When used alone, Protec led to a 70 percent lower mortality rate. Combined with PD vaccination, mortality was reduced by 85 percent.
In a controlled challenge trial conducted at VESO Vikan in Norway during spring of 2010, Skretting ARC researchers measured the effect of combining specific PD vaccination with Skretting Protec. At the end of the trial, total mortality of the fish receiving both vaccine and Protec was reduced to 3.2 percent, compared with 21.5 percent of those that received neither. Feeding Protec alone also led to a marked improvement in fish survival, with only 6.5 per cent mortality.
"This study confirms results from previous trials showing that Protec supports the salmon's natural defence mechanisms sufficiently to significantly reduce the degree of heart damage," said Dr Charles McGurk at Skretting ARC.
A lower level of infection
In the study, one group of Atlantic salmon was vaccinated with Compact PD vaccine while two other groups were injected with saline solution to ensure consistent handling. Each group comprised three tanks, giving nine tanks in total. Two weeks later, two groups were switched to Protec while the remaining group continued on the control diet. After four weeks, all groups were exposed to PD virus by introducing fish that were experimentally infected with PD. These individuals then spread the infection further. Experiments conducted in this manner are often referred to as cohabitant challenge trials. This is the preferred method as it closely simulates the manner in which PD virus spreads through a site.
"Measurements made at the peak of infection showed that while 93 percent of fish that received neither vaccine nor health diet were infected, only 67 percent of the fish that had received both vaccine and Protec tested positive for PD virus," says McGurk.
Mortality period later and shorter
The onset of deaths was at day 15 in the control group, at day 20 in fish fed Protec but not vaccinated and at day 24 in those given the combination of Protec and vaccine—nine days later than the unvaccinated fish on control diet.
Both Protec and PD vaccination reduced the period of mortality. The period was 26 days in the unvaccinated control diet group. In the unvaccinated Protec group it was 17 days and in the group receiving both vaccine and Protec the mortality period was further reduced to six days.
"This tells us that the combination of a specific vaccine against PD and feeding Protec is a very effective strategy to reduce the effect of a PD outbreak", says John Roberts, Product Manager, Skretting UK.
Cumulative mortality in VESO Vikan Protec trial 2010