in an area only if all these prerequisites are present and, consequently, the life cycle and transmission of the parasite are broken if only one of these prerequisites is eliminated (von Bonsdorff and Bylund 1982).
Raw fish and fish products are highly appreciated dietary components in many population groups. Slightly salted or marinated filets of several freshwater fish species are traditional components in the meals in many regions in the Baltic area. The fillets are salted for one or a few days only and are eaten without further preparation. In addition, slightly salted hard roe is a delicacy served at home as well as in the most pretentious restaurants in the same area, a hazardous delicacy, however, as the roe of pike and burbot, for example, may contain larvae of D. latum. Dietary habits are usually local; thus, the infection is attributable to different fish dishes in different regions. Raw or insufficiently heated fish products, however, are always the components responsible for transmission of the infection to man. The present hygenic practice of flushing faecal products through water closets and sewage systems into lakes and rivers, greatly facilitates the spread of tapeworm infection, especially as the sewage purification plants in many smaller communities and villages are imperfect or absent. Water biotopes suitable for development of the larval stages exist in large parts of the temperate and subarctic areas. The embryonic development in the egg and hatching of the first larval stage normally takes place in fresh waters at temperatures between 4 and 25°C. However, the parasite life cycle is not strictly bound to a freshwater biotope; the larval development can take place in brackish water with a salinity up to about 0.5-0.6%. Thus, the larvae of D. latum was previously frequently encountered in coastal areas of the Baltic Sea where the salinity is low. Shallow littoral zones with water temperatures of 15-20°C provide the most favourable conditions for the parasite development. As the host specificity is not very pronounced, suitable species of intermediate hosts are present in most parts of the temperate and subarctic regions. Fish species like pikes, perches and burbots, most susceptible to the infection, have a more or less circumpolar range in the northern Hemisphere.
As the ecological conditions of the water biotopes in very large areas fulfil the requirements of the parasite, the dietary habits mainly are decisive for the spread of the disease; this also gives diphyllobothriasis its nature of an endemic disease. The reproductive potential of the parasite is very high. Due to this even sporadic worm carriers can give rise to high prevalence of larvae in a fish population. There are numerous examples where population transfers or immigrants retaining their dietary habits have induced new endemic foci of D. latum infection (Almer 1974; von Bonsdorff 1977). The role of domestic and wild animals in the epidemiological patterns of diphyllobothriasis is somewhat controversial. Although it has been firmly shown that a number of animals can harbour the parasite and release viable eggs, all available data indicate that the parasite is absent in regions where all ecological prerequisites for the parasite are fulfilled, including the presence of susceptible fish eating mammals, but human population is absent. Thus it seems reasonable to assume that animals are of minor or no importance for the dissemination of the infection.
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