Outbreaks of Granulomatous Disease Associated With Vibrio harveyi Infection in Grouper (Epinephelus fuscoguttatus × E. lanceolatus) Aquaculture in Taiwan.

Between 2015 and 2024, outbreaks of muscular and visceral granulomatous disease, resulting in moderate-to-severe mortality in farmed groupers, occurred in southern Taiwan. Initially, affected groupers were screened for granuloma-associated pathogens, including Mycobacterium spp., Nocardia spp., Photobacterium damselae and Edwardsiella spp., using PCR methods at local animal disease prevention and control centres. Twenty-five hybrid groupers (Epinephelus fuscoguttatus × E. lanceolatus) that tested negative for these pathogens were submitted to our laboratory for further examination. Infected fish exhibited multiple grey-white nodules on the body surface and visceral organs, particularly the kidneys. Large numbers of Gram-negative bacilli, measuring 2-3 μm in length, were observed in organ smears. Microscopic examination revealed multifocal granulomatous lesions in the subcutaneous muscle, kidneys, spleen and liver. Sequencing of the 16S rRNA gene and the Vibrio harveyi hemolysin (vhh) gene from all isolates confirmed their identity as V. harveyi. The pathogen was definitively localised within the lesions via in situ hybridisation using a digoxigenin (DIG)-labelled probe. This is the first report of natural V. harveyi infection causing muscular and visceral granulomatous disease in Taiwanese grouper farms. These findings underscore the emerging pathological relevance of V. harveyi and provide a practical diagnostic framework for granulomatous diseases in aquaculture.