Anisakis simplex, known as 'herring-worm' and 'whale-worm', is only one larval type among several, occurring in marine fishes (Figure 16.1). The other important one is Pseudoterranova
decipiens - 'cod-worm' or 'seal-worm'- formerly known as Porrocaecum decipiens, Terranova decipiens, and Phocanema decipiens (Figure 16.2). A third larval type is Contracaecum/ Phocascaris (Figure 16.3). Species in these genera live as sexually mature adult males and females in the stomach of their definitive warm-blooded hosts. Anisakis simplex occurs mainly in porpoises and whales (cetaceans). It is also found in seals (pinnipeds), P. decipiens is found in various seals, and Contracaecum/Phocascaris also in seals (Contracaecum species also occur in birds, the taxonomy of these genera is not resolved). Having similar morphology and biology, they are all placed in the family Anisakidae, and their larvae are potentially able to cause anisakiosis in abnormal hosts. For this reason the less specific anisakidosis is now gaining acceptance in the literature, although A. simplex larva is responsible for the majority of human cases.
Eggs are voided into the sea with host faeces, and in the egg the tiny larva develops and moults. It has been generally assumed that the second stage larva hatches from the egg, but Koie et al. (1995) found a second very thin cuticle surrounding the larva; thus, the larva hatching from the egg is a third stage one. This view is contested by Measures and Hong (1995), who by electron microscopy did not see this second sheath.
Suitable small invertebrates, mainly small crustaceans, which transfer the larvae to higher levels in the food chain, must ingest the sinking eggs/larvae. For A. simplex, krill (euphausids) are the main hosts, transferring them to various fishes, such as herring, blue whiting, mackerel
and many others. Pseudoterranova decipiens eggs/larvae are transferred to fish via small bottom-dwelling crustaceans. Arriving in the fish gut, the larvae bore through into its viscera, body cavity, or muscles. The fish reacts by producing a connective tissue capsule around each larva, which remain dormant until it is damaged or digested. Being regularly challenged by new larvae, the infection in any fish tends to build up slowly; consequently large and old fish usually have a larger larval burden than small and young fish. If a fish, or fish viscera thrown overboard, is eaten by another fish, the larvae will repeat this process of boring into viscera and muscles. In this way old and large carnivorous fish, such as cod, hake, and ling, may harbour very high numbers of encapsulated third stage A. simplex larvae. Anisakis simplex larvae not only lodge on and in the viscera of their fish hosts, but also in the somatic muscles, mainly in their belly flaps. When gutted, most larvae will be removed.
Pseudoterranova decipiens larvae are mainly found in the thick muscles, and where seals are common;
this 'seal-worm' is also common in many commercial fishes, causing problems for the fishing industry. Due to such serious problems in Atlantic Canada, much work on its population biology has been done in that country (see Bowen 1990).
When the 'correct' final host - seal or whale - eats the fish host, the larvae become free, bore partly into the stomach wall, grow and moult twice before reaching the final fifth adult stage. Often, many specimens form tight clusters in craters in the stomach wall.
The anisakid larvae found in fish are all third stage. Lips are absent, but a small boring tooth is present on the head, and there are no visible reproductive organs. The shape of the head, tail, and anterior digestive tract is used for distinguishing each larva. Anisakis simplex (Figure 16.1) forms a 3-4 mm tight flat spiral, while P. decipiens (Figure 16.2), being larger and yellowish-brown, is shaped more like a corkscrew. Both have a glandular ventricle, but P. decipiens also has an intestinal caecum. Contracaecum/Phocascaris larvae have opposed intestinal caecum and ventricular appendix, and near the head, the cuticle has characteristic transverse striae and longitudinal ribs, giving it a tessellated appearance (Figure 16.3). Similar drawings are found in the literature, but to actually see these characters in whole worms through a microscope, fixed/dead specimens should be made transparent by clearing them in lactic acid, glycerol, phenol-ethanol, or lactophenol.
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