Associations between Anisakis simplex (Nematoda: Anisakidae) and red vent syndrome in wild Atlantic salmon Salmo salar.

The relationship between the anisakid nematode Anisakis simplex and red vent syndrome (RVS) was investigated through parasitological, clinical and pathological examinations of 124 wild adult Atlantic salmon Salmo salar, sampled from the River Dee, Wales, River Tyne, England, and River Tyne estuary between 2009 to 2013. All S. salar were infected with A. simplex, but those exhibiting RVS lesions displayed significantly higher infection intensities of A. simplex than unaffected individuals. S. salar exhibiting RVS lesions also displayed significantly higher infection intensities of A. simplex in the discrete location of the vent than unaffected fish. Up to 95% of total A. simplex infection intensities were found within the vent, with up to 83.6% localised within the outer ~5-10 mm of vent tissue. This suggests that the vent is a discrete and potentially favoured site of A. simplex. Intensity of infection of A. simplex in the vent did not always correlate with vent pathology. Moreover, internal inflammation and marked fibrogranulomatous changes were observed in the absence of associated haemorrhage, suggesting that visual reddening of the vent may not always occur in spite of significant A. simplex infections. Whilst fish exhibiting clinical RVS displayed significantly higher vent infection intensities, no correlation was identified between the severity of symptoms and A. simplex intensity.