Aeromonas hydrophila induces intestinal inflammation in grass carp (Ctenopharyngodon idella): An experimental model

• The work describes the experimental induction of enteritis in a grass carp model by the administration of Aeromonas hydrophila.
• Previous reports did not sufficiently study intestinal microvilli effacement at sites of bacterial attachment.
• Anal intubation is critical for successful bacterial infection in the proposed model.
• A disease activity index (DAI) scoring system was specifically designed to evaluate the severity of intestinal inflammation in fish.

Bacterial enteritis is a widespread inflammatory disease in most farmed fish, yet its pathogenesis is largely unknown, primarily because of the paucity of reproducible experimental models of intestinal inflammation in fish. The aim of the present study was to develop a model of intestinal inflammation in the grass carp by challenging it with a strain of Aeromonas hydrophila. To induce intestinal inflammation, the fish were challenged with A. hydrophila via anal intubation at 2.7 × 106 cfu/fish, an appropriate dose based on the disease activity index for inflammatory signs. The pathological changes occurring in the intestines over the 21 day experimental period were examined with light microscopy and scanning electron microscopy. Histopathological examinations showed that A. hydrophila infection caused severe intestinal lesions, including intestinal villus fusion and shedding, and induced heavy inflammatory cell infiltration on day 3 after challenge. Ultrastructural observations also revealed that bacterial infection caused microvillus effacement. The injured intestinal villi were gradually repaired during the subsequent 18 days of the experimental period. Our results also indicated that the inflammatory responses induced by A. hydrophila infection were closely associated with the expression of pro-inflammatory cytokines interleukin 1β (IL-1β), IL-8, and tumor necrosis factor α (TNF-α) and myeloperoxidase (MPO) activity. The highest mRNA levels of these cytokines and highest MPO activity in the intestine were observed on day 3 after challenge. These results show that the intestinal lesions caused by A. hydrophila infection and their repair were completely consistent with the time course of inflammation-related genes or biomarkers. Thus, we have developed an experimental model of intestinal inflammation in an intensively farmed fish. This model is expected to be useful in clarifying the pathogenesis of bacterial enteritis in the grass carp and other farmed fish, and in testing antimicrobial drugs for potential use in aquaculture.