Koi herpesvirus causes mass mortality events in carp and koi populations, with significant welfare implications for affected fish and biosecurity challenges for pond keepers.
Koi herpesvirus mortality events cause acute suffering on a large scale. Fish experiencing severe gill necrosis are unable to extract adequate oxygen from water, causing respiratory distress comparable to asphyxiation. The rapid progression of disease means that affected fish deteriorate over days in ways that are visible to owners who are often unable to intervene effectively. The welfare obligation is primarily preventive: rigorous quarantine of new fish, testing before introduction, and isolation of any sick individuals. The emotionally difficult decision of euthanising affected fish to prevent further suffering requires access to appropriate euthanasia methods including clove oil.