Piscine lactococcosis caused by Lactococcus petauri is one of the most serious emerging threats to wild and farmed fish in the Americas. This bacterial disease commonly presents as a peracute-acute hemorrhagic septicemia, resulting in high mortality rates and substantial losses in animal life and financial revenue. There are no commercial vaccines for piscine lactococcosis in the United States, and treatment options are limited and understudied. Florfenicol (Aquaflor®) is a broad-spectrum antibiotic approved for use in finfish aquaculture and erythromycin (Aquamycin 100®) an Investigational New Animal Drug (INAD) with activity against gram-positive bacteria. The effectiveness of these antimicrobials in controlling lactococcosis was investigated through cohabitation challenges in a susceptible host species, rainbow trout (Oncorhynchus mykiss).
Shedder fish were intracoelomically injected with L. petauri (~1.5 x 104 CFU) and introduced to naïve tank populations maintained at either 13 or 18°C. Medicated feed treatments were initiated after the first observed mortality. Experimental tanks received either 15 mg/kg Aquaflor® for 10 days or 100 mg/kg Aquamycin 100® for 21 days. Negative and positive control tanks received antimicrobial-free feed. Both antimicrobials were effective in halting disease progression. At 18°C, cohabitant survival was significantly higher (p < 0.05) in the florfenicol (100%) and erythromycin (93%) treatment tanks compared to the positive controls (60%). There were no mortalities in shedder or cohabitant fish at 13°C. Bacterial persistence in surviving fish was assessed by culture and quantitative PCR (qPCR). By qPCR, L. petauri DNA was detected in 21% of positive control fish, 7% of erythromycin-treated fish, and 0% of florfenicol-treated fish at 13°C, compared to 29%, 14% and 14% in the respective groups at 18°C. However, culturable L. petauri was only re-isolated from 14% of the untreated positive control fish at either temperature. These results indicate that early intervention with florfenicol or erythromycin can limit mortality and spread of L. petauri, and that lower water temperatures may reduce disease onset, improving options for managing lactococcosis in aquaculture.