Pasteurellosis (Photobacteriosis): causes, symptoms and treatment

Pasteurellosis (Photobacteriosis) (Photobacterium damselae subsp. piscicida) — etiology, symptoms, diagnosis, active-substance medication, recovery and prevention; mortality without treatment: high.

Overview

Marine Gram-negative pathogen causing chronic systemic infection with white granulomas in spleen and kidney. Major problem in marine aquaculture, occurs in ornamentals. Causative agent: Photobacterium damselae subsp. piscicida. Transmission: water. Incubation: 7-30 days. Reported mortality without treatment: high.

Symptoms

chronic wasting
darkening
lethargy
white nodules in internal organs (necropsy)
anorexia
delayed mortality

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 Photobacterium damselae subsp. piscicida.

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

Florfenicol or oxytetracycline. Florfenicol 10 mg/kg or oxytetracycline 75 mg/kg in feed for 10-14 days; resistance increasing in aquaculture strains. (duration: 10-14 days)

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

avoid summer temperature peaks
quarantine marine fish
good biosecurity
vaccination available for aquaculture