Video-based identification of surrogate endpoints in experimental bacterial infections of rainbow trout (Oncorhynchus mykiss)

To identify visually perceptible clinical signs that would enable timely distinction between survivors and nonsurvivors in infection experiments requiring death as experimental endpoint, rainbow trout were recorded on video after being subjected to bacterial challenge with Aeromonas salmonicida and Yersinia ruckeri. Deviations from normal morphology, behaviour and movement patterns were analysed for their potential to predict death in infected animals kept in groups of 10 fish inside small volume holding tanks (15 l). It was found that clinical signs reflecting a highly debilitated physiological state, like inability to hold an upright position or being passively dragged by a current, offer high sensitivity and specificity, but are largely inefficient in reducing the overall time an animal spends inside the experiment. A change in body shape detected in fish infected with A. salmonicida, provided high sensitivity and earlier identification of nonsurvivors. Anorexia was identified as a promising death predictor in terms of sensitivity and timely identification, but its specificity was likely to be confounded by a high level of social aggression observed between the fish. As expression of early clinical signs in form of sickness behaviour might have been strongly influenced by experimental husbandry conditions, it is assumed that optimisation of those could present an effective approach for improving applicability of surrogate endpoints in this species.