Lesions caused by virulent Aeromonas hydrophila in farmed catfish (Ictalurus punctatus and I. punctatus × I. furcatus) in Mississippi

In 2009, a virulent strain of Aeromonas hydrophila caused severe disease and high mortalities (motile aeromonad septicemia variant [MASv]) in farmed channel catfish (Ictalurus punctatus) and hybrid catfish (I. punctatus × I. furcatus) in eastern Mississippi and Alabama. As is common in MAS, there is severe hemorrhagic dermatitis with ulceration, as well as abdominal hyperemia, petechiation, and mild ascites. Additional findings in MASv cases include panophthalmitis and orbital cellulitis, leading to ocular rupture, and brains are often hyperemic with mild random acute hemorrhage. In MASv infections, there is consistently also marked hemorrhage and edema in the submucosa and muscularis of the stomach, with lymphangitis and a few bacteria, plus splenomegaly with infarcts. Microscopically, spleens have necrosis of ellipsoids with macrophage infiltration and small numbers of bacteria; however, large infarcts are filled with bacteria. Other organs, such as liver, kidneys, and intestine, which are typically associated with MAS caused by various aeromonad species, are less affected. The findings in the stomach have not been reported in MAS in farmed catfish, to our knowledge, and the splenic changes are highly characteristic of MAS compared to infection with other gram-negative bacteria, including Edwardsiella ictaluri and other aeromonad species and strains.