Pathological investigation of Photobacterium damselae causing big belly disease in barramundi Lates calcarifer (Bloch 1790): case study and experimental infection

Abstract

The purpose of this research was to investigate the pathological alterations of Photobacterium damselae associated with big belly disease in barramundi Lates calcarifer, using case study and experimental infection. A total of twenty natural sick barramundi showing big belly symptoms were sampled for parasites and bacterial identification in abdominal and faecal samples. For experimental infection, a total of 150 healthy fish with an average length of 9.46 ± 0.41 cm and a bodyweight of 12.92 ± 1.28 g were divided into five replicates (10 fish per replicate) of three treatments; phosphate buffer saline as control, 1.53 x 107 cfu mL-1 and 1.28 x 103 cfu mL-1 P. damselae. Clinical symptoms, mortality, histology and blood profile of fish from each replicate of experimental challenge was observed. The DNA sequencing results from naturally infected big belly disease fish indicated 100% homolog of P. damselae. The experimental infection results demonstrated that P. damselae caused mortality of up to 72.50 ± 9.57% at 9 days post infection. Pathological investigation of fish infected with P. damselae in both natural and experimental conditions revealed a swollen abdomen with reddish hazy liquid and a red patch on the liver. All of the examined tissue showed necrosis and haemorrhage. Severe intestinal peritonitis was shown by the accumulation of reddish hazy fluids in the abdominal cavity. Barramundi fish infected with higher concentration of P. damselae showed a higher number of white blood cells and lymphocytes (P<0.05) than fish from a lower concentration infection or control group. The findings of this study suggest that P. damselae is pathogenic to juvenile barramundi and causes significant mortality within two weeks from the onset of infection.