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Epidemiology of infectious pancreatic necrosis virus (IPNV) in Atlantic salmon marine farms in Chile, from 2010-2014

Fernando O. Mardones1*, Joaquin Escobar-Dodero1, Rolando A. Ibarra2, Alfredo Tello2 and Gustavo E. Monti3
  • 1 Universidad Nacional Andres Bello, Escuela de Medicina Veterinaria, Chile
  • 2 SalmonChile-Instituto Tecnológico del Salmón, Department of Fish Health, Chile
  • 3 Universidad Austral de Chile, Facultad de Ciencias Veterinarias, Chile

Infectious pancreatic necrosis virus (IPNV) is the causative agent of infectious pancreatic necrosis (IPN) disease, a highly contagious viral disease with a high incidence in world aquaculture. This double-stranded RNA virus belong to the Aquabirnavirus genus within the Birnaviridae family (Dobos and Roberts, 1983). IPNV is known to infect > 63 marine species, including mollusks and crustaceans (Saint-Jean et al., 2003). IPNV causes disease in young salmonids, with clinical signs including darkening of the skin, exophthalmia, abdominal swelling and aberrant swimming behavior. IPN mortality in salmonids is inversely proportional to the age of the fish, being typically higher in fry and fingerling salmonids and rare in older fish, in which infections can be unapparent (Roberts and Pearson, 2005). IPN is among the most important viral diseases in Chile and is considered endemic in the country, causing yearly losses over US$20 million attributed to the disease (Bravo and Midtlyng, 2007; Tapia et al., 2015). Circumstantial evidence suggests that IPNV was introduced in association with eggs imported from North America (Mcallister and Reyes, 1984) but the presence of IPN was not confirmed until 1998 (Tapia et al., 2015). Oral, immersion and injectable vaccines in Atlantic salmon (Salmo salar L.) pre and post-smolts are available and have been reported to give well but not complete protection (Munro and Midtlyng, 2011). Since 2002, vaccination has showed an important increase, however, it is unclear vaccines' efficacy against virulent Chilean isolates (Bravo and Midtlyng, 2007). IPNV can be transmitted horizontally and vertically and usually the host develops a carrier state (Saint-Jean et al., 2003). Risk factors for clinical outbreaks of IPN include the use of many smolts suppliers, increasing cohort size, cohort type (spring vs. autumn smolts), species (salmon vs. trout), decreasing temperature at sea transfer, IPN on farm within the last 2 years and decreasing mean weight at sea transfer. Geographic location of a farm and increasing infection pressure (exposure to infection by proximate infectious farms) also affected the risk of IPN (Jarp et al., 1995; Murray, 2006; Jensen and Kristoffersen, 2015). A retrospective cohort study was carried out at group and production cycles levels at the two most important regions for salmon production (10th and 11th). A group was considered as a batch of Atlantic salmon (AS) typically raised at the same fresh water farm, and transferred to seawater at the same time, whereas a production cycle was considered as a stock of AS raised at the same farm transferred at the same time but including a variable number of groups. Retrospective information for cohorts of farmed AS that were transferred to the sea after December 2010 and harvested before June 2014, and that were held on the same farm for at least 6 months were included to fit mixed effects logistic regression models. The dependent variable was the report of mortalities attributed to IPNV at the farm and risk factors included on-farm (stocked population, smolt weight, farmed year-class and infectious and non-infectious diseases conditions) and spatial (minimum distance to the nearest farm, number of farms and average distance to farms within 5, 10, 15, 20 and 25 kilometers (km) of radius) variables. Random effect were farms for the group level model and management areas for the farm level. In addition, the spatial scan test (Kulldorff and Nagarwalla, 1995), a local cluster test, was used to examine whether clinical outbreaks were distributed randomly over space and time. A space–time Poisson model (Kulldorff, 1997) was selected to compare the disease rate inside a cylindrical space–time window to the population at risk outside the window. The report of IPN at the farm was considered as an indication of a clinical outbreak. A total of 294 AS farms provided information of 306 production cycles and 629 groups (average 2.1 groups per production cycle). The average time from stocking to the first report of IPN was 69 days, and the prevalence at production cycle and group levels were 30.7% and 33.2%, respectively. IPN mortalities at the production cycle ranged from 0.01% to 13% with an average of 0.89%. The production cycle level model included the positive association with the number of farms within 15 km of radius and farmed year-class (2012 at higher risk than 2010). The group level fitted model also retained the previous two variables but decreasing mean weight at sea transfer and the occurrence of mortalities due to mycosis contributed considerably to the fit of the final model (Table 1). Neither spatial nor spatiotemporal clusters were identified at both study regions. IPN occurred in an important proportion of AS farms, and for the study period, its magnitude was increasing on time. Results here are in line with previous studies that have identified risk factors for clinical outbreaks of IPN. Despite the disease does not define a spatial and/or temporal pattern; an effect similar to the previously described in Norway, i.e., the exposure to infection by proximate infectious farms was also found here. For an increment of one marine salmon farm within a 15 km radius from an AS farm, the risk of having clinical outbreaks due to IPNV increased between 22 to 34%. Similarly, heavier fish at sea transfer was identified as a protective factor, and for every 10 grams the transferred smolts are, the risk was reduced in 36%. One of the consequences of IPNV in infected fish is weight loss (Munro and Midtlyng, 2011), and together with the report of mortalities associated to mycosis (a fungal infection) (OR = 7.6), results here provides new clues that may help to evaluate the quality of smolts and the risk to IPN. This is the first observational study that evaluates risk factors for clinical IPN in the Chilean AS industry. Birnaviruses are the most ubiquitous microorganisms in aquatic species and IPNV is one of the most resistant to disinfection procedures in aquaculture making eradication difficult (Munro and Midtlyng, 2011). Considering risk factors that promote preventive measures and/or help the spatial planning of the salmon industry are important contributors to disease management strategies.

Acknowledgements

Fondecyt postdoctoral grant # 3140235

References

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Keywords: Infectious pancreatic necrosis virus, IPN, Atlantic salmon farming, Disease Management, Risk factors

Conference: AquaEpi I - 2016, Oslo, Norway, 20 Sep - 22 Sep, 2016.

Presentation Type: Oral

Topic: Aquatic Animal Epidemiology

Citation: Mardones FO, Escobar-Dodero J, Ibarra RA, Tello A and Monti GE (2016). Epidemiology of infectious pancreatic necrosis virus (IPNV) in Atlantic salmon marine farms in Chile, from 2010-2014. Front. Vet. Sci. Conference Abstract: AquaEpi I - 2016. doi: 10.3389/conf.FVETS.2016.02.00048

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Received: 31 May 2016; Published Online: 14 Sep 2016.

* Correspondence: DVM, PhD. Fernando O Mardones, Universidad Nacional Andres Bello, Escuela de Medicina Veterinaria, Santiago, Chile, fernando.mardones@unab.cl