Maria E. Markiw

Biology Contravenes Taxonomy in the Myxozoa: New Discoveries Show Alternation of Invertebrate and Vertebrate Hosts

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.

Experimentally induced whirling disease I; Dose response of fry and adults of rainbow trout exposed to the Triactinomyxon stage of Myxobolus cerebralis

Abstract 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 of fish 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 triactin...

Experimentally Induced Whirling Disease II. Determination of Longevity of the Infective Triactinomyxon Stage of Myxobolus cerebralis by Vital Staining

Abstract Effects of temperature and aging on viability and infectivity of laboratory-produced actinosporean triactinomyxon spores (infective stage of the organism causing whirling disease) were studied. In vitro staining of triactinomyxon spores with vital fluorescein diacetate correlated with the ability of the spores to infect fry of rainbow trout Oncorhynchus mykiss. The experimentally produced actinosporean stage of Myxobolus cerebralis was short-lived, persisting for only 3–4 d at 12.5°C and for less time at warmer temperatures. The vital staining method has potential for screening therapeutants intended to control myxosporean infection of fish.

Whirling disease: earliest susceptible age of rainbow trout to the triactinomyxid of Myxobolus cerebralis

Abstract 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 μ m 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.

Communications: The Occurrence of Myxobolus neurobius (Myxosporea) in Wild Young Atlantic Salmon and Arctic Char in Newfoundland

Abstract Spores identified as Myxobolus neurobius were found in head homogenates of both wild young Atlantic salmon Salmo salar and Arctic char Salvelinus alpinus. Formalin-preserved spores from Atlantic salmon measured an average of 12.9 × 9.4 μm and contained two polar capsules that measured 6.4 × 2.9 μm. The spores from the Arctic char appeared morphologically identical by bright-field microscopy to those from the Atlantic salmon except they measured 13.7 × 9.6 μm and had polar capsules that measured 6.9 × 3.4 μm. Myxobolus neurobius has not previously been reported from Atlantic salmon or Arctic char.