The aquatic oligochaete Tubifex tubifex parasitized by Myxobolus cerebralis releases triactinomyxon (TAM) actinospores that can infect some species of salmonids and cause salmonid whirling disease. Silica sand was tested as a filtration medium for removal of TAMs from water containing the parasite. Laboratory tests indicated sand filtration removed > 99.99% of TAMs. In 2 different field tests, groups of 1 mo old rainbow trout Oncorhynchus mykiss were exposed for 2 wk to filtered and unfiltered water from a spring-fed pond enzootic for M. cerebralis. In November 2000, the exposure dose was estimated as between 3 and 5 TAMs fish . Fish were held for 6 mo post exposure (p.e.) in laboratory aquaria for observation and evidence of clinical signs of whirling disease. We used 4 diagnostic techniques to assess the prevalence and severity of infection by M. cerebralis among fish exposed to filtered and unfiltered water. These included polymerase chain reaction (PCR) for genomic DNA of the parasite, histological evaluation for tissue damage, tissue digestion for quantification of cranial myxospores of the parasite, and total non-sampling mortality that occurred over 6 mo p.e. All diagnostic tests verified that the prevalence and severity of infection was significantly reduced among fish in treatment groups exposed to filtered water compared to those exposed to unfiltered water in both the low-dose and high-dose exposures. KEY WORDS: Salmonid whirling disease · Myxobolus cerebralis · Sand filtration Resale or republication not permitted without written consent of the publisherDis Aquat Org 57: [77][78][79][80][81][82][83] 2003 thesized that the large size of the TAM might facilitate its removal from small amounts of flowing water using a silica-sand-based passive filtration system. Indeed, passive sand filtration for removal of various waterborne pathogens has been in use in England, Germany and the eastern United States since the 1800s (Hazen 1913, Bellamy et al. 1985. A passive flow-through mechanical filtration system could be useful for removal of TAMs from the small volumes of water needed to augment water supplies for aquaculture or from effluents of earth-bottom ponds contaminated with the parasite.In 1995, Myxobolus cerebralis infections were detected in Age 1 (16 mo old) wild rainbow trout Oncorhynchus mykiss in the Fryingpan River drainage in central Colorado. By 1998, the abundance of Age 1 rainbow trout had declined 90% from the levels observed in 1994 (Nehring & Thompson 2001). Water filtration studies as described by Thompson & Nehring (2000) were conducted throughout the drainage from August 1998 through August 2002 to identify potential sources of TAMs responsible for the M. cerebralis infections in Age 1 wild rainbow trout in the river. The water filtration studies provided strong empirical evidence that the effluent emanating from a series of private fish ponds was the primary source of TAMs flowing into the Fryingpan River. The pond discharge flowed into the river 15 km upstream of the lower...