Arrested development of the myxozoan parasite, Myxobolus cerebralis, in certain populations of mitochondrial 16S lineage III Tubifex tubifex.

Laboratory populations of Tubifex tubifex from mitochondrial (mt)16S ribosomal DNA (rDNA) lineage III were generated from single cocoons of adult worms releasing the triactinomyxon stages (TAMs) of the myxozoan parasite, Myxobolus cerebralis. Subsequent worm populations from these cocoons, referred to as clonal lines, were tested for susceptibility to infection with the myxospore stages of M. cerebralis. Development and release of TAMs occurred in five clonal lines, while four clonal lines showed immature parasitic forms that were not expelled from the worm (non-TAM producers). Oligochaetes from TAM- and non-TAM-producing clonal lines were confirmed as lineage III based on mt16S rDNA and internal transcribed spacer region 1 (ITS1) sequences, but these genes did not differentiate these phenotypes. In contrast, random amplified polymorphic DNA analyses of genomic DNA demonstrated unique banding patterns that distinguished the phenotypes. Cohabitation of parasite-exposed TAM- and non-TAM-producing phenotypes showed an overall decrease in expected TAM production compared to the same exposure dose of the TAM-producing phenotype without cohabitation. These studies suggest that differences in susceptibility to parasite infection can occur in genetically similar T. tubifex populations, and their coexistence may affect overall M. cerebralis production, a factor that may influence the severity of whirling disease in wild trout populations.

Evaluation of a range of doses of ultraviolet irradiation to inactivate waterborne actinospore stages of Myxobolus cerebralis.

The ability of a range of doses of ultraviolet irradiation (UV) to inactivate the waterborne actinospore or triactinomyxon stages (TAMs) of Myxobolus cerebralis was evaluated by infectivity for juvenile rainbow trout Oncorhynchus mykiss. TAMs were UV-irradiated using a low pressure mercury vapour lamp collimated beam apparatus. All doses 40, 80, 120 and 160 mJ cm(-2) were found to completely inactivate the TAMs as demonstrated by the absence of microscopic lesions, myxospores and parasite DNA detected by quantitative PCR (qPCR) among rainbow trout 5 mo post-exposure. In contrast, rainbow trout receiving the same concentrations of untreated TAMs (1000 fish(-1)) developed clinical signs of whirling disease at 2 mo post-exposure and had severe microscopic lesions, high myxospore counts and high qPCR values when examined at 5 mo following exposure to the parasite.

Ultraviolet irradiation inactivates the waterborne infective stages of Myxobolus cerebralis: a treatment for hatchery water supplies.

The effects of ultraviolet (UV) irradiation on the viability of the waterborne triactinomyxon stages of Myxobolus cerebralis were evaluated by vital staining and the infectivity for juvenile rainbow trout Oncorhynchus mykiss. A dose of 1300 mWs cm-2 was required to inactivate 100% of the triactinomyxons held under a static collimated beam of UV as determined by vital staining. Juvenile rainbow trout were protected from infections with M. cerebralis when exposed to 14,000 or 1400 triactinomyxon spores per fish that had been treated with the collimating beam apparatus (1300 mWs cm-2). Among all fish receiving UV-treated triactinomyxons, none had clinical signs of whirling disease, or evidence of microscopic lesions or spores of M. cerebralis after 5 mo at water temperatures of 15 degrees C. In contrast, 100% of the fish receiving the higher dose of untreated triactinomyxons developed clinical signs of whirling disease and both microscopic signs of infection and spores were detected in all of the high and low dose trout receiving untreated triactinomyxon exposures. Two additional trials evaluated the Cryptosporidium Inactivation Device (CID) for its ability to treat flow-through 15 degrees C well water to which triactinomyxons were added over a 2 wk period. CID treatments of a cumulative dose exceeding 64,000 triactinomyxons per fish protected juvenile rainbow from infections with M. cerebralis. Rainbow trout controls receiving the same number of untreated triactinomyxons developed both microscopic lesions and cranial spore concentrations up to 10(4.6) per 1/2 head, although no signs of clinical whirling disease were observed. UV (126 mWs cm-2, collimated beam apparatus) was also effective in killing Flavobacterium psychrophilum, the agent causing salmonid bacterial coldwater disease, as demonstrated by the inability of bacterial cells to grow on artificial media following UV treatment.