Fisheries Research Services is an agency of the Scottish Government

FRS Marine Laboratory PO Box 101 375 Victoria Road Aberdeen AB11 9DB UK

tel +44 (0)1224 876544 fax +44 (0)1224 295511

enquiries@marlab.ac.uk http://www.frs-scotland.gov.uk

Fisheries Research Services

Samples analysed a few weeks after fish had entered fresh watershow some recovery of the skin. Focal reddening, developingscar tissue and some localised scale loss is common, but noerosion or deep swelling is evident.

Severe cases show pronounced swellingand obvious erosion; tissue may protrudefrom the vent and bleeding may occur ifgentle pressure is applied.

Normal vent for comparison

Mildly affected fish might show a numberof small red spots (petechial haemorrhage)and reddening around the vent.

Moderately affected fish show an obviouswidespread redness surrounding the ventand initial swelling; haemorrhage, skinerosion and scale loss might be observed.

Mild Moderate Severe

Normal

Red Vent Syndrome (RVS) in Wild Atlantic Salmon

BackgroundReports of wild Atlantic salmon (Salmo salar) returning toScottish rivers with inflamed, swollen bleeding vents werereceived in early June 2007. By the end of July over 50rivers had reported similar observations. Apart from theaffected vent, the fish appear to be in good physicalcondition. So far, the syndrome appears to be mainlyrestricted to adult male and female Atlantic salmon onesea-winter fish (i.e. grilse), and particularly prevalentamong ‘fresh run’ fish. A smaller number of two sea-winterfish were also reported showing signs of the condition.Neither sea trout (S. trutta) nor farmed Atlantic salmonhave been affected by the condition. Investigations werecarried out by Fisheries Research Services (FRS) in anattempt to establish the cause and extent of the problem.

External signsThe following series of photographs show the conditionin varying degrees of severity and at different timesfollowing return return to rivers.

After 6-8 months in fresh water the vent area of a spawned femalefish (i.e. a kelt) is raised and slightly protruding, but with noreddening, and full recovery of the skin.

Fisheries Research Services is an agency of the Scottish Goverment

FRS Marine Laboratory PO Box 101 375 Victoria Road Aberdeen AB11 9DB UK

tel +44 (0)1224 295525 fax +44 (0)1224 295620

fishhealth@marlab.ac.uk http://www.frs-scotland.gov.uk © Crown copyright Printed on Revive Silk,a recycled paper

Fisheries Research Services

Anisakis sp. life cycle in the North Atlantic(after Buchmann, Bresciani & Beyerholm 2001, modified by Levsen 2007)

Investigation into the causeAll fish examined by FRS were screened for fish pathogens.The results from a suite of bacteriological, virological andmolecular tests were all negative. However, parasites werefound both in the body cavity and particularly in the ventregion. The parasite is a small round nematode calledAnasakis, which can be observed with the naked eye andresembles a watch spring, measures a few millimetres indiameter, but up to 200mm long when uncoiled (Plate 7).

ConclusionsThe very high number of Anisakis larvae within thediscreteregion of the vent, and the associated tissuereaction correlates with the externally observed ‘red vent’,and is considered to be the most likely causal agent ofthe Atlantic salmon ‘red vent syndrome’. Otherwise, allof the fish examined showed a good general health status,and there is no evidence of any bacterial, viral or fungalagent. Moreover, all internal organs and tissues otherthan the vent appeared to be in a normal healthy condition.

The presence of Anisakis larvae in wild adult salmon isnot novel as the parasite occurs naturally in many fishspecies. However, the concentration of parasites in thevent tissues is unusual. The impact of the parasite on thefish is not fully known, but from the first years observations,there is no evidence that the condition has either led tomortality, or prevented salmon from spawning successfully.Consequently, there is no basis to recommend that fishshowing signs of RVS are removed or killed.

The final natural host of the Anisakis worm are marinemammals such as whales, dolphins or seals. However,the parasite may also affect humans who become infectedfrom eating raw fish (for example sushi) or fish that isun d e r - co o ke d . Fo r m o re i n fo r ma t i o n s e e :http://www.food.gov.uk/multimedia/pdfs/guidsalmonanisakis.pdf

1 cm

5 mm

Parasite

Anasakis larvae were consistently found in the region ofthe vent of all fish examined and in very high numbers,embedded in the dermal, sub-dermal and muscle tissueswithin the vent. Microscopically, changes includeepidermal erosion, scale loss, encapsulation, severedermatitis, inflammatory response and haemorrhage,which all contribute towards the appearance of the ventexternally.

Contact usAnyone concerned about RVS should contact the FishHealth Inspectorate at FRS Marine Laboratory. Telephone01224 876544 and ask for the Duty inspector. Alternativelye-mail: fishhealth@marlab.ac.uk.

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