Effects of temperature and age on immune response to infectious pancreatic necrosis (IPN) in salmonids .
Introduction.
The growth of the salmon farming industry in the last couple decades has stimulated interest in the diseases of farmed Atlantic salmon, Salmo salar L. Infectious pancreatic necrosis (IPN) has become of particular concern because of the increasing prevalence of IPN virus (IPNV) in salmon farms. IPN has been identified in nearly all trout and salmon producing countries. In Norwegian salmon farms, losses of millions of dollars annually in Atlantic salmon have been due to outbreaks of IPN. In 1995, it was estimated that 5% of Atlantic salmon set to sea in Norwegian fish farms were lost due to IPN, giving economic losses of about 60 million USD yearly (Christie, 1997). .
IPNV occurs in salmonids in both freshwater and marine environments. The disease can be acquired through horizontal transmission, from feces of infected fish, and by vertical transmission, from parent to progeny via reproductive products (Wolf, 1988). The virus causes acute infection, primarily in the liver, but it is also found in high concentrations in the kidney and spleen (Smail & Munro, 1995).
In salmonid populations, prevalence of IPNV and the impact of the disease vary with the pathogenicity of the virus strain, water temperature, stocking densities and other stress-related factors. Most of the experimental work has been done on brook trout, Salvelinus fontinalis Mitchill, and in rainbow trout, Salmo gairdneri Richardson. Brook trout are the most susceptible, followed by the rainbow trout and Atlantic salmon (Sadasiv, 1995). .
IPN was traditionally thought to be confined to first feeding stage of salmonids. As the newly hatched fry mature rapidly over the first few months of life, so does the immune system of. This increased resistance with age has been demonstrated in the case of IPN (Frantsi, 1971, Dorson, 1981).