Farm-level reproduction number during an epidemic of infectious salmon anemia virus in southern Chile in 2007–2009

• Published in: Preventive veterinary medicine 2011-12, Vol.102 (3), p.175-184
• Main Authors: Mardones, F.O., Perez, A.M., Valdes-Donoso, P., Carpenter, T.E.
• Format: Article
• Language: English
• Subjects: Animals; Aquaculture; Basic Reproduction Number; Chile; Chile - epidemiology; Cluster Analysis; Communicable Diseases, Emerging - epidemiology; Communicable Diseases, Emerging - transmission; Communicable Diseases, Emerging - veterinary; disease control; disease outbreaks; Epidemics - veterinary; Farm level reproduction number; farm numbers; fish culture; Fish Diseases - epidemiology; Fish Diseases - transmission; fish farms; fish production; Infectious salmon anemia virus; ISAV; Isavirus; Modeling infectious diseases; Orthomyxoviridae Infections - epidemiology; Orthomyxoviridae Infections - transmission; Orthomyxoviridae Infections - veterinary; Population Surveillance; reproduction; Salmon; Salmon farming; World Health Organization
• ISSN: 0167-5877; 1873-1716
• DOI: 10.1016/j.prevetmed.2011.07.005

An epidemic of infectious salmon anemia virus (ISAV) has greatly impacted salmon production in Chile with devastating social and economic consequences. The epidemic is analyzed here and is likely the largest ISAV outbreak reported affecting one of the most productive regions for salmon farming activities in the world. After re-emerging in 2007, ISAV rapidly expanded the following two years, both in magnitude and geographic range, affecting about 65% and 50% of salmon farms located at the 10th and 11th regions of Chile, respectively. A useful metric for the control of infectious diseases that quantifies the progression of an epidemic is the reproduction number at the farm level ( R f), which describes the mean number of secondary cases generated by an infectious farm. The parameter in this study was estimated for individual farms ( R f i ), specific phases ( R tf), and for the entire epidemic ( R f) by using several analytical approaches based on the characterization of the epidemic curves for the two regions. For the initial spread and the epidemic growth phase, initial and intrinsic growth rates were used to estimate R tf. In addition, two approaches (epidemic final size and nearest neighbor analyses) were used to obtain an individual ( R f i ) and overall estimate of R f for the complete epidemic. In general, two distinct regional patterns of spread were identified. In the 10th region, after an explosive initial spread of ISAV in which R tf reached 12.0–16.9, a smaller epidemic growth of 1.6 ≤ R tf ≥ 2.5 and a final burnout with R tf < 1 were observed. For the 11th region, R tf only reached 2.4 during the initial spread phase, ranged from 1.6 ≤ R tf ≥ 4.4 during the epidemic growth phases and ended when R tf was