15 May 2013
A recent extensive study comparing Pancreas Disease (PD) vaccinated and unvaccinated farmed Atlantic salmon (Salmo salar) populations in the PD (SAV3) endemic zone in Norway has shown that PD vaccination has made a positive impact on accumulated mortality and discards since the widespread introduction of NORVAX Compact PD from 2008 to 2011 (Jensen et al 2012). Up until now the number of annual PD outbreaks diagnosed by the National Veterinary Institute (NVI) has been used as a measure of the effect of PD vaccination but this very simplistic approach does not take into account the severity of the reported outbreak. The number of PD outbreaks has decreased by 18‐30% since the introduction of PD vaccination in 2008, but a more robust interrogation of the data was required to reveal the real impact of vaccination.
A PD outbreak is defined as a fish population showing clinical signs (increased mortality, reduced feed intake) and a positive result of PCR screening, in association with typical PD tissue lesions in the same population. The study compared PD vaccinated and unvaccinated cohorts put to sea between 2007 and 2009 and slaughtered before mid–February 2011 in the PD endemic zone. A cohort is defined as all fish put to sea at one site and followed from sea transfer to slaughter.
This study aimed to look at other production parameters such as mortality and growth in PD vaccinated versus unvaccinated cohorts. With the aid of the PD‐free project, 193 inputs or cohorts were studied which constituted 53% of sites in the PD endemic zone and therefore was representative of the entire Norwegian Atlantic salmon farming population in that area. One hundred and eleven of these cohorts were diagnosed as PD positive and they were evenly
distributed in the endemic zone.
Of the 193 cohorts studied, 123 were PD vaccinated and PD was diagnosed in 59 of these vaccinated groups. The mean time from sea transfer to diagnosis of PD was 9 months with an approximate range of 2.8‐16 months. The mean cumulative mortality for cohorts with PD was 22.6%, compared to 11.7% in cohorts without PD. When all other factors are kept constant, the cumulative mortality at PD sites decreased from 22.5% to 15% if fish were vaccinated. Thus vaccination decreases mortality of a PD outbreak almost to the same level as if the cohort had not experienced an outbreak. The growth rate increased from 0.72% to 0.75% when cohorts were vaccinated and the percentage of discards was reduced from 2.74‐1.28%.
The risk factors for PD which were considered in this study were, not vaccinating, other diseases, such as IPN, HSMI, CMS, infection pressure (km‐1 ), no of fish (106), fish density (m‐3), spring or autumn smolt or mixed, cumulative mortality, growth rate, feed factor and the number of fish discarded at slaughter.
Sophisticated multivariate statistical analysis was used to interrogate and quantify these risk factors for PD. The main objective of this analysis is to study how the variables are related to one another, and how they work in combination. The study found that the variables which had an effect on the probability of PD were; PD vaccination, IPN infection, infection pressure and cohort index (i.e. smolt type Spring vs. Autumn). If more than 0% of the fish in a cohort were vaccinated, the odds of a PD outbreak were 3 times lower than cohorts with no vaccinated fish. The odds of a PD outbreak were 2.7 times higher if IPN was present and increased with infection pressure, where for a given site the infection pressure depends on the number of sites with PD around the site and the seaway distance to the PD sites. Spring input smolts had half the probability of developing PD compared to autumn or mixed smolt groups.
The study also suggested that infection pressure and vaccination might be inversely correlated, i.e. as vaccination increased, infection pressure was decreasing, which is what one would expect from an effective vaccine.
Overall this is the first factual evidence that current PD vaccination is a strong preventative tool in the control of this serious infectious disease of farmed Atlantic salmon in Norway.
Britt Bang Jensen, Anja B Kristoffersen, Camilla Myr and Edgar Brun (2012) Cohort Study of effect of vaccination on pancreas disease in Norwegian salmon aquaculture Diseases of Aquatic Organisms Vol 102: 23‐31
For more information about NORVAX COMPACT PD, go to http://aqua.merck‐animal‐health.com/, e‐mail email@example.com or contact your local MSD Animal Health representative.
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