Establish and apply ISI pathological method to evaluate the intestinal health of largemouth bass ( Micropterus salmoides ) and channel catfish ( Ictalurus punctatus ) challenged with Aeromonas hydrophila

Gut health is essential for animal growth because it can absorb nutrients more efficiently, promote growth, and protect the body from invasion by pathogenic microorganisms. Temperature, dissolved oxygen, water quality, nutrition level, pathogen, net fishing, and so on will affect the gut health of aquatic animals. Intestinal diseases in aquatic animals were often not easily detected, making them difficult to prevent and control. Therefore, establishing a stable and scientific evaluation standard of gut health was of great significance. This study evaluated the effects of gross pathological and histological parameters of largemouth bass and channel catfish infected by Aeromonas hydrophila . A total of 150 largemouth bass and 150 channel catfish were randomly divided into 5 groups: 1 control group (NC) and 4 challenge groups (CH, fish were infected with A. hydrophila by intraperitoneal injection with a concentration of 200 µL 1x108 CFU/mL, 1x107 CFU/mL, 1x106 CFU/mL, and 1x105 CFU/mL, respectively). After the challenge, largemouth bass’ gross pathological and histological changes were observed on the 2nd, 6th, 9th, 12th, and 14th days. In contrast, channel catfish’s gross pathological and histological changes were observed on the 3rd, 6th, 9th, 12th, and 15th day. In this study, regarding the “I See Inside” (ISI) methodology of poultry, a pathological evaluation method of fish intestinal health was established to comprehensively evaluate the intestinal health status of fish by gross pathological and histopathological score. In gross pathology, it was scored by the body surface and intestinal lesions. In histology, it was a metric evaluation of histological alterations in the intestine, converting macroscopic and microscopic alterations into numbers and then scoring. Each evaluation parameter sets up an “influence factor” (IF) according to the degree of influence of its pathological changes on intestinal function. The final score was the sum of the IF product and this parameter’s score. The results of two kinds of fish showed that the gross pathological and histopathological scores correlated with the infection dose and time. And the significant difference in the score between the enteritis and control groups was observed. The results showed that the higher score, the more serious the pathological changes of fish infected by A. hydrophlia . From this, it can be concluded that the pathological evaluation method established can be used to evaluate the gut health of fish.