Rapid and slow sand filtration techniques and their efficacy at filtering triactinomyxons of Myxobolus cerebralis from contaminated water

Slow and rapid sand filtration methods were tested for their ability to remove triactinomyxon actinospores (Tams), the waterborne infective stage of the salmonid parasite Myxobolus cerebralis, from contaminated water. Within the rapid sand filtration treatments, two backflush protocols were tested. The first consisted of extended backflush duration, and the second consisted of diverted flow past the aquaria with fish for 5 min after backflushing. A slow sand filter treatment served as a nonbackflushed control to the two rapid sand filters and also as its own unique filtration technique. Negative and positive controls were run simultaneously and served both slow and rapid sand filters. The sand used consisted only of particles greater than 180 mum (diameter). Triactinomyxon actinospores were regularly introduced to the fish-rearing systems above the sand filters. After 60 d. clinical signs of whirling behavior and black tails were seen among the positive controls. A polymerase chain reaction (PCR) assay for M. cerebralis conducted at the study's conclusion indicated no infection among the negative controls and both of the rapid sand filter treatments. In the slow sand filter treatment 1.6% of all fish were infected, whereas 98% of the positive controls were infected. Portions of the same tissue samples used for the PCR analysis were also assayed according to the pepsin-trypsin digest (PTD) test. Within the rapid sand filters, 2.9% of fish within the long back flush treatment were infected, as were 100% of the positive controls. The diverted backflush, slow sand filter, and negative controls were all negative according to the PTD test. These results demonstrate that the backflush technique is important in the proper function of rapid sand filters used to remove Tams and that both rapid and slow sand filtration could be viable options in treating hatchery water supplies that are contaminated with whirling disease.