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 microsporidlan parasite. Experimentally infected and control fish were held at water temperatures of 9, 12, 15, 18 and 21°C for 12 wk and then one half of the exposed and control groups of fish a t 9 and 12°C were shifted to 15°C and held for a n additional 8 wk. Among fish held a t constant water temperatures, severe infections occurred among exposed fish a t 15 and 18°C resulting in 90.0% cumulative mortality in both groups. Disease and significant mortality was also observed at 21°C (47.5 %). The parasite and signs of the disease slo\vly developed over time at 12'C and the cumulative mortality reached 73.7% between 13 and 20 wk. Although the development of the microspondian was not arrested at a water temperature of 9"C, infections in chinook salmon were not severe and cumulative mortalities were low (10.0%). However, parallel groups of exposed chinook salmon at g0C which were shifted to 15°C showed a cumulative mortality of 60 0% by 8 wk after transfer to the hlgher water temperature. Shifting the exposed fish from 12 to 15°C did not increase the mortality rate from that of fish kept constantly at 12OC. The control fish (not exposed to E. salmonis) in all temperature groups did not show signs of the disease nor mortality throughout the study.KEY WORDS: Enterocytozoon salmonis -Water temperature Chinook salmon . Microsporea Enterocytozoon salmonis Chilmonczyk, Cox, Hedrick 1991 is an intranuclear microsporidian associated with a severe anemic condition in salmonid fish, particularly, chinook salmon Oncorhynchus tshawytscha (Hedrick et al. 1990, Baxa-Antonio et al. 1992. Spontaneous infections with the microsporidian have also been reported in steelhead trout 0. mykiss (MacConnell et al. 1991) and among cultured populations of golden trout 0 . aquabonita and brook trout Salvelinus fontinalis (R. Hedrick unpubl. obs.). The progress of infection with the microsporidian has been described in chinook and kokanee salmon Oncorhynchus nerka (Hedrick et al. 1990(Hedrick et al. , 1991. Earlier studies documented the occurrence of similar infections in chinook salmon raised in sea water or fresh water (Elston et al. 1987, Morrison et al. 1990). We have demonstrated transmission of the parasite and the progress of infections in chinook salmon held in fresh and sea water (Antonio & Hedrick unpubl.).Enterocytozoon salmonis has been successfully propagated jn vitro using a newly developed culture medium (Wongtavatchai et al. 1994). Using parasites from in vltro cultures of infected leucocytes as an inoculum, the effect of infections on chinook salmon at 5 water temperatures was examined.Materials and methods. Juvenile chinook salmon (mean wt 20.0 g) were acclimated for 14 d at water temperatures of 9, 12, 15, 18 and 21°C in 132 1 a q u a~l a receiving f...
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