Microbiological Purification of Fish Grown in Fecally Contaminated Commercial Fish Pond

The digestive tracts of tilapia reared in commercial fish ponds with animal manure and/or municipal wastewater generally were contaminated with the fecal indicators E.coli and enterococci to levels greater than those in the environmental water. When the indicator densitites in the water were sufficiently high, the liver and muscle tissue also became contaminated, albeit to much lower levels. Thermotolerant aeromonas were recovered from the environmental water and fish tissues at levels 2-4 logs higher than those of the fecal indicators. Accumulation of the indicators in the tissues of the naturally contaminated (pond) fish generally was greater than that in the artificially contaminated fish.

The indicator levels in the tissues did not appreciably decrease when the pond-reared fish were maintained for 5-8 days under starvation conditions in tanks containing water which was not exchanged. However, the E.coli and virus levels in the tissues of experimentally contaminated tilapia were appreciably and significantly reduced under the same conditions. The results point out the limited value of conventional “purification” methods as applied to tilapia reared in fecally fertilized waters and of data obtained from studies with experimental animals. Since fish are cooked prior to consumption, the major public health concern could be the risk of Aeromonas wound infections among individuals who handle and process fish.