Search

Myxobolus cerebralis is the parasite causing whirling disease, which has dramatic ecological impacts due to its potential to cause high mortality in salmonids. The large-scale efforts, necessary to underpin an effective surveillance program, have practical and economic constraints. There is, hence, a clear need for models that can predict the parasite spread. Model development, however, often heavily depends on knowing influential variables and governing mechanisms. We have developed a graphical model for the establishment and spread of M. cerebralis by synthesizing experts’ opinion and empirical studies. First, we conducted a series of workshops with experts to identify variables believed to impact the establishment and spread of the parasite M. cerebralis and visualized their interactions via a directed acyclic graph. Then we refined the graph by incorporating empirical findings from the literature. The final graph’s nodes correspond to variables whose considerable impact on M. cerebralis establishment and spread is either supported by empirical data or confirmed by experts, and the graph’s directed edges represent direct causality or strong correlation. This graphical model facilitates communication and education of whirling disease and provides an empirically driven framework for constructing future models, especially Bayesian networks.

We provide the first documented case of whirling disease (WD) impacts to wild, self-sustaining rainbow trout (RNTR, Oncorynchus mykiss) populations in Canada. Myxobolus cerebralis (Mc), the causative agent of WD, was first confirmed in Alberta in 2016. However, evidence of disease in local fish populations was unknown. Using a weight-of-evidence approach, we examined multiple parasite life cycle stages in the Crowsnest River, Alberta. Percentage of infected Tubifex tubifex worms actively shedding triactinomyxons (TAMs) exceeded known thresholds of Mc establishment and TAM densities instream exceeded thresholds known to cause ≥90% declines in RNTR populations. Mc was detected at 5 of 6 study sites in water, fish, and worms. Disease severity was highest in the lower watershed where 100% of sentinel fish tested positive for Mc 7 to 14 days post-exposure; up to 85% of wild fingerling RNTR showed clinical signs of disease and yearling trout were largely absent from the river suggesting reduced survival. Our findings indicate conditions necessary for outbreak of WD exist in Alberta, highlighting the need to consider this disease as an emerging threat to wild salmonid populations.

Myxobolus cerebralis is the causative agent of whirling disease in salmonid fishes. In 2016, this invasive parasite was detected in Alberta, Canada, for the first time, initiating a comprehensive 3 yr monitoring program to assess where the parasite had spread within the province. As part of this program, a qPCR-based test was developed to facilitate detection of the environmental stages of M. cerebralis and from the oligochaete host, Tubifex tubifex. During this program, ~1500 environmental samples were collected and tested over 3 yr. Fish were collected from the same watersheds over 2 yr and tested as part of the official provincial monitoring effort. Substrate testing identified sites positive for M. cerebralis in 3 of 6 watersheds that had been confirmed positive by fish-based testing and 3 novel detections where the parasite had not been detected previously. Testing of individually isolated Tubifex from each sample site was used to further confirm the presence of M. cerebralis. DNA barcoding of the cytochrome oxidase I (cox1) gene of 567 oligochaete specimens collected from 6 different watersheds yielded 158 unique sequences belonging to 21 genera and 37 putative species. Phylogenetic analyses of sequences assigned to the genus Tubifex predicted 5 species of Tubifex arising from this assessment. Based on our results, we propose that environmental and worm samples can be a valuable complement to the gold-standard fish testing and will be especially useful for monitoring in areas where fish collection is challenging or prohibitive because of site accessibility or vulnerability of the fish populations.

Myxobolus cerebralis is the causative agent of whirling disease in salmonid fishes. In 2016, this invasive parasite was detected in Alberta, Canada, for the first time, initiating a comprehensive 3 yr monitoring program to assess where the parasite had spread within the province. As part of this program, a qPCR-based test was developed to facilitate detection of the environmental stages of M. cerebralis and from the oligochaete host, Tubifex tubifex. During this program, ~1500 environmental samples were collected and tested over 3 yr. Fish were collected from the same watersheds over 2 yr and tested as part of the official provincial monitoring effort. Substrate testing identified sites positive for M. cerebralis in 3 of 6 watersheds that had been confirmed positive by fish-based testing and 3 novel detections where the parasite had not been detected previously. Testing of individually isolated Tubifex from each sample site was used to further confirm the presence of M. cerebralis. DNA barcoding of the cytochrome oxidase I (cox1) gene of 567 oligochaete specimens collected from 6 different watersheds yielded 158 unique sequences belonging to 21 genera and 37 putative species. Phylogenetic analyses of sequences assigned to the genus Tubifex predicted 5 species of Tubifex arising from this assessment. Based on our results, we propose that environmental and worm samples can be a valuable complement to the gold-standard fish testing and will be especially useful for monitoring in areas where fish collection is challenging or prohibitive because of site accessibility or vulnerability of the fish populations.

Summary 1. Individuals that live in groups benefit from increased foraging success and decreased predation. Protection from some types of parasites may provide an additional benefit of groupliving. For fish, the extent to which shoaling can reduce an individual’s risk of exposure to the infective stages of parasites is unknown. 2. We tested for antiparasite benefits of shoaling in fathead minnows exposed to larvae (cercariae) of two of their most common species of trematode, Ornithodiplostomum ptychocheilus and Posthodiplostomum minimum. As developing stages (metacercariae) of these trematodes cause reductions in minnow activity, growth and survival, natural selection should favour the evolution of cercariae-avoidance behaviours. 3. We evaluated shoal dimensions in groups of minnows exposed to O. ptychocheilus and to other chemical/physical stimuli within aquaria. To compare risk of exposure in shoaling vs. non-shoaling fish, we confined groups of minnows into mesh cages in outdoor mesocosms, exposed them to cercariae, then compared mean worm numbers in grouped vs. solitary fish. Lastly, we tested whether fish located within the centre of an artificial shoal reduced their risk of cercariae exposure compared with those along peripheral edges. 4. Minnows distinguished infective cercariae from other potential aquatic threats and responded with activity that reduced the 2-dimensional area of their shoals 15-fold compared with water-only controls. Fish confined within artificial shoals had 3-fold fewer worms than single fish and minnows located within the centre of artificial shoals had significantly fewer worms than those without peripheral minnows. 5. These results show that shoaling reduces a minnows’ risk of exposure to cercariae, either directly via detection of cercariae in the water column followed by behavioural avoidance or indirectly via behaviour-mediated differences in exposure between shoaling vs. non-shoaling fish.

Epidermal club cells of fishes in the superorder ostariophysi have puzzled evolutionary biologists because they were historically linked to chemical alarm signalling and relied on group selectionist explanations. Alternative hypoth- eses to explain the existence of these cells include the possibility of an anti-pathogenic or anti-parasitic function. If this is so, individual fish should invest in increased numbers of club cells after exposure to parasites, and club cells should contain components that reduce the infectivity of skin-penetrating larvae. Infectivity of cercariae of the trematode Ornithodiplostomum sp. was significantly reduced when exposed to the skin extract of fathead minnows (an ostariophysan), but also to skin extract of mollies (a non-ostariophysan that lacks club cells), respectively, compared to controls. Moreover, club cell density was not affected by exposure to cercariae. Taken together, these results are inconsistent with an anti-parasite function for these cells and instead suggest a generic role in response to injury. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 884-890. ADDITIONAL KEYWORDS: alarm substance - anti-predation - Ornithodiplostomum - Ostariophysi - infectivity - Pimephales.

Many fishes possess specialized epidermal cells that are ruptured by the teeth of predators, thus reliably indicating the presence of an actively foraging predator. Understanding the evolution of these cells has intrigued evolutionary ecologists because the release of these alarm chemicals is not voluntary. Here, we show that predation pressure does not influence alarm cell production in fishes. Alarm cell production is stimulated by exposure to skin-penetrating pathogens (water moulds: Saprolegnia ferax and Saprolegnia parasitica ), skin-penetrating parasites (larval trematodes: Teleorchis sp. and Uvulifer sp.) and correlated with exposure to UV radiation. Suppression of the immune system with environmentally relevant levels of Cd inhibits alarm cell production of fishes challenged with Saprolegnia . These data are the first evidence that alarm substance cells have an immune function against ubiquitous environmental challenges to epidermal integrity. Our results indicate that these specialized cells arose and are maintained by natural selection owing to selfish benefits unrelated to predator–prey interactions. Cell contents released when these cells are damaged in predator attacks have secondarily acquired an ecological role as alarm cues because selection favours receivers to detect and respond adaptively to public information about predation.