Pathogenicity and infectivity of Saprolegnia species in Atlantic salmon (Salmo salar L.) and their eggs

In the past decades the global population has becoming increasingly dependent on aquaculture due to over-fishing of oceans and rivers. Consequently, fish farming has become the fastest growing food sector in the world. Increased production intensity constituting intensive culturing of fish coupled with the ban in the use of malachite green, an effective remedy against Saprolegnia infections, accounts for the dramatic re-emergence of Saprolegnia infections in aquaculture which fish farmers are struggling to control. This precarious situation inevitably created a sense of urgency for researchers to explore the possibility of developing novel and sustainable treatments to control this devastating pathogen of fish. Saprolegnia infection in fish is characterized by white or grey patches of filamentous mycelia on the host. The infection initially appears on epidermal tissues of the head, tail and fins and subsequently spreads to the rest of the body. Lesion areas may be soft, necrotic and ulcerated and the surrounding areas may show oedema and necrosis. Fish eggs infected with Saprolegnia species appear like tufts of cotton wool, as the Saprolegnia hyphae form a white cotton wool or felt-like mat on the surfaces of the eggs.