The Ecology of Tubifex tubifex in two Myxobolus cerebralis Enzootic Streams in Pennsylvania

Whirling disease is the result of acute infections by Myxobolus cerebralis, a myxozoan parasite that was introduced to the United States over 50 years ago. The life cycle of M. cerebralis involves two alternate hosts, a trout and the oligochaete Tubifex tubifex. It is a widely held belief that M. cerebralis epizootics are unlikely in Pennsylvania, though no investigation has been undertaken to provide an explanation for this hypothesis. In this study, we chose two streams known to be enzootic for M. cerebralis to investigate the spatial distribution and abundance of the tubificid host. Tubifex tubifx was not widely distributed in either stream, and the density of this species was strongly related to proximity to gross organic enrichment. Ilyodrilus templetoni, a likely competitor, was well distributed and dominant in both systems. We suggest that the restricted distribution and abundance of T. tubifex in Pennsylvania may limit transmission of M. cerebralis between infected worms and susceptible trout and may explain why whirling disease epizootics have not recently been observed in Pennsylvania streams enzootic for M. cerebralis.