For 80 years the infectivity of salmonid whirling disease has eluded discovery. New findings now show that this myxosporean disease of fish is initiated by what is regarded as an actinosporean produced in a tubificid oligochaete. Experimental results provide evidence that, instead of being considered as representatives of separate classes in the phylum Myxozoa, the myxosporean and actinosporean are alternating life forms of a single organism.

An effective sequential procedure for recovery of Myxosoma cerebralis spores from infected trout was developed, and quantification of spores was carried out at each step of release and concentration. Methods are described for fresh and frozen material. Effective concentration of from 1100- to 9000-fold and an estimated efficiency of recovery of about 80% has been achieved. Tabular and graphic data are presented with recommendations for diagnostic applications. The immediate applications of these procedures are in implementing more effective detection and in preparing antigen for the immunologic studies that should provide the most sensitive detection.

Studies of the life cycle of Myxosoma cerebralis showed that development of infectivity did not occur endogenously but that the spore “aging” process required participation of an aquatic tubificid oligochaete. Data suggestive of such involvement were derived from trials in which spores were “aged” in an array of inert, sterilized, pasteurized, or natural aquatic substrates and from examination of aquatic soils from trout hatcheries in which whirling disease was epizootic. The role of the aquatic oligochaete was confirmed two ways. First, signs of whirling disease developed, and M. cerebralis spores were produced in young rainbow trout (Salmo gairdneri) that had been fed oligochaetes harvested from pond soil taken from two hatcheries where whirling disease was epizootic. Second, when containers of pasteurized soil were populated with four genera of oligochaetes–Aeolosoma, Dero, Stylaria, or Tubifex– from a biological supply house, or with tubificid worms from trout hatcheries free of whirling disease, and then seeded with M. cerebralis spores and “aged” for 4 months, whirling disease occurred only in trout held with Tubifex and with hatchery tubificids.

The intensity and prevalence of whirling disease was tested by exposure of 2-monthold fry and 1-, 2-, and 3.5-year-old adults of rainbow trout Oncorhynchus mykiss to a known number of laboratory-produced Myxobolus cerebralis at the actinosporean triactinomyxon stage. Fry exposed to graded concentrations of infectivity (triactinomyxons) for 3 h were individually examined for spores of Myxobolus cerebralis 5 and 6 months later. Exposure offish to the lowest doses, 1 and 10 triactinomyxons per fish, did not result in detectable myxosporean spores. Fish that became lightly infected by a dose of 100 triactinomyxons per fish experienced a decrease in the incidence of infection between 5 and 6 months after exposure. A linear relationship was found between the numbers of recovered myxosporean spores and doses of 100-10,000 triactinomyxons per fish, and the spore burden appeared to plateau at doses of 10,000-100,000 triactinomyxons per fish. Adult fish continuously exposed to the highest dose of triactinomyxons for 3.5 months were infected and asymptomatic; however, the severity of myxosporean infection decreased with increased age of fish. This information may help in controlling whirling disease in salmonids.

Portals of entry via the skln, fins, buccal cavlty and dlgestlve tract have been demonstrated for the myxozoan that causes salmonld whirling disease Exper~mentally, young rainbow trout Salmo ga~rdner~ were evposed to tnactlnomyxon spores W~thin 10 nun after ~nitial exposure intracellular aggregates of trlactinomyxon sporoplasms appeared in the e p~t h e h a of exposed fish Dunng several hours following penetrat~on, the sporoplasms moved or were trdnsported from the external ep~thelial layers into deeper strata Unexposed control trout were free of the organlsm The aggregates of intracellular triactinomyxon sporoplasms were idenufied serologically as belng Myxobolus cerebrahs