Red Vent Syndrome.
In 2007, a number of anglers reported fish caught from local rivers with damaged and bleeding vents. The Scottish Government’s Fisheries Research Services investigated this phenomenon, which was widespread across Scotland and reported from 59 different rivers. The condition is characterised by swollen and bleeding vents in both female and male wild salmon and is commonly known as red vent syndrome (RVS). In more recent years the problem has been far less common, and RVS seems to be present only in certain runs of grilse.
There is no evidence at present to suggest that the condition affects the survival of returning Atlantic salmon or that it impacts on spawning success. The likely cause of the condition is a nematode worm infestation and Anisakis sp. and Hysterothylacium sp., both very common parasites, have been found around the vent area of all fish examined with RVS.
The nematode has a complex life cycle with the adult worms found in seals, whales and other marine mammals. Wild salmon become infected at sea when they ingest prey items which are infected with the nematode larvae. These symptoms have not been reported in farmed Atlantic salmon or sea trout (S. trutta) and seem to be restricted to wild Atlantic salmon.
Whilst the parasite can be carried into freshwater by migrating fish, it is unable to be transmitted between fish in the freshwater environment, so it is safe to return infected salmon to the river.
The full information leaflet is available for download from the Fisheries Research Services website:
Parasites in fish, particularly Anisakis, can, if eaten alive, cause serious health problems. Therefore, the Food Standards Agency has issued new guidance for anglers who may want to eat their own catch.
- Visually inspect the wild salmon to detect and remove parasites. Those fish which remain obviously contaminated should not be consumed.
- If wild salmon is to be eaten raw or almost raw it should be frozen in all parts for at least 24 hours, at a temperature of –20 deg C or colder. This will ensure that any non-visible parasites or undetectable larvae of nematodes are destroyed.
- This freezing advice also extends to wild salmon that are to undergo a cold smoking process or to be eaten after marinating or salting i.e. as in Gravadlax.
- Where wild salmon is to be hot smoked (internal temperature above 60 deg C), which is sufficient to kill any parasites present, then it is safe to eat without freezing first.
- Where it is not possible to carry out adequate freezing it is advisable to cook the wild salmon. A temperature of 70 deg C for two minutes will kill any parasitic contamination present. As there is no infallible method of detecting and removing larvae, this advice is particularly relevant for pregnant women and elderly people, where ingestion of live parasites from fish could pose a serious health risk.
This guidance document has been issued because of an increased prevalence of wild salmon in UK rivers infected with the parasite and can be downloaded from the Food Standards Agency's website: