Abstract
The present work aimed to investigate the causes of summer mortality syndrome affecting cultured European seabass (Dicentrarchus labrax) by examining physiochemical farm water characteristics, isolation, and identification of recovered bacterial pathogens from diseased fish studying the effect of water temperature on stress biomarkers and disease severity. Studied water parameters were normal except ammonia and dissolved oxygen was higher and lower than the standard value. Sixty-two bacterial isolates were recovered from moribund fish and identified as 31 Vibrio fluvialis, 23 Pseudomonas aeruginosa and 8 Staphylococcus aureus isolates. The calculated LD50 of V. fluvialis, P. aeruginosa and S. aureus for D. labrax fingerlings were 4.67 × 107, 2.37 × 106 and 1.38 × 107, respectively. There was a direct correlation between water temperature and mortality rate of fish challenged with V. fluvialis as the mortality rate was 44.44, 50, 66.66, and 83.33% for fish maintained at 27, 30, 33, and 36°C. Plasma cortisol, superoxide dismutase, catalase and malondialdehyde significantly increased when the water temperature exceeded 30°C. The experimentally infected fish showed similar clinical signs and postmortem lesions of naturally diseased fish with no boundary between different pathogens. Antibiogram test indicated that florfenicol was the most effective antibiotic against all the recovered bacterial isolates while all isolates resisted sulfamethoxazole-trimethoprim. Massive degenerative changes observed in the hepatopancreas, posterior kidney and gill tissues of experimentally infected fish.
Highlights
European seabass (Dicentrarchus labrax) is considered the most economically important fish in Mediterranean mariculture (Vandeputte et al, 2019)
Thirty-one V. fluvialis isolates were grown on thiosulphate citrate bile salts sucrose agar (TCBS) as yellow, medium-size colonies about 2–3 mm in diameter
V. fluvialis was susceptible to doxycycline, florfenicol, and erythromycin; it was intermediately sensitive to spiramycin, but resisted oxytetracycline, sulfamethoxazole-trimethoprim, and amoxicillin
Summary
European seabass (Dicentrarchus labrax) is considered the most economically important fish in Mediterranean mariculture (Vandeputte et al, 2019). Egypt produces 14% of cultured D. labrax and Sparus aurata in the Mediterranean region, and it is considered the third major producer after Turkey and Greece (Muniesa et al, 2019). Many bacterial pathogens are responsible for severe diseases affecting farmed seabass in the Mediterranean. Muniesa et al (2019) reported that Vibriosis was the most frequently reported bacterial infection in D. labrax, followed by tenacibaculosis and photobacteriosis. Uzun and Ogut (2015) isolated Aeromonas veronii, Photobacterium damselae subsp. Damselae, Vibrio vulnificus, V. harveyi and V. rotiferianus from cage cultured D. labrax. V. anguillarum, V. harvey, Photobacterium damsela subsp. Piscicida, Tenacibaculum, Aeromonas spp. and Mycobacterium spp. represents the primary pathogens in European seabass aquaculture in Europe (Zrncic, 2020) V. anguillarum, V. harvey, Photobacterium damsela subsp. piscicida, Tenacibaculum, Aeromonas spp. and Mycobacterium spp. represents the primary pathogens in European seabass aquaculture in Europe (Zrncic, 2020)
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