EFFECT OF WATER TEMPERATURE ON INFECTIONS WITH THE MICROSPORIDIAN ENTEROCYTOZOON SALMONIS IN CHINOOK SALMON

The effect of water temperature on the progress of infections associated with Enterocytozoon salmonis Chilmonczyk, Cox, Hedrick 1991 was examined in chinook salmon Oncorhynchus tshawytscha after intraperitoneal injections of mononuclear leukocytes infected with the microsporidian parasite. Experimentally infected and control fish were held at water temperatures of 9, 12, 15, 18 and 21 degrees C for 12 wk and then one half of the exposed and control groups of fish at 9 and 12 degrees C were shifted to 15 degrees C and held for an additional 8 wk. Among fish held at constant water temperatures, severe infections occurred among exposed fish at 15 and 18 degrees C resulting in 90.0% cumulative mortality in both groups. Disease and significant mortality was also observed at 21 degrees C (47.5%). The parasite and signs of the disease slowly developed over time at 12 degrees C and the cumulative mortality reached 73.7% between 13 and 20 wk. Although the development of the microsporidian was not arrested at a water temperature of 9 degrees C, infections in chinook salmon were not severe and cumulative mortalities were low (10.0%). However, parallel groups of exposed chinook salmon at 9 degrees C which were shifted to 15 degrees C showed a cumulative mortality of 60.0% by 8 wk after transfer to the higher water temperature. Shifting the exposed fish from 12 to 15 degrees C did not increase the mortality rate from that of fish kept constantly at 12 degrees C. The control fish (not exposed to E. salmonis) in all temperature groups did not show signs of the disease nor mortality throughout the study.