The effects of Myxobolus cerebralis on Apache and Gila trout in laboratory exposures

Whirling disease has been implicated in salmonid population declines in several western states. To determine the risk of a species or strain of salmonid to whirling disease it is critical to establish its relative susceptibility to Myxobolus cerebralis infection. Gila trout Oncorhynchus gilae and Apache trout Oncorhynchus gilae apache were exposed to various doses of M. cerebralis triactinomyxons (TAMs) in laboratory experiments. In trials conducted in consecutive years, fish were exposed to TAMs in doses ranging from 25 to 2,000/fish at ages ranging from 66 to 201 d posthatch (dph). All fish were held for 900 degree-days, and then the infection intensity of each fish was determined by histological examination. In 2002, 98% of the Gila trout died during exposure or within 48 h postexposure. Seventy-four percent of the Apache trout died before the end of the 90-d study period. Those that survived the entire study period had an average histological score of more than 4.0. In subsequent trials, the TAM dosage was decreased to 25-1,000/fish. Also, the age of the fish was increased from 66-72 dph to 89-201 dph. The survival rate increased from 16.0% to 49.1%. Average histological grades ranged from 1.6 to 4.8. Based on this data, it can be concluded that both Apache and Gila trout are highly susceptible to M. cerebralis in laboratory trials.