Various developmental stages of eyed eggs and newly hatched sac fry of rainbow trout (Oncorhynchus mykiss) were exposed to several concentrations of laboratory produced spores of the triactinomyxon stage of Myxobolus cerebralis, the infective stage of salmonid whirling disease. Exposed eggs and sac fry and appropriate unexposed controls were examined microscopically immediately after challenge for the presence of initial forms of the disease and 4 months later for the presence of spores of the myxosporean stage of Myxobolus cerebralis in resulting fingerlings. Although initial forms of whirling disease shown as intracellular aggregates of small sporozoites (sporoplasms) 1.5–2.0 μm in diameter were found in the epithelium of eyed eggs a few hours before hatching and in 1-day-old sac fry, the resulting fingerlings examined 4 months later were free of M. cerebralis spores. The youngest trout that became infected with whirling disease and yielded spores of M. cerebralis was the 2-day-old sac fry. This result indicates that the distribution of eyed eggs from water contaminated with whirling disease should not pose a threat of spreading the disease into non-endemic areas.
