Identification and Characteristics of Aeromonas Hydrophila from Diseased Asian Swamp Eel (Monopterus Albus) with Hemorrhagic Sepsis

Abstract

Hemorrhagic sepsis caused by Aeromonas hydrophila is one of the most important deadly diseases in cultured Asian swamp eel (Monopterus albus). Here, we reported the identification and characteristics of the A. hydrophila MaAh001 strain from the blood of eels with hemorrhagic sepsis in Shanghai. The results of artificial infection showed that the main symptoms of the sick fish were bleeding on the body surface, swelling of the head, bulging of the anus, and visceral congestion and blackness. Histopathological examination showed the mucosal epithelium shedding, the lamina propria exfoliation, absence of glandular structures, and a small amount of vascular congestion in the intestinal tissue. In renal tissue, fewer renal tubules arranged loosely and irregularly, with a large number of visible necrotic cell fragments, and a small amount of renal tubule epithelial cells were swollen. The cytoplasm was loose and pale, arranged irregularly, and large areas of bleeding could be seen in the renal interstitium. Hepatocytes are arranged irregularly in the liver tissue, most of which have water degeneration, cells become swollen, loose cytoplasm, hepatocytes undergo necrosis, karyopyknosis and fragmentation. The boundary between red and white pulps in the spleen tissue was not obvious, brown-yellow particles gathered around a few blood vessels, and a small amount of necrotic cell fragments were visible. The arrangement of local muscle cells in muscle tissue was irregular. Occasionally, spot-like necrosis of muscle cells was seen. The median lethal dose (LD50) of MaAh001 to eels per gram was 3.9×106 CFU/g. Virulence gene test showed that MaAh001 carried 14 virulence factor genes (act, aer, aexT, aha, ahh, alt, ast, ela, eprA1, hly, lip, luxS, ompA, and ser). Drug sensitivity study showed that MaAh001 was highly sensitive to 16 antibiotics, including ceftriaxone, florfenicol and gentamicin etc., moderately sensitive to imipenem, sulfamethoxazole, ciprofloxacin, cephalexin, erythromycin and norfloxacin, and resistant to polymyxin B, lincomycin, oxacillin, vancomycin, penicillin, ampicillin and clindamycin. Meanwhile, the drug resistance gene detection showed that MaAh001 carried ant (3′′)-I, intl1, sul1 and sul2. In summary, this study reported a typical case of A. hydrophila MaAh001 strain infection in eels, which laid a theoretical foundation for the prevention and treatment of eel sepsis in aquaculture and the development of subsequent vaccines.

Keywords: Asian swamp eel (Monopterus albus), Hemorrhagic disease, Aeromonas hydrophila, Virulence gene, Drug resistance