Florfenicol- and erythromycin-medicated feeds are similarly efficacious in reducing mortality from Lactococcus petauri infections in Rainbow Trout.

Objective: Piscine lactococcosis associated with Lactococcus petauri is a serious emerging threat to fish populations in the Americas. This bacterial disease commonly presents as a hemorrhagic septicemia, resulting in high mortality rates and substantial financial losses. There are no commercial vaccines in the United States, and treatment options are limited and understudied. Florfenicol (Aquaflor) is a broad-spectrum antibiotic approved for finfish aquaculture, and erythromycin (Aquamycin 100) is an investigational new animal drug. This study aimed to compare the effectiveness of florfenicol- and erythromycin-medicated feed against lactococcosis through cohabitation challenges in Rainbow Trout Oncorhynchus mykiss.

Methods: Shedder fish were intracoelomically injected with L. petauri (∼1.5 × 104 CFU) and introduced to naïve populations at 13°C or 18°C. Treatments were initiated immediately after observation of mortality, with fish receiving florfenicol at 15 mg/kg for 10 d, erythromycin at 100 mg/kg for 21 d, or the control diet.

Results: At 18°C, cohabitant survival was significantly higher in florfenicol-treated (100%) and erythromycin-treated (93%) tanks compared to untreated positive controls (60%). There were no mortalities at 13°C or in the negative control tanks. In sampled survivors, L. petauri was detected by quantitative PCR in 29% of positive control fish, 14% of erythromycin-treated fish, and 14% of florfenicol-treated fish at 18°C compared to 21, 7, and 0%, respectively, at 13°C. Culturable bacteria were only recovered from 14% of positive control fish at both temperatures.

Conclusions: These results indicate that early intervention with florfenicol or erythromycin can limit the spread of L. petauri and that lower water temperatures reduce disease onset, improving options for managing lactococcosis in aquaculture.