In Chilean salmonid farmers’ worst nightmares, Piscirickettsia salmonis must be a regular feature. This bacterium causes Salmon Rickettsial Septicaemia (SRS), a highly infectious disease whose symptoms include haemorrhaging, lesions, ulcers, anorexia, and many cases death. SRS is an epidemic in Chile, costing the salmon aquaculture industry over US $300 million each year. Vaccines have been largely ineffective and antibiotics have given mixed results. Mandatory three-month fallowing, on the other hand, seems to routinely reduce the chance of reinfection.

The underlying principle behind fallowing is fairly straightforward. Net-pen systems commonly used by finfish farmers allow pathogens to spread into the wider environment. Some are carried away by currents, some remain suspended in the water column or on the seafloor under and near the farm. In all cases, they wait for a new host to come close enough for reinfection. Fortunately for the fish, and the farmer, pathogens can’t survive forever without a host – which is where fallowing comes in. Take the hosts away and all the equipment out of the water for disinfection, and the odds of contagion rapidly drop...

This article was written for The Fish Site - please continue reading here.

Image Credit: Thomas Bjørkan/Flickr (CC BY-SA 2.0)