| Whirling disease of trout, caused by the parasite Myxobolus cerebralis , was first observed in the United States during the late 1950s at several fish culture facilities in Central Pennsylvania. Although infected trout were subsequently distributed throughout the state, outbreaks of this disease among wild trout populations have not been observed and are believed to be unlikely in Pennsylvania. Recent outbreak of whirling disease in the Intermountain West has called attention to deficiencies in knowledge of the ecology of the oligochaete Tubifex tubifex, an obligate host in the parasite life cycle, and in our understanding of factors associated with parasite establishment and disease outbreak in natural systems. In order to improve our contemporary knowledge of the parasite in Pennsylvania, and to identify factors that may prevent disease outbreak, we examined the distribution, abundance, and genetics of Tubifex tubifex populations, the prevalence of infection among wild trout, and features of the stream environment through an analytic survey of 20 stream sites. The enrichment of fine sediment habitat below point-sources of organic pollution appeared to be a primary requirement for the occurrence of T. tubifex populations. In addition, subclinical infection of wild trout was only detected at sites typified by organic point-sources. We speculated that T. tubifex and infected trout may only be found near such point-sources, and gathered evidence supporting this notion during an in-depth survey of two of the parasite-infected systems. We suggest that the restricted distribution and abundance of T. tubifex in Pennsylvania, relative to disease affected systems in the West, may limit transmission of M. cerebralis between infected worms and susceptible trout, thus precluding whirling disease outbreak in this region. |
