Plesiomonas Septicemia: causes, symptoms and treatment

Plesiomonas Septicemia (Plesiomonas shigelloides) — etiology, symptoms, diagnosis, active-substance medication, recovery and prevention; mortality without treatment: moderate.

Overview

Gram-negative bacterium causing septicemia and intestinal disease in stressed warm-water fish. ZOONOTIC — implicated in human gastroenteritis from contaminated aquarium water. Causative agent: Plesiomonas shigelloides. Transmission: water. Incubation: 3-14 days. Reported mortality without treatment: moderate.

Symptoms

hemorrhages
ulcers
swollen abdomen
lethargy
loss of appetite
intestinal inflammation

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 Plesiomonas shigelloides.

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

Sulfa-trimethoprim or fluoroquinolones. TMP-SMX 30 mg/kg in feed daily for 10 days; enrofloxacin 5 mg/kg as alternative. (duration: 10 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

wash hands after aquarium contact
avoid wounds when working in tanks
good water quality
quarantine new fish