Whirling Disease: Reviews and Current Topics

<EM>ABSTRACT. </EM>Anadromous fish were excluded above Pelton Round Butte Hydroelectric Project (PRB Project), located midway (RM 100) on the Deschutes River in central Oregon, beginning in 1968. Reintroduction of these fish above the PRB Project is proposed to meet conservation concerns that arise from lack of natural production and separation of populations. One consideration, when moving fish groups that have been isolated one from the other for thirty years, is that of disease. The health of the fish populations above Round Butte Dam could be seriously jeopardized by the introduction of whirling disease. Straying hatchery steelhead trout <em>Oncorhynchus mykiss </em>were detected with <em>Myxobolus cerebralis </em>spores, in 1987, at Warm Springs National Fish Hatchery, below the PRB Project. <em>Myxobolus cerebralis </em>is established in tributaries of the upper Columbia River basin and of the Snake River basin, where some of these straying hatchery and wild steelhead trout may have originated. From 1997 to 2000, fish from the Deschutes River basin have been sampled for the presence of <em>M. cerebralis</em>. The parasite has been found in both straying hatchery and unmarked adult chinook salmon <em>O. tshawytscha </em>and steelhead trout. Presently there is no evidence of infection of resident fish or in returning adult fish originating from Round Butte Hatchery, although the potential for establishment of <em>M. cerebralis </em>in the Deschutes River watershed cannot be ruled out.

Whirling Disease: Reviews and Current Topics

<em>ABSTRACT. Myxobolus cerebralis </em>possesses unique phenotypic and genotypic characteristics when compared with other histozoic parasites from the phylum Myxozoa. The parasite infects the cartilage and thereby induces a serious and potentially lethal disease in salmonid fish. Comparisons of the small subunit ribosomal DNA (ssu rDNA) sequences of <em>M. cerebralis </em>to other myxozoans demonstrate that the parasite has evolved separately from other <em>Myxobolus </em>spp. that may appear in cartilage or nervous tissues of the fish host. <em>Myxobolus cerebralis </em>has a complex life cycle involving two hosts and numerous developmental stages that may divide by mitosis, endogeny, or plasmotomy, and, at one stage, by meiosis. In the salmonid host, the parasite undergoes extensive migration from initial sites of attachment to the epidermis, through the nervous system, to reach cartilage, the site where sporogenesis occurs. During this migration, parasite numbers may increase by replication. Sporogenesis is initiated by autogamy, a process typical of pansporoblastic myxosporean development that involves the union of the one cell (pericyte) with another (sporogonic). Following this union, the sporogonic cell will give rise to all subsequent cells that differentiate into the lenticular shaped spore with a diameter of approximately 10 µm. This spore or myxospore is an environmentally resistant stage characterized by two hardened valves surrounding two polar capsules with coiled filaments and a binucleate sporoplasm cell. In the fish, these spores are found only in cartilage where they reside until released from fish that die or are consumed by other fish or fish-eating animals (e.g., birds). Spores reaching the aquatic sediments can be ingested and hatch in susceptible oligochaete hosts. The released sporoplasm invades and then resides between cells of the intestinal mucosa. In contrast to the parasite in the fish host, the parasite in the oligochaete undergoes the entire developmental cycle in this location. This developmental cycle involves merogony, gametogamy or the formation of haploid gametes, and sporogony. The actinosporean spores, formed at the culmination of this development, are released into the lumen of the intestine, prior to discharging into the aquatic environment. The mechanisms underlying the complex development of <em>M. cerebralis</em>, and its interactions with both hosts, are poorly understood. Recent advances, however, are providing insights into the factors that mediate certain phases of the infection. In this review, we consider known and recently obtained information on the taxonomy, development, and life cycle of the parasite.

Whirling Disease: Reviews and Current Topics

<EM>ABSTRACT</EM>. The culture of the aquatic worm <em>Tubifex tubifex</em>, the alternate host of whirling disease, is necessary to conduct research regarding triactinomyxon (TAM) viability, controlled infection studies, and methods of disease control. Presumed infected worms collected from the field may produce TAMs for several months, but production generally decreases after a few months. To ensure a stable supply of TAMs, we investigated the effects of rearing substrate on TAM production and worm survival. We also analyzed the time of TAM release during 24-h periods divided into 12 h light:12 h dark.

Whirling Disease: Reviews and Current Topics

<EM>ABSTRACT. </EM>Laboratory challenges to determine the susceptibility of indigenous Deschutes River, Oregon, salmonids to <em>Myxobolus cerebralis </em>were conducted as part of a study to assess the risk of reintroducing anadromous salmon above a migration barrier on that river. Replicate groups of progeny from wild rainbow trout <em>Oncorhynchus mykiss</em>, steelhead (anadromous rainbow trout), kokanee <em>O. nerka</em>, and chinook salmon <em>O. tshawytscha</em>, were exposed to doses of 0, 200, or 2,000 triactinomyxons per fish. Fish were evaluated at 5 months postchallenge for spore concentration in the cranial cartilage, severity of microscopic lesions in the cartilage, and clinical signs of disease. The wild rainbow trout (0.7 g at exposure) were most susceptible to infection, with infection prevalence and spore concentrations similar to those of a susceptible (Mt. Lassen) control rainbow trout strain (0.7 g at exposure), although clinical disease signs were less common in the wild strain. Two year classes of steelhead, exposed at different sizes (0.3 g and 1.0 g), both showed fewer clinical disease signs, a lower prevalence of infection, a lower spore concentration, and a decreased mean lesion score, compared with the control rainbow trout (0.6 g and 1.2 g). Kokanee (1.5 g at exposure) became infected but less severely than the control rainbow trout (1.8 g at exposure). Clinical signs were not evident in the kokanee or the susceptible rainbow trout, possibly because of the large size at exposure. No signs of infection were detected in the chinook salmon (1.0 g at exposure) at either dose, despite high infection prevalence in the control rainbow trout (0.6 g at exposure). These results demonstrate that the indigenous salmonids present in the Deschutes River, both above and below the barrier, are susceptible to infection, but the rainbow trout would be most at risk should introduction of the parasite occur in this system.

Whirling Disease: Reviews and Current Topics

<em>ABSTRACT. </em>In Colorado, Windy Gap Reservoir is a focus of <em>Myxobolus cerebralis </em>infectivity of greater intensity than may be explained by the potential contribution of <em>M. cerebralis </em>myxospores by dead fish. One mechanism that would help explain this situation is the expulsion of viable <em>M. cerebralis </em>myxospores by living infected fish. We conducted laboratory experiments to see if <em>Tubifex tubifex</em>, purged of infection by incubation at 26°C for a minimum of 30 d, could become reinfected by exposure to feces and wastes from aquaria containing <em>M. cerebralis</em>-infected brown trout <em>Salmo trutta</em>. In two separate experiments, replicate experimental units of <em>T. tubifex </em>were thoroughly infected in this manner. By comparison, evidence of infection in negative control replicates was much weaker in both experiments. It is possible that the purging process used to remove initial infection was not 100% effective, yet the differences between experimental and negative control replicates were dramatic. Positive control replicates, intentionally exposed to harvested myxospores of <em>M. cerebralis</em>, became heavily infected in both experiments. These results strongly support the hypothesis that brown trout are capable of expelling viable <em>M. cerebralis </em>myxospores.

Whirling Disease: Reviews and Current Topics

<EM>ABSTRACT. </EM>The potential for <em>Myxobolus cerebralis</em>, the cause of salmonid whirling disease, to affect resident populations of spring chinook salmon <em>Oncorhynchus tshawytscha </em>in the Lostine River, Oregon, was investigated in this study. Spring chinook salmon and rainbow trout <em>O. mykiss </em>fry were held in the Lostine River for 14 d in late March 1999, when resident chinook salmon alevins naturally emerge. After exposure, fry were held in pathogen-free water in the laboratory. The prevalence of infection at 5 months postexposure, as determined by PCR, was equivalent in both species (37.5% and 41%, respectively). Only rainbow trout developed cranial lesions (average lesion severity 0.4 on a 5-point scale; 4 of 10 fish examined were positive), and no spores were detected in homogenates of cartilage from fish of either species. Comparison of data on chinook salmon spawning sites (1996–2000) with known distribution of <em>M. cerebralis </em>in the Lostine River demonstrated that the majority of chinook salmon spawn in the middle section of the river, where levels of <em>M. cerebralis </em>exposure were reduced. Results of this study indicate that juvenile chinook salmon may become infected with <em>M. cerebralis</em>, when naturally exposed to the parasite, but suggest that the timing and location of their emergence may mitigate the negative impacts of <em>M. cerebralis </em>infection in this river.

Whirling Disease: Reviews and Current Topics

<EM>ABSTRACT. </EM>The myxosporean parasite <em>Myxobolus cerebralis </em>is the causative agent of salmonid whirling disease. Containing its spread and limiting its effects in the Intermountain West will require judicious management programs, but such actions await a comprehensive understanding of the biology and ecology of this parasite and its hosts and how these elements interact; we do not yet know the weaknesses of this organism. To better guide efforts aimed at such an understanding, we assembled available information on the ecology of the parasite, organizing it into a conceptual model of its life cycle, to help foster understanding, focus future research, and lead eventually to a mathematical model for evaluating control measures. <em>Myxobolus cerebralis </em>has a complex life cycle with two obligate hosts, a salmonid fish and the oligochaete <em>Tubifex tubifex</em>, parasitized by the myxosporean and the actinosporean, respectively, and two infective “spore” stages, the myxospore and the triactinomyxon. This complexity is enhanced by the variable suitability of multiple salmonid species to serve as hosts, varying host suitability of genetic variants of <em>T. tubifex</em>, relatively recent introduction of <em>M. cerebralis </em>to North America, and unique traits of the parasite that preclude easy classification into conventional modeling categories. Much is known about the anatomy and function of myxospores and triactinomyxons from laboratory studies, but information on their distribution, abundance, and dispersal in natural systems is limited and based on indirect observations. Similarly, we understand development of the parasite within its hosts and resulting pathologies well but know little about host immune reactions and other mechanisms controlling proliferation within hosts or how environmental factors affect these defenses. Population-level effects on fish in natural systems have been quantified only rarely, where good prewhirling disease data exist, and effects on <em>T. tubifex </em>populations are unknown. Most rates and frequencies needed to infer relationships and model system dynamics have not been directly quantified in natural systems, but rapid progress is being made. Larger issues, including effects of <em>M. cerebralis </em>on community dynamics and ecosystem structure and function, have yet to be explored.

Whirling Disease: Reviews and Current Topics

<EM>ABSTRACT. </EM>We obtained data from several sources to explore selected habitat compartments of a more complex epizootic model of factors affecting <em>Myxobolus cerebralis </em>in free-ranging populations of fish. We examined histological sections from branchial and cranial tissues from Yellowstone cutthroat trout <em>Oncorhynchus clarki bouvieri </em>and rainbow trout <em>O. mykiss</em>, naturally exposed to <em>M. cerebralis </em>at nine locations within three river drainages in Idaho, for evidence of characteristic pathology of whirling disease. Mean water temperature during exposure, temperature coefficient of variation, and the density of two groups of benthic macroinvertebrates that can thrive in habitats of high organic content were significantly positively correlated with the mean lesion severity of infected fish. We used stepwise multiple linear regression models to test combinations of variables as predictors of disease severity. Models with average water temperature or average temperature variation during exposure and the density of oligochaetes and chironomids accounted for more than 80% of the variation.

Whirling Disease: Reviews and Current Topics

<em>ABSTRACT. </em>The ability of several compounds to discharge the polar filaments of polar capsules of the triactinomyxon stage of <em>Myxobolus cerebralis </em>was tested. Premature polar filament discharge may provide a means for preventing the infective stage of myxozoan parasites from attaching to fish hosts. The discharge regimes evaluated included high and low pH, chloride and phosphate salts, calcium chelators, direct current, mucus, tricaine methanesulfonate anesthetic, neurochemicals, and chemosensitizing agents that are effective discharge agents for members of the phylum Cnidaria. Polar filament discharge, in response to HCl or NaOH, did not differ from controls until pH levels dropped to 1.1 or increased to 11.7. Among the chloride salts tested (NaCl, KCl, CaCl₂, NH₄Cl, MgCl₂), discharge increased at concentrations ranging from 3.1 to 100‰. Discharge varied among the salts tested, peaking at 71% for 100‰ KCl; however, the phosphate salts K⁺and Na⁺did not differ in discharge ability. Comparison among KCl, KI, and KPO₄indicated that Cl^-was significantly more effective at both 6.2‰ (45.6% discharge) and 12.5‰ (57.8%) than the other anions. The calcium chelators sodium citrate and EGTA did not induce any significant increase in discharge, nor did the neurochemicals angiotensin, bradykinin, and acetylcholine chloride. Compounds, such as N-acetyl neuraminic acid, proline, and glutathione, that have been reported as chemosensitizers for cnidae discharge among cnidarians, were ineffective discharge agents for triactinomyxon polar capsules. Mucus from rainbow trout or bovine submaxillary gland failed to significantly increase discharge. Attempts to combine mucus with force (stirring rod) or a 0.45 Gauss magnetic field did not increase discharge rates. However, using an electroporator to administer direct current, the discharge rate increased with pulse length (up to 99 µsec) and the number of pulses (0–25). Maximum discharge (98%) and mortality (100%) was observed after 25 99-µsec pulses of 3 kV. Results with electricity indicate a potential for using direct current as a means of disinfection. The data suggest some similarities and differences with similar research on Cnidaria that is discussed.