Abstract
A highly virulent clonal population of Aeromonas hydrophila (vAh) has been the cause of recent motile Aeromonas septicemia epizootic in channel catfish (Ictalurus punctatus) farms in the Southeastern United States. The pathology of the disease caused by vAh has not been studied well yet. Thus, our aim was to determine histopathological and ultrastructural changes in channel catfish following vAh challenge. To accomplish this, catfish fingerlings were challenged with vAh (strain ML09-119) by bath. Six fish per each time point were collected at 1, 3, 5, 6, 24, and 48 h for light microscopy, and six fish were collected at 48 h for transmission electron microscopy (TEM). The first pathological lesions were detected in the spleen and stomach at 1 h post-challenge (HPC) while intestine, gills, kidney, and liver lesions were observed at 24 and 48 HPC. Histopathological examination revealed degenerative changes, necrosis, extensive edema, and inflammation in internal organs. The TEM showed severe tissue destruction with multiple bacterial cells secreting outer membrane vesicles, especially in spleen and gills and far number in the stomach. Degenerated bacterial cells were observed in the intestinal lumen and the phagosomes of phagocytic kidney cells. We identified, for the first time, degranulate eosinophilic granular cells, and dendritic cells like (DC-like) cells in the necrotic intestinal epithelium. These findings suggest that vAh rapidly proliferated and spread through the catfish organs following bath challenge.
Highlights
Aeromonas hydrophila is a Gram-negative, facultative anaerobic, motile bacterium that is the causative agent of motile Aeromonas septicemia (MAS) in fish
The disease in fish has two forms: acute hemorrhagic septicemia characterized by generalized edema, hemorrhage, and diffuse necrosis; and chronic ulcerative syndrome marked by Virulent A. hydrophila Pathogenesis in Channel Catfish the formation of deep dermal ulcers (Huizinga et al, 1979; Cipriano et al, 1984)
Fish mortality in the virulent A. hydrophila strain (vAh) challenge group was significantly higher than that of control group (P < 0.05; 62.4% vs. 0%), and most of the mortalities occurred within 5–48 h post-challenge (HPC)
Summary
Aeromonas hydrophila is a Gram-negative, facultative anaerobic, motile bacterium that is the causative agent of motile Aeromonas septicemia (MAS) in fish. This bacterium is widely distributed in aquaculture and can cause significant losses in the presence of predisposing stressor (Plumb and Hanson, 2010). Since 2009, a highly virulent A. hydrophila strain (vAh) has caused severe acute mortality in cultured catfish in Alabama, Mississippi, and Arkansas (Hemstreet, 2010; Da Silva et al, 2012). The disease has cost about three million pounds of food-size catfish annually (Hemstreet, 2010). These epizootics have continued to affect catfish production adversely (Bebak et al, 2015)
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