Susceptibility and Clinicopathological Findings in Three Amazonian Fish Species Experimentally Infected with Lactococcus spp.: A Preliminary Study

Bacterial diseases cause significant losses in the aquaculture of Amazonian fishes (Arapaima gigas, Brycon amazonicus, and Colossoma macropomum). Lactococcosis-causing bacteria (LCB) have emerged as important pathogens in global aquaculture, and although they have already been isolated from these fish species, their pathogenic role remains unconfirmed. This study evaluated the susceptibility of three Amazonian fish species to Lactococcus spp. through experimental infection and performed a detailed examination of the associated pathological damage. Lactococcus formosensis, L. garvieae, and L. petauri strains were used to intracoelomically infect juvenile Amazonian fish. Following infection, the fish were monitored for 15 days to evaluate clinical signs and mortality. Clinical signs were observed in the L. garvieae-challenged A. gigas, with one fish dying at five days post-infection. The remaining experimental animals, regardless of fish species, survived the challenge. All mortalities and surviving fish at the end of the trial underwent bacteriological and histopathological examinations. Bacteriological examination confirmed bacterial re-isolation from asymptomatic animals in the L. garvieae-challenged A. gigas (2/7) and L. formosensis-challenged C. macropomum (3/7) groups using MALDI-TOF MS and multiplex qPCR analysis. No bacterial growth or histological alterations were observed in the control groups or in the groups infected with L. petauri. Microscopic examination revealed L. garvieae-induced fibrinoid-necrotic hepatitis, lymphohistiocytic myocarditis, and myositis in diseased or asymptomatic A. gigas, while L. formosensis caused lymphohistiocytic pericarditis in surviving C. macropomum. These findings provide preliminary evidence suggestive of susceptibility of A. gigas and C. macropomum to infection by L. garvieae and L. formosensis, respectively, highlighting their potential to act as asymptomatic carriers. Furthermore, the histopathological findings were suggestive of species-associated lesion patterns of piscine lactococcosis in Amazonian fish, rather than indicative of high mortality.