The life cycle of D. latum involves the final host, principally man, and two intermediate hosts. (Figure 17.1). Although man is, without doubt, the most suitable final host for D. latum, it has been shown that several species of domestic and wild animals (dog, cat, pig, wolf, fox, otter seals, etc.) can harbour the parasite and release viable tapeworm eggs (von Bonsdorff 1977).
The adult parasite resides in the small intestine of the final host. The fish tapeworm is the largest parasite of man (Figure 17.2). Usually the length of the worm ranges from 5 to 10m (maximum width 2cm) but specimens reaching a length of 25m have been recorded. The small (45 x 65 •m), ovoid, operculate eggs are released in enormous numbers with the faeces from the definitive host. Each worm carrier expels 2040 million eggs a day. If the egg is discharged into fresh water the first larval stage, the coracidium, containing the six-hooked oncosphere, develops and hatches from the egg within 8-12 days at 16-20°C. The free-swimming coracidium larva hatches from the egg only in the presence of light. The coracidium has to be ingested by an appropriate planktonic copepode in order to continue the life cycle. The host specificity is not very pronounced at this stage and about 40 different species of freshwater copepodes have been recorded as first intermediate hosts of D. latum (von Bonsdorff 1977). Calanoids of the genera Diaptomus and Eudiaptomus and cyclopoids of the genus Cyclops are highly liable to infection and many species in these groups
have a circumpolar distribution. The larva penetrates into the hemocoel of this first intermediate host, where the next larval stage, the procercoid measuring 300-500 •m, develops. This development takes 26 weeks, depending on the water temperature.
If the copepode is ingested by a plankton-eating fish of a particular species the larva penetrates the intestinal wall and migrates to the muscles or various visceral organs of the fish and develops into the plerocercoidlarvae, the stage infective for the definitive host (Figure 17.3). The whitish plerocercoids are on average 5-15mm long but may reach even 40-50mm in length (Figure 17.4). They may occur in almost any organ of the fish, frequently also free in the abdominal cavity. Usually, the plerocercoids lie unencysted in the tissues of the fish but they may sometimes be enclosed in thin connective tissue cysts. From an epidemiological point of view, the occurrence of larvae in the muscle tissue and gonads of the fish are of main importance. If the plankton-eating fish is eaten by an appropriate predatory fish, the larvae may migrate to different organs of this fish and retain their infectivity. The larvae may survive and retain their viability for years in the fish.
Numerous species of freshwater fish may serve as second intermediate hosts for D. latum (von Bonsdorff 1977). In the endemic regions in the northwestern Europe the plerocercoids are mainly recorded from northern pike (Esox lucius), perch (Perca fluviatilis), burbot (Lota lota) and ruff (Acerina cernua) and these fish species must be considered the most important transmitters of fish tapeworm infection to man. Plerocercoids are rarely recorded from salmonid fish and when occurring in these fish they are encysted on the intestinal tract of the fish and thus are without significance for transmission of the infection to man as the entrails are discarded when the fish is cleaned. Contrary to previous statements it seems to be firmly established that coregonid fish do not harbour plerocercoids of D. latum although they frequently harbour larvae of other Diphyllobothrium species. Diphyllobothrium latum larvae are sporadically reported from some other freshwater fish species too but the validity of these findings is questionable; erroneous identification of larvae of this parasite group previously caused a considerable confusion concerning the true fish hosts for D. latum larvae (Bylund 1975).
In North America, the infection is referred to pike (Esox lucius), wall-eyed pike (Stizostedeon vitreum), sand pike (S. canadense), burbot (Lota maculosa), yellow perch (Perca flavescens), and possibly also Onchorhynchus species.
When fish tissues containing viable larvae are eaten by man (or another suitable host), the parasite establishes itself in the small intestine. Within 3-4 weeks it grows and develops into an adult, egg-producing worm. The growth rate during this phase is very fast, up to 22cm per day, that is, almost 1cm per hour.
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