Retrospective analysis of antibiotic treatments against piscirickettsiosis in farmed Atlantic salmon Salmo salar in Chile

Abstract

Piscirickettsiosis is the most prevalent salt-water infectious disease in farmed salmonids in Chile. Antimicrobials are used to treat this disease; however, there is growing concern about the poor response to therapeutants on some fish farms. The objective of this study was to assess whether factors such as type of antibiotic used, average fish weight, temperature at the beginning of the treatment, and mortality at the time of treatment administration affect the probability of treatment failure against piscirickettsiosis. Pen-level treatment and production information for the first treatment event from 2014 pens on 118 farms was used in a logistic mixed model to assess treatment failure. We defined a failed treatment as when the average mortality 3 wk after the treatment was above 0.1%. Farm and company were included in the model as random effects. We found that the antibiotic product, mortality level before the treatment, and fish weight at the start of the treatment all had a significant effect on treatment outcome. Our results suggest that antibiotic treatment success is higher if the treatment is administered when mortality associated with piscirickettsiosis is relatively low. We discuss the effect of weight on treatment success and its potential relationships with husbandry practices and drug pharmacokinetics.

Improving the use of data to support management of piscirickettsiosis in Chile

“…As a consequence, we included mortalities attributed to unknown causes in both the end-of-outbreak threshold and the outcome measure (next section). This is consistent with the approach used in previous studies on SRS, where total mortality was used after a diagnosis of SRS was reached on a farm (Jakob et al 2014;Price et al, 2016).…”

“…Consequently, the salmon industry in Chile is seeking ways to move towards more prudent antimicrobial use (Barton and Fløysand, 2010). The rate of antimicrobial use in the Chilean salmonid industry reflects the high rate of occurrence of SRS, relatively poor responses to antimicrobial treatment (Almendras and Fuentealba, 1997;Price et al, 2016; and limited effectiveness of other control measures such biosecurity, vaccination, reducing fish stress and early harvest (Evensen, 2016). Antimicrobial resistance of P. salmonis to florfenicol or oxytetracycline appears uncommon (Henriquez et al, 2016) and it is critical to identify other factors that may explain the limited effectiveness of treatments.…”

“…Consequently, there are few published studies on the effectiveness of antimicrobial treatment to control SRS in farmed salmon (Price et al, 2016;Mardones et al, 2018), despite the importance of SRS and strong reasons to reduce antimicrobial use. Alternatively, retrospective studies of industry-generated data may provide insights without the need for resource-intensive data collection (Hutchison et al, 2018).…”

A 12‐point checklist for surveillance of diseases of aquatic organisms: a novel approach to assist multidisciplinary teams in developing countries

“…2020) and the use of antimicrobials (Price et al . 2016), amongst others. The development and establishment of comprehensive surveillance systems supported by participative salmon farmers and interacting with researchers from the academia has resulted in the advancement of updated science‐based policies in Chile (Gallardo Lagno et al .…”

Cost‐effectiveness of longitudinal surveillance for Piscirickettsia salmonis using qPCR in Atlantic salmon farms (Salmo salar) in Chile

“…Although antibiotics are available to treat SRS, limited effectiveness of treatments has been reported in farmed salmonids (Price et al., 2016; Rozas‐Serri & Enríquez, 2014). The main reason for poor treatment response is loss of appetite in P. salmonis ‐infected salmon which leads to insufficient consumption of medicated feed to reach therapeutic tissue concentrations of antibiotics (Price et al., 2018; Rozas‐Serri & Enríquez, 2014); and factors such as antibiotic type, disease incidence in the population and time‐to‐treatment (Price et al., 2016; Rozas‐Serri & Enríquez, 2014). Consequently, in order to achieve the successful SRS treatment and control, early detection of piscirickettsiosis is crucial.…”