We conducted a quantitative analysis of the spread of ISAV in Scotland and its relationship to the movements of four well boats that serviced the farmed-salmon harvesting center of Loch Creran in the year preceding the epidemic (May 1997 to May 1998).
Movement of the well boats from Loch Creran may then have rebroadcast ISAV.
Infected wild salmonids, which have been found throughout Scotland (33) could act as an ISAV reservoir (31).
In 1999, sites not visited by these well boats (and also Loch Broom, which was reportedly only visited once) became ISAV suspect (20).
The evidence presented here supports a very strong quantitative link between the number of visits by well boats and the probability of ISAV detection in an area.
ISAV (but not infectious salmon anemia) was detected by PCR methods in wild salmonids throughout Scotland in 1999 (33), but although present, was much less prevalent in 2000 (21).
At the time of the epidemic, effluent from the Loch Cretan processing plant was not fully disinfected, so ISAV could have been present in ballast water taken up after disinfection.