The "low-intensity" management practices used prior to the 1980's generally resulted in good pond water quality and lower overall stress on fish populations. Lower fish densities also meant less efficient transmission of disease organisms. Over the years, stocking and feeding rates steadily increased and producers adopted a multiple-batch cropping system wherein new populations of fingerlings were stocked into ponds with existing populations of larger fish. These production practices lead to the emergence of infectious diseases as the primary limiting factor in catfish production, and disease outbreaks are not uncommon even on well-run facilities.
About 45 percent of inventory losses on catfish fingerling farms are attributable to infectious diseases. Corresponding survey data for food-sized fish are lacking. Of the overall catfish losses caused by infectious disease, approximately 60 percent are the result of single or mixed bacterial infections, 30 percent result from parasitic infestation, 9 percent from fungal infections, and 1 percent are of viral etiology. Multiple or mixed infections often occur in pond raised channel catfish making treatment decisions difficult.
Economic losses resulting from infectious diseases are difficult to quantify because record keeping varies among farmers and many diseases go unreported. Nevertheless, infectious disease is believed to cost producers many millions of dollars in direct fish losses each year. In addition, infectious diseases influence profitability by increasing treatment costs, reducing food consumption by fish, increasing feed conversion ratios, and causing harvesting delays. Fish-eating birds may also be attracted to ponds with sick and dying fish causing further losses.
There are several disease syndromes for which the etiology remains in question, such as channel catfish anemia (CCA), which has also been referred to as "no blood disease". Another syndrome is visceral toxicosis of catfish (VTC), believed to be caused by a toxin.
Once a disease outbreak occurs, effective health management requires three basic steps: problem identification, diagnosis, and corrective management--all of which must be performed in a timely manner to avoid further losses. Whenever multiple factors contribute to the disease process, it makes the diagnosis more difficult and often complicates corrective management. source : http://msucares.com/aquaculture/catfish/disease.html