Internally, edema (fluid buildup in organs and in the body
cavity) and inflammation in many organs may be present.
Pinpoint hemorrhages are often observed in the wall of the
gas bladder and other internal organs. The intestine may be
severely inflamed and may contain significant amounts of
mucus. The spleen is often enlarged.
Concurrent infection with bacteria, particularly Aeromonas
(A. salmonicida or A. hydrophila), may confuse the
diagnosis because fish with this bacterial disease alone will
show similar signs of systemic infection such as ascites and
hemorrhages.