Pseudomonas fluorescens and Edwardsiella tarda Isolated from Diseased Tilapia

Abstract

Chronic mortarities up to 0.2-0.3 % a day occurred in Osaka, Aichi and Nara Prefectures from 1980 to 1981 at some farms of tilapia, Sarotherodon niloticus.
Two types of clinical features were observed among the diseased fish. One was characterized by fine white nodules in the spleen and abscesses in the swimming bladder. The other was characterized by hemorrhagic lesions in the gonad, especially ovary. Pseudomonas fluorescens and Edwardsiella tarda were associated with the respective types. These bacteria were proved to kill tilapia by intramuscular injection and to cause essentially the same features as those naturally infected.
Ps. fluorescens infection occurred mainly in winter and spring while E. tarda infection took place at warmer seasons. These epizootiological phenomena were supported by experimental infections performed at selected water temperatures. The mortalities of tilapia due to Ps. fluorescens and E. tarda became highest at 15-20°C and at 20-30°c, respectively.