PSEUDOMONIASIS

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Causative Agent(s)

Several species of Pseudomonas have been implicated in disease outbreaks in fish. Examples are Pseudomonas chloraphis, P. fluorescens, P. plecoglossicida, P. pseudoalcaligenes and P. putida (Austin and Austin, 2007). In their compilation of species of Pseudomonas affecting fish, P. aeruginosa was not considered a pathogen, let alone being mentioned as opportunistic. Hossain et al. (2006) however had isolated Pseudomonas aeruginosa from naturally infected sick tilapia in Rajshahi, Bangladesh.

Pseudomonas aeruginosa is a highly motile Gram-negative rod that is strongly oxidase positive. Clinical signs, gross and microscopic lesions

Clinical Presentation and Epidemiology

Experimental infection of Clarias gariepinus with P. aeruginosa resulted in lethargy and anorexia. Distinct gross and microscopic lesions were observed. Grossly, diffuse cutaneous depigmentation, corneal opacity, broken barbels, loss of body slime, extensive branchial pallor with serosanguinous fluid below the arborescent organs, necrotic hepatitis with petechiation and liquefactive necrosis of the kidneys, were observed (Oladele et al., 2019). Microscopically, lesions induced by P. aeruginosa infection in Clarias gariepinus included diffuse degeneration of the ova and colonies of the bacterium were also found in the ovarian blood vessels. Pseudomonas aeruginosa induced severe diffuse tubular degeneration and necrosis with severe thrombosis in renal and hepatic vessels (Oladele et al., 2019). This suggests that P. aeruginosa destroys fish via disseminated intravascular coagulation.

Diagnostic Consideration

Affected organs (e.g. liver, kidney, spleen) observed during necropsy are harvested for microbial analysis and the bacterial isolate is subjected to Gram staining and several biochemical tests for identification of the organism. As for Pseudomonas aeruginosa, it typically produces Pyocyanin (a bluish-green pigment) on Nutrient agar.

Management and Treatment

This is best done by first identifying the source of infection (e.g., contaminated feed) and tackling it from this point. The use of antibiotics should be based on outcome of antibiotic susceptibility test. Drugs like Enrofloxacin are effective in controlling this condition. It can be administered at 10mg/kg body weight via feed for 5 to 7 consecutive days.