A case investigation of mortality factors in Japanese Koi (Cyprinus carpio) from a recreational pond.

An investigation covering water quality, parasitology, bacteriology, and virology was conducted to address cumulative mortality rates exceeding 80% among Japanese koi (Cyprinus carpio) in a recreational pond at AkuaTAR, Fisheries Research Institute, Penang. Low dissolved oxygen levels (DO < 4 mg/L) were identified as a critical environmental stressor likely contributing to the high mortality rates. Gross examinations of the Japanese koi revealed generalised hemorrhages on the body, tail, and fin bases, alongside clinical signs of fin rot and scale loss. The parasite Argulus japonicus was identified based on 18S rRNA with a 100% prevalence and a mean intensity of 79 parasites per fish. Conventional plate culture of the fish and parasite's internal tissue resulted in the isolation of three distinct bacterial strains, identified as Aeromonas sobria, Aeromonas hydrophila, and Enterobacter aerogenes based on biochemical tests. However, further taxonomic validation using whole genome sequencing and in-silico genome-to-genome hybridization approach reclassified both A. sobria and A. hydrophila as Aeromonas jandaei. The presence of multiple bacterial species within the fish parasite indicates its potential role as a vector in disease transmission. The pathology observed in the affected Japanese koi was primarily driven by tissue injury from A. japonicus attachments and bites, and alongside generalised haemorrhage associated with secondary Aeromonas spp. infections. The high parasite burden and tissue damage at the infection site suggest that A. japonicus potentially impairs host defenses and facilitates secondary bacterial infections, ultimately contributing to the mortality event. The findings suggest that the high mortality rate was associated with a multifactorial event involving low dissolved oxygen levels, a severe A. japonicus infestation, and subsequent secondary bacterial infections.