Enteric Redmouth Disease: causes, symptoms and treatment

Enteric Redmouth Disease (Yersinia ruckeri) — etiology, symptoms, diagnosis, active-substance medication, recovery and prevention; mortality without treatment: high.

Overview

Gram-negative pathogen primarily of salmonids, causing characteristic hemorrhages around mouth and isthmus. Major aquaculture pathogen with available vaccines. Causative agent: Yersinia ruckeri. Transmission: water. Incubation: 5-19 days. Reported mortality without treatment: high.

Symptoms

red hemorrhages around mouth
darkening
exophthalmia (popeye)
petechiae on belly
lethargy
anorexia

Causes

Outbreaks are typically triggered by chronic stress, poor water quality, temperature swings, overcrowding, or the introduction of unquarantined fish. The pathogen spreads via free-swimming or waterborne stages in shared water.

Diagnosis

Diagnosis is based on clinical signs (lesions, hemorrhages, behaviour) combined with bacterial culture and Gram-staining where available. Differentiate from co-infections with other Gram-negative pathogens; antibiotic sensitivity testing improves treatment success against Yersinia ruckeri.

Treatment

Effective treatment requires isolating affected fish in a quarantine tank, identifying the pathogen, administering the appropriate active substance at the correct dose and duration, and supporting recovery with stable water parameters and nutrition.

Step 1: Quarantine

Set up a bare-bottom quarantine tank with a mature sponge filter, heater, and aeration. Match temperature and pH to the display tank, and acclimate fish slowly. A bare bottom simplifies daily siphoning and prevents medication from being absorbed by substrate.

Step 2: Medication

Oxytetracycline / florfenicol. Oxytetracycline 75 mg/kg or florfenicol 10 mg/kg in feed daily for 10 days. (duration: 10 days)
ERM vaccination. Immersion or oral vaccine for trout fry — primary preventive measure in aquaculture. (duration: single dose)

Step 3: Recovery

After medication, perform a 30-50% water change and run fresh activated carbon for 24-48 hours to remove residues. Continue feeding a high-quality, varied diet with vitamins and immunostimulants. Reintroduce fish to the display tank only after at least one week without recurrence of symptoms.

Prevention

vaccinate trout fry
avoid temperature stress
minimize handling stress
quarantine new fish