Whirling Disease: Reviews and Current Topics

Relative Susceptibility of Various Salmonids to Whirling Disease with Emphasis on Rainbow and Cutthroat Trout

E. Richard Vincent

doi: https://doi.org/10.47886/9781888569377.ch10

ABSTRACT. There are many factors that govern the susceptibility of salmonids to the parasite Myxobolus cerebralis, such as fish size and age, the water temperature in which the fish is exposed to the infective spore stage (triactinomyxon), and the dosage (concentration level) of the infective spore. This study examined changes in infection intensity (degree of cartilage loss and cell inflammation in cranial cartilage) following exposure to 100–4,000 triactinomyxons per fish in 7.6 L of water for 2 h. The infection intensity in each fish was determined by histological examination of the cranial cartilage of each exposed fish, and then, a numerical score of zero to five was assigned to each fish based on this cartilage tissue damage. A total of ten rainbow trout Oncorhynchus mykiss strains, three strains of cutthroat trout O. clarki, and five other salmonid species that included brown trout Salmo trutta, bull trout Salvelinus confluentus, chinook salmon O. tshawytscha, eastern brook trout Salvelinus fontinalis, and kokanee salmon O. nerka were examined. Rainbow trout was the most susceptible species tested, having the most severe cranial cartilage damage and cell inflammation at all dosage levels. The only exception within the rainbow trout strains tested was the DeSmet strain, which showed a much lower infection severity, especially at the lower triactinomyxon dosages (100 triactinomyxons/fish and 1,000 triactinomyxons/fish), but even this strain showed severe infection intensities when exposed to higher triactinomyxon dosages (>2,000 triactinomyxons/ fish). Of the nine other salmonids tested, only eastern brook trout showed an infection intensity similar to rainbow trout.