as far as i know, and atleast for the majority of freshwater catfish they are not venomous, however there "stingers" i guess you could call them which are on the leading adges of there pectoral and dorsal fins are more like barbs and do carry a large amount of dangerous bacteria, much as what is in a lizards mouth such a komodo dragon or monitor lizard, any wound sustained from a catfish should be immediatley flush with sterile water and cleaned out thuroughly. a good course of atleast topical antibiotics and possibly even oral antiobitics would be advised as well.
on the other hand however ive been stung a couple times by the barbs and havent cleaned them out, they are definatley sore for a few days to a few weeks though and i could easily see how a deep puncture wound from one of these could esily entire the blood stream and cause major problems.
almost all catfish have barbs atleast on there dorsal fin if not there pectoral(side) fins. it is the hard leading edge of these fins, if you look closely you can see how this forward edge is much thicker and harder than the rest of the fin. in addition when netter these fins can easily poke through the net and cause problems, getting the fish entangled damaging the fins. that is why most catfish keepers use the semi hard rubber nets or very fine mesh nets, much like those used for brine shrimp.
