Epidemiology and host-parasite interactions between Myxobolus cerebralis, the causative agent of salmonid whirling disease, and its oligochaete host, Tubifex tubifex

In this study the prevalence of M. cerebralis infections in T. tubifex was monitored for several years at sites in the Rock Creek drainage of western Montana. These sites were chosen based on the location of cages housing sentinel fish which were used to assess the severity of disease in salmonids. Results of this study showed that the severity M. cerebralis infections in sentinel trout increased significantly throughout the Rock Creek Drainage between June of 1998 and 2000, and that the range of M. cerebralis is still expanding. However, the effect the parasite is having on wild salmonid populations in Rock Creek remains unknown. Fish population surveys have not been conducted in this drainage since M. cerebralis was discovered here in 1997. There was not a significant increase in the prevalence of M. cerebralis infections in T. tubifex populations observed during this study, but this may have been due to sample size. Laboratory exposures of non-infected T. tubifex collected revealed that T. tubifex resistant to infection with M. cerebralis are present in this drainage, and both living specimens, and DNA samples from these worms have been made available to other investigators. The monitoring of experimentally and naturally-infected T. tubifex using PCR revealed that T. tubifex remain infected with M. cerebralis for the duration of their natural lifespan and are capable of releasing viable TAMs, nearly 2 years after exposure to M. cerebralis myxospores, which is significant because it opens the possibility of a seasonal periodicity in TAM release. Limited support for a seasonal periodicity in TAM release was obtained from this study using chambers designed to monitor TAM release by naturally infected worms. In addition, these experiments identified problems likely to be faced by other researchers attempting to quantify M. cerebralis TAMs (e.g. the necessity of using molecular methods in addition to morphology to confirm the specific identity of M. cerebralis). Finally, this is the first report of TAMs being released in fecal packets, which we believe to be the natural mechanism used by the parasite to exit T. tubifex.