The Parasite that Causes Whirling Disease, \u3cem\u3eMyxobolus cerebralis\u3c/em\u3e, is Genetically Variable Within and Across Spatial Scales

Understanding the genetic structure of parasite populations on the natural landscape can reveal important aspects of disease ecology and epidemiology and can indicate parasite dispersal across the landscape. Myxobolus cerebralis (Myxozoa: Myxosporea), the causative agent of whirling disease in the definitive host Tubifex tubifex, is native to Eurasia and has spread to more than 25 states in the USA. The small amounts of data available to date suggest that M. cerebralis has little genetic variability. We examined the genetic variability of parasites infecting the definitive host T. tubifex in the Madison River, MT, and also from other parts of North America and Europe. We cloned and sequenced 18S ribosomal DNA and the internal transcribed spacer-1 (ITS-1) gene. Five oligochaetes were examined for 18S and five for ITS-1, only one individual was examined for both genes. We found two different 18S rRNA haplotypes of M. cerebralis from five worms and both intra- and interworm genetic variation for ITS-1, which showed 16 different haplotypes from among 20 clones. Comparison of our sequences with those from other studies revealed M. cerebralis from MT was similar to the parasite collected from Alaska, Oregon, California, and Virginia in the USA and from Munich, Germany, based on 18S, whereas parasite sequences from West Virginia were very different. Combined with the high haplotype diversity of ITS-1 and uniqueness of ITS-1 haplotypes, our results show that M. cerebralis is more variable than previously thought and raises the possibility of multiple introductions of the parasite into North America

Is Habitat Type a Useful Predictor of the Outcome of Interactions between Tubifex tubifex and Myxobolus cerebralis, the Causative Agent of Salmonid Whirling Disease?

The aquatic oligochaete Tubifex tubifex is the intermediate host for the parasite, Myxobolus cerebralis, which causes salmonid whirling disease (WD). Although the relationship between WD severity in salmonids and infection in T. tubifex is not well understood, previous research suggests that variation within local stream populations of T. tubifex may be an important determinant of parasite success. Our goals were to examine relationships among habitat features, abundance, infection prevalence, genetic diversity and susceptibility of T. tubifex, and WD risk in Yellowstone cutthroat trout spawning tributaries in Yellowstone National Park, where M. cerebralis was detected in 1998. Abundance of tubificids and T. tubifex, and infection prevalence in T. tubifex were higher in unconfined habitat types than in confined habitat types. Tubifex tubifex belonging to mtDNA lineages III, which are considered moderately susceptible to M. cerebralis, were also more abundant in unconfined habitats than confined habitats. We assessed the susceptibility of four genetically distinct strains of lineage III T. tubifex to M. cerebralis in the laboratory. Strains were established from field collected T. tubifex. All strains were susceptible to infection by M. cerebralis and strains from unconfined habitats amplified the parasite only slightly more than strains from confined habitats. These results suggest habitat type may influence variability in WD risk by affecting the outcome of interactions between T. tubifex and M. cerebralis in the field

If you\u27ve seen one worm, have you seen them all? Spatial, community, and genetic variability of tubificid communities in Montana

Genetic studies are recognized increasingly as important for understanding naturally occurring disease dynamics and are used to predict host genetic diversity and coevolutionary processes and to identify species composition in ecological communities. Tubifex tubifex, the definitive host of the whirling disease parasite Myxobolus cerebralis, comprises 6 known lineages that vary widely in parasite susceptibility. We used 16S ribosomal DNA (16S rDNA) to identify relationships among genetic variability of 3 oligochaete genera (T. tubifex, Rhyacodrilus spp., and Ilyodrilus spp.; Oligochaeta:Tubificidae), oligochaete assemblage composition, and the presence of whirling disease in 9 locations across 4 watersheds in Montana, USA. We assessed genetic variability among 183 tubificid worms from locations classified as positive or negative for whirling disease based on 5 to 8 y of monitoring by the Montana Department of Fish, Wildlife, and Parks. Within genera, we found 2 groups of T. tubifex (lineages I and III), 2 groups of Rhyacodrilus spp., and 4 groups of Ilyodrilus spp., possibly suggesting cryptic species. The maximum genetic variability within taxa was relatively high (~10% sequence divergence) for all 3 genera, but haplotype diversity within groups with \u3e5% sequence divergence was greater for Ilyodrilus spp. (0.719) than for Tubifex spp. (0.246) and Rhyacodrilus spp. (0.143). The variation was nonrandomly distributed over the landscape. Oligochaete genetic composition was more similar among locations in the same watershed than among locations with or without whirling disease. Thus, oligochaete assemblage composition did not appear to be related to the presence of the disease at this watershed spatial scale

Avian Piscivores as Vectors for Myxobolus cerebralis in the Greater Yellowstone Ecosystem

Myxobolus cerebralis, the cause of whirling disease in salmonids, has dispersed to waters in 25 states within the USA, often by an unknown vector. Its incidence in Yellowstone cutthroat trout Oncorhynchus clarkii bouvieri within the highly protected environment of Yellowstone Lake, Yellowstone National Park, is a prime example. Given the local abundances of piscivorous birds, we sought to clarify their potential role in the dissemination of M. cerebralis. Six individuals from each of three bird species (American white pelican Pelecanus erythrorhynchos, double-crested cormorant Phalacrocorax auritus, and great blue heron Ardea herodias) were fed known-infected or uninfected rainbow trout O. mykiss. Fecal material produced during 10-d periods before and after feeding was collected to determine whether M. cerebralis could be detected and, if so, whether it remained viable after passage through the gastrointestinal tract of these birds. For all (100%) of the nine birds fed known-infected fish, fecal samples collected during days 1–4 after feeding tested positive for M. cerebralis by polymerase chain reaction. In addition, tubificid worms Tubifex tubifex that were fed fecal material from known-infected great blue herons produced triactinomyxons in laboratory cultures, confirming the persistent viability of the parasite. No triactinomyxons were produced from T. tubifex fed fecal material from known-infected American white pelicans or double-crested cormorants, indicating a potential loss of parasite viability in these species. Great blue herons have the ability to concentrate and release viable myxospores into shallow-water habitats that are highly suitable for T. tubifex, thereby supporting a positive feedback loop in which the proliferation of M. cerebralis is enhanced. The presence of avian piscivores as an important component of aquatic ecosystems should continue to be supported. However, given the distances traveled by great blue herons between rookeries and foraging areas in just days, any practices that unnaturally attract them may heighten the probability of M. cerebralis dispersal and proliferation within the Greater Yellowstone Ecosystem

Relative acute effects of low pH and high iron on the hatching and survival of the water mite (Arrenurus manubriator) and the aquatic insect (Chironomus riparius)

We investigated the relative effects of low pH and high iron on a water mite, Arrenurus manubriator and an aquatic insect, Chironomus riparius. Eggs and active stages were exposed in static renewal toxicity tests to pH 6, 5, 4, 3, and 2, made by adding sulfuric acid to reconstituted soft water, or to iron levels of 200, 400, 600, 800, and 1,000 mg/L, made by adding ferrous sulfate to soft water at pH 4. Experiments were conducted at 22°C with a 16:8-h photoperiod, and treatments were replicated three times with at least nine individuals per treatment. Data were analyzed with a logistic response function and one-way ANOVA for pH and iron tests, respectively. Egg hatching was reduced at pH 2 for midges and at pH 3 for mites. Iron had no effect on hatching for either species. Survival of midge larvae was partially reduced at pH 4, and survival of mite deutonymphs, larvae, female and male adults was reduced at pH 3. Survival of midge larvae, and mite deutonymphs and male adults was reduced at 400, 200, and 1,000 mg Fe/L, respectively. Mite female adults and larvae were unaffected by iron. Higher metabolic requirements of unfed immature stages, the gelatinous covering of mite and insect eggs, the longer incubation period of mite eggs, and the greater osmoregulatory potential of adult mites may have contributed to the differences observed