Whirling Disease: Reviews and Current Topics

Review: The Effects of Myxobolus cerebralis on the Salmonid Host

Elizabeth MacConnell and E. Richard Vincent

doi: https://doi.org/10.47886/9781888569377.ch9

ABSTRACT. Whirling disease is a chronic inflammatory disease in salmonid fish caused by the myxosporean parasite Myxobolus cerebralis. The disease, first recognized in cultured rainbow trout Oncorhynchus mykiss, is characterized by the radical tail-chasing behavior of infected fish. The parasite is enzootic in free ranging and cultured populations of trout in areas of salmonid aquaculture, except South America. Recent reports of severe population declines among wild trout associated with whirling disease has caused great concern and renewed interest in this pathogen. The parasite has a sequential affinity for the skin, nerves, and, finally, skeletal cartilage, in the fish host. Abundant cartilage in the skeleton of young trout renders them extremely susceptible to the effects of the disease. Most salmonid species are susceptible to infection with M. cerebralis, but susceptibility varies among species and may also vary among strains and individual fish within a population or a similarly exposed group. Rainbow trout and anadromous steelhead are highly susceptible to whirling disease. The severity of the disease and mortality are related to age of the fish, when first exposed, and parasite dose. Trophozoite stages of the parasite lyse cartilage, cause an inflammatory response, and may interfere with normal bone deposition in the salmonid host. In severely infected fish, growth rates are depressed during active infection, and behavioral effects and severe skeletal deformities compromise functions such as swimming and feeding. The effects of M. cerebralis on the salmonid host are determined by factors such as species, age, size, parasite dose, immune response, and water temperature. Understanding the interaction of these factors in waters where M. cerebralis is present is critical to the future management of healthy wild trout populations.