Susceptibility and Clinicopathological Findings of Three Amazonian Fishes Experimentally Infected with <em>Lactococcus</em> spp.

This study evaluated the susceptibility of three Amazonian fish species, Arapaima gigas, Brycon amazonicus and Colossoma macroporum to Lactococcus spp., through experimental infection, and performed a detailed examination of the pathological damage. While lactococcosis-causing bacteria have already been isolated from these fish species, their pathogenic role remains unconfirmed. 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. Fish that died or survived until the end of the experiment underwent bacteriological and histopathological examinations. 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, did not die. Bacteriological examination confirmed re-isolation of asymptomatic animals in the L. garvieae-challenged A. gigas and L. formosensis-challenged C. macropomum groups. No bacterial growth or histological alterations were observed in the control groups nor in the groups infected with L. petauri. Microscopic examination revealed L. garvieae-induced fibrinoid-necrotic hepatitis, lymphohistiocytic myocarditis and myositis in A. gigas, while L. formosensis caused lymphohistiocytic pericarditis in C. macropomum. These findings demonstrate that A. gigas and C. macropomum are susceptible to L. garvieae and L. formosensis, respectively, and that they can become bacterial carriers. The distinct histopathological profiles—particularly myositis (L. garvieae) and pericarditis (L. formosensis)—highlight species-specific manifestations of piscine lactococcosis in Amazonian fish species.