Filtering Myxobolus cerebralis Triactinomyxons from contaminated water using rapid sand filtration

Rapid sand filtration was explored as a means of removing Triactinomyxon actinospores (Tams), the waterborne infective stage of the salmonid parasite Myxobolus cerebralis that causes whirling disease, from contaminated water. Preliminarily, a batch of sand was sieved to create 12 size ranges from 180 to 2000 μm. These individual ranges were tested for their efficacy of removing Tams through sand beds either 2 cm or 4 cm deep. The critical size at which no Tams passed through the sand bed was 300 μm at 2 cm depth and 425 μm for 4 cm bed depth. Additional tests evaluated the passage of Tams through filter beds comprised of sand that had all particles smaller than 180 μm removed. With this sand, 0.2±0.5% of Tams passed through a 2 cm bed, and 0.0±0.0% with a 4 cm sand bed. Based on these preliminary results, small (61 cm×15 cm) rapid sand filters were placed in-line with aquaria containing rainbow trout fry. The sand bed depth was 10 cm under which lay 10 cm of aquarium gravel. Four treatments were (1) negative control, (2) positive control, (3) sand of >180 μm, (4) sand of >300 μm. Tams were regularly introduced to the rearing systems above the sand filters. After 60 days, clinical signs of whirling behavior and black tails were seen among the positive controls. A polymerase chain reaction assay for Myxobolus cerebralis 1 month after exposure proved negative for negative controls and the >180 μm group, whereas 10% of the >300 μm group and 71% of the positive controls were infected. Results from the PCR assay at the study’s conclusion indicated the negative controls and >180 μm group were still disease free. All positive control fish were infected, and 49% of >300 μm fish were infected. These results were mirrored by those obtained from a pepsin–trypsin digest assay, except one fish among the >180 μm group was found to be infected. These results demonstrate that sand filtration may be a viable option in treating hatchery water supplies that are contaminated with whirling disease.