Aeromonas hydrophila is considered an important pathogen of fish causing substantial economic loss in aquaculture worldwide. The objective of the current study was to assess the virulence potential of A. hydrophila strains isolated from gills of apparently healthy fish using phenotypic assays, PCR based genotypic tests, and in-vivo pathogenicity assays in Indian catfish, Clarias magur. For this, motile aeromonads (n = 50) were isolated from gills of apparently healthy freshwater food fish (n = 50) collected from domestic fish market. Gill swabs were incubated for primary enrichment in alkaline peptone water supplemented with cephalothin (10 mg/mL) for 12 hours followed by plating on Starch Ampicillin Agar. A single big yellow colony was selected from each sample, purified, and presumptively identified as Aeromonas with the help of Aerokey-II. Out of the fifty isolates of Aeromonas, five isolates of diverse origin (9C, 10G1, 7C, 10C and 4P) were confirmed to be A. hydrophila by automated bacterial identification system VITEK 2 with confidence interval of 90-98%. In the phenotypic assays, strains 9C and 10G1 showed high serum resistance, swimming and swarming activity and low biofilm producing capabilities, which were indicative of putative virulence. Both the strains belonged to genotype aah + act + alt + ascV + eno + lip + ast + on the basis of detection of virulence genes by PCR, i.e. extracellular haemolysin (aah), cytolytic enterotoxin (act), heat-labile cytotonic enterotoxin (alt), type-III secretion system (ascV), enolase (eno), lipase (lip) and heat-stable cytotonic enterotoxin (ast). In-vivo pathogenicity assays confirmed that both the strains were pathogenic to magur fish. The LD 50 for 9C strain was 6.81 × 10 4 CFU/mL and 7.62 × 10 5 CFU/mL for 10G1strain. In conclusion, our phenotypic and genotypic findings showed that A. hydrophila isolated from apparently healthy fish harbour number of important virulence genes/factors and could have important implications in triggering disease in farmed fish under stress.

The use of chemicals, biologicals and veterinary medicinal products (VMPs) helps in healthy and sustainable fish production. Information on the use of these products is essential for assessing farming practices, potential human health and environmental risks. A questionnaire‐based nationwide survey covering aquaculture farms (n = 2936) producing carps, tilapia, pangasius and rainbow trout in freshwater and shrimp in brackishwater estimated the use of 52 different types of inputs which included disinfectants (597 g t−1), probiotics (2.28 kg t−1), environmental modifiers (22.82 kg t−1), nutritional supplements (1.96 kg t−1), natural anti‐infective agents (293 g t−1), herbicide and piscicides (844 g t−1), antibiotics (2 mg PCU−1), antifungal (4 mg PCU−1), and antiparasitic (14 mg PCU−1) agents. The bulk of these inputs was used for soil and water quality improvement and had low environmental and human safety concerns. The multivariate analysis revealed significant variation in the frequency and quantity of compounds use among farm groups. Redundancy analysis revealed a significant association between the number of products used and stocking density. The survey also showed a considerable influence of education and farming experience on the usage pattern of aquaculture inputs. Results of the study indicated greater reliance of farmers on the use of disinfectants for biosecurity, nutritional supplements for enhanced growth and environmental modifiers for maintaining soil and water quality in culture systems. Though there was no use of restricted antibiotics and antiparasitic agents, the development and implementation of standard regulatory guidelines are essential for safe and effective use of inputs for sustainable aquaculture.

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