Geographic and seasonal distribution of the infectious stage of Ceratomyxa shasta Noble, 1950, a myxozoan salmonid pathogen in the Fraser River system

Abstract

The infective stage of C. shasta was demonstrated to be present at two lower Fraser River sites. However, it was absent in the McGregor River, a tributary of the upper Fraser, and rarely present in the upper Fraser River near Prince George, B.C. Seasonally, the infective stage occurred in the lower Fraser River from late April to mid-November. In April 1983, 12% of 25 fish became infected with C. shasta, indicating a low onset of activity. In experiments in 1981, 10- and 20-day exposures of 216 juvenile chinook salmon resulted in increased infections of C. shasta, from 72 to 75% in May to 96 and 98% in July. Time to death decreased from a mean of 54 to 30 days from first exposure. In 1982, 5- and 10-day exposures resulted in deaths of 98% of 181 fish from August to November. Mean time to death increased from 27 to 48 days and was probably due to decreasing river temperatures. In November, 90% of the fish became infected at water temperatures of 4 – 6 °C.