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Kudoa Parasites in Fish

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Kudoa is a microscopic parasite found in several marine fish species worldwide. Enzymes produced by these parasites can make fish flesh soft or even gel-like, a condition commonly referred to as soft flesh. The parasite does not infect humans. NIFES has previously detected one Kudoa species, Kudoa thyrsites, in Mackerel from the North Sea. The parasite has not been detected in other species of wild or farmed fish in Norway.

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Kudoa does not give any visible symptoms in living fish, but one to two days post mortem, potent enzymes produced by the parasite may degrade the muscle. This results in a soft fillet having a gel-like appearance, thus the name soft flesh. The presence of the parasite is mainly of concern for the quality of the fish, and does not pose a health-risk for consumers.

Kudoa in Norwegian Mackerel

In examinations conducted by NIFES, the proportion of Mackerel that developed soft flesh was low, and Kudoa could only be detected in Mackerel weighing above 400 g. Approximately nine per cent of the Mackerel in the largest weight group (over 600 g) developed soft flesh.

The Kudoa detected in Mackerel in this study were identified as Kudoa thyrsites by a combination of DNA-analysis and also by the size and morphology of the parasite spores. The parasites could be detected in 17.8 per cent of 952 examined Mackerel by applying DNA-based methods.

Only a fraction of the fish that have the parasite will develop symptoms, and in our study 1.3 per cent of 1908 examined fish showed initial signs of muscle degradation after storage for two days.

In the fish the parasite undergoes several developmental stages, and in the initial phase of the infection it is not possible to detect parasites by microscopy. By DNA-based methods, researchers at NIFES are now examining if even smaller Mackerel may harbour the parasite.

Kudoa in other fish species

The parasite has been detected in more than 20 fish species worldwide. This also includes Atlantic Salmon in aquaculture, where the parasite is a known problem for Atlantic Salmon in British Columbia. So far K. thyrsites has not been detected in Norwegian farmed Salmon.

Unknown life cycle

The life cycle of K. thyrsites is still unknown, but it probably involves at least one other host organisms. A hypothesis is that alternative host organisms are not present in the North-Sea or the Norwegian sea, including the coast of Norway, thus Mackerel is probably infected by the parasite in the spawning grounds south of the British Isles. Researchers at NIFES follow the Kudoa-situation in Mackerel. Furthermore, the status in Norwegian farmed fish should be closely monitored also taking into account possible increase in seawater temperature and following migration of possible alternative hosts for the parasite into Norwegian waters.

October 2012