Changes in prevalence of clinical infectious pancreatic necrosis among farmed Scottish Atlantic salmon, Salmo salar L. between 1990 and 2002

Abstract

Infectious pancreatic necrosis virus (IPNV) induces an economically important disease that causes a pancreatic and gut-associated pathology in freshwater fish. Currently IPN is also recognised as an emerging problem for sea water cultured salmonids. This study reports clinical IPN outbreaks in farmed Atlantic salmon, Salmo salar, in Scottish marine waters among S1 and S1/2 stock, as determined by light microscopy from 1990 to 2002. The IPN diagnosis by light microscopy was supported by isolation of the causative virus in cell culture. Prevalence was calculated from the number of positive IPN cases relative to the total number of marine reared salmon examined histologically each year. Data for Scotland indicate an overall increasing trend, but with prevalence changes between years. The prevalence of IPN in post-smolt Atlantic salmon in Scotland has increased from 1.2% in 1990 to 12.5% to date. In the Shetland Isles, IPN prevalence has increased from 1995 to 1996. Prevalence of IPN for areas of Scotland other than the Shetland Isles shows little change up to 2000, but with an increasing trend thereafter. IPN mortality and histological evidence of infection was first recorded in S1/2 salmon in 1996. IPN was recorded among S1 post-smolts during the spring months several weeks after transfer to sea. Outbreaks of IPN reached a peak in July following spring seawater transfer, and clinical was IPN uncommon after October. Mortality at sites with confirmed IPN varied between 0.03% and 0.1% per day in May and 0.5% per day in July, and then declined to early spring levels. IPN in both S1 and S1/2 fish groups showed affected acinar cells with acute necrosis and pyknosis with basophilic inclusions. IPN pathology extended into the gut mucosa and necrotic areas replaced with fibrous/fatty tissue and cell sloughing in combination with increased mucous resulting in an acute, catarrhal gut exudate. Salmon pancreas disease (SPD) was diagnosed by light microscopy in 1990 with a prevalence of 11.6%, declining to zero by 2002.