Whirling Disease: Reviews and Current Topics

<em>ABSTRACT. Myxobolus cerebralis</em>, the causative agent of salmonid whirling disease, was described nearly a century ago as a one-host parasite. It was not until 1984 that the true two-host life cycle of the parasite was discovered. Consequently, numerous studies have examined interactions between the parasite and its fish hosts but, in comparison, there has been relatively little work conducted on interactions between <em>M. cerebralis </em>and its obligate, oligochaete host <em>Tubifex tubifex</em>. Recently, though, several research groups have begun examining the interplay between the parasite and the oligochaete in hopes of determining the parameters necessary for the successful transmission of <em>M. cerebralis </em>by <em>T. tubifex</em>. These studies can be broken into three broad categories: 1) individual host factors that affect the transmission of <em>M. cerebralis</em>, such as species-specific host susceptibility and an individuals’ capacity to support and release varying numbers of the parasite; 2) <em>T. tubifex </em>population factors that affect the transmission of the parasite, including genetic and geographic variations in susceptibility; and 3) ecological and epidemiological parameters that influence the transmission of <em>M. cerebralis </em>by <em>T. tubifex</em>, such as habitat conditions, time of season, water temperature, and distribution of infected oligochaetes within a watershed. Therefore, the goals of this review are to discuss what is known for each category and to evaluate these results in context of their role in the transmission of <em>M. cerebralis</em>. By doing so, continued and future research, necessary to completely understand the epidemiology of whirling disease, should become apparent.