Biological and mathematical modeling of dynamics of furunculosis in chinook salmon (Oncorhynchus tshawytscha) and infectious hematopoietic necrosis (IHN) in rainbow trout (Oncorhynchus mykiss)

A series of experiments with Aeromonas salmonicida and infectioushematopoietic necrosis virus (IHNV) were carried out to determine dynamics of thespread of infection in chinook salmon (1.2-1.98g) and rainbow trout (1.2-3.1g). It wasfound in experiments with A. salmonicida that fish infected by bath immersion becameinfectious at 4 days postexposure (dpe), one day prior to dying from furunculosis. Incohabitation experiments with a single infected fish donor, an average of 75% diseasespecific mortality was obtained. There was suggestive evidence that there is a positiverelationship between holding volumes and furunculosis prevalence in cohabitationexperiments with single donor fish. Median day to infection was inversely correlatedwith density. The threshold density at density of 1.97 fish/L was approximately 30 timesless than the density of 0.08 fish/L, 13.33 and 320 fish respectively. Reproductive ratio(R₀) and transmission coefficient (β) in the furunculosis epizootic were 3.23 and 0.021(individuals*day)⁻¹ respectively. The mortality rate (α) of infected animals was 28.7%per day. The models constructed successfully mirrored the results of laboratoryexperiments. Data produced by simulation of the models were significantly associatedwith the data obtained from laboratory experiments for susceptible (S) class and also forinfected class.In similar experiments carried out with IHNV, it was found that donor fishbecame infectious 3 dpe. The virus levels in donor fish and prevalence levels were alsohighly associated. Smaller volumes of that led to higher prevalence levels than observedin bigger volumes with 60 or 30 fish in each. A single donor fish was able to transfervirus to recipient fish. However, unlike the A. salmonicida experiment, transmission wasinsufficient to initiate a full-scale infectious hematopoietic (IHN) epizootic. Estimatedparameters for dynamics of infection were approximately half of the values for A.salmonicida (R₀=2.57,β=0.008 (individuals*day)⁻¹ and α=0.15). The modelsconstructed for IHNV spread were used to simulate the results of density experiment.However, it was not possible to test the association between susceptible and infectedclasses due to inadequate number of infected fish.