Modelling management strategies for a disease including undetected sub-clinical infection: Bacterial kidney disease in Scottish salmon and trout farms

Abstract

Disease is a major constraint on animal production and welfare in agriculture and aquaculture. Movement of animals between farms is one of the most significant routes of disease transmission and is particularly hard to control for pathogens with subclinical infection. Renibacterium salmoninarum causes bacterial kidney disease (BKD) in salmonid fish, but infection is often sub-clinical and may go undetected with major potential implications for disease control programmes. A Susceptible-Infected model of R. salmoninarum in Scottish aquaculture has been developed that subdivides the infected phase between known and undetected sub-clinically infected farms and diseased farms whose status is assumed to be known. Farms officially known to be infected are subject to movement controls restricting spread of infection. Model results are sensitive to prevalence of undetected infection, which is unknown. However, the modelling suggests that controls that reduce BKD prevalence include improve biosecurity on farms, including those not known to be infected, and improved detection of infection. Culling appears of little value for BKD control. BKD prevalence for rainbow trout farms is less sensitive to controls than it is for Atlantic salmon farms and so different management strategies may be required for the sectors.

Case definition for clinical and subclinical bacterial kidney disease (BKD) in Atlantic Salmon (Salmo salarL.) in New Brunswick, Canada

“…A fourth category has been used previously by differentiating the subclinical category into ‘unknown infected’ and ‘known infected’ (Murray et al . ) Although test positives may have been classified as known cases and field observations as unknown cases to further subdivide our case definitions, simplicity and reflecting industry practice did not warrant expanding beyond three categories.…”

Applicability of a food chain analysis on aquaculture of Atlantic salmon to identify and monitor vulnerabilities and drivers of change for the identification of emerging risks

“…Using a model tailored towards sea lice management, Murray and Salama (2016) showed that synchronization of management measures such as medication and fallowing within the same management areas is key to highly effective protection. Murray et al (2011) describe how for infections with bacterial kidney disease caused by Renibacterium salmoninarum, management could be more effective. Towards this end, these authors concluded that not only farms with clinically diseased animals, which are already put under oversight and shut off (no fish movement), but also measures should be taken on other farms with potentially sub-clinically infected salmon, as well as improved surveillance .…”

Estimating the prevalence ofRenibacterium salmoninarum‐infected salmonid production sites

“…These results are in agreement with previous reports indicating that both the number (Bruno ) and proportion (Bruno ; Munro ; Murray et al . , ) of R. salmoninarum ‐infected rainbow trout sites are greater than for Atlantic salmon sites. The statistics assume, however, that the biennial site testing is completely reliable with perfect diagnostic sensitivity (Se) and specificity (Sp).…”

“…Summaries of the number of reported wild fish with BKD in Scotland (Smith ; Bruno ) or, following the widespread adoption of aquaculture, the number and/or proportions of production sites (hereafter referred to as sites) infected with R. salmoninarum have been provided (Bruno , ; Munro ; Murray et al . ; Murray et al . ) although uncertainty regarding the prevalence remains among fish health professionals (Murray & Soje ).…”