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Dangerous Parasites Discovered in the North Sea

UK - Researchers at NIFES have detected the parasite Kudoa thyrsites in 0.8 % of Atlantic mackerel from the northern North Sea. Enzymes released by the parasite gradually turn the fish flesh soft and jelly-like. Therefore, the condition is commonly known as "soft flesh".

Most abundant in large mackerel

A single Kudoa thyrsites spore from the musculature of Atlantic mackerel. (Scale bar: 0.005 mm)

Kudoa gives no visible symptoms in live mackerel, but 24-56 hours after catch, powerful enzymes released by the parasite gradually break down the fish's musculature. This results in a soft and jelly-like fish filet, and therefore, the condition is commonly known as "soft flesh". The presence of this parasite in fish is primarily a quality-related problem. Kudoa-infections are neither treatable nor preventable. -The proportion of mackerel developing "soft flesh" was low, and Kudoa was only detected in larger mackerel weighing more than 450 grams," says Dr Arne Levsen, researcher at the National Institute of Nutrition and Seafood Research (NIFES) in Norway. “Approximately 9% of the analyzed mackerel in the largest size group (more than 600 grams) developed soft flesh". However, "soft flesh" was not found in smaller mackerel weighing less than 450 grams.

What about symptom-free fish?

In order to investigate whether symptom-free mackerel also may carry the parasite, blood and muscle samples were taken from mackerel immediately after catch. There were no signs of infection, i.e. no Kudoa spores were present, in any of the samples. However, this does not exclude the possibility of infections in these fish as the parasite undergoes several developmental stages in fish, and especially the earliest stages are not readily detectable by common microscopy. By using DNA-analysis, however, researchers from NIFES are currently investigating whether smaller mackerel may carry the parasite as well.

Kudoa in Norwegian waters

The Kudoa species that was detected in the current survey was identified as Kudoa thyrsites based on DNA-analysis and the shape and size of the spores. The parasite is known from more than 20 species of fish worldwide, including farmed Atlantic salmon. In Norwegian waters, however, it has so far only been found in Atlantic mackerel.

What is a spore?

Kudoa belongs to a group of parasites called myxosporean parasites, which all undergo several stages in their life cycles. The parasite's most characteristic stage is the spore, which also forms the basis for the classification of the species. The spores, which may number millions in infected mackerel, are the parasite's infective stage to the next host organism. However, the life cycle of Kudoa is still not completely resolved.