Elizabeth MacConnell

Whirling disease: re-emergence among wild trout

Summary: Whirling disease of rainbow trout is caused by Myxobolus cerebralis, a myxozoan parasite possessing a life cycle well adapted to the natural environments where salmonid fish are found. Whirling disease was first described in Europe in 1898 among farmed rainbow trout but recent occurrences have been devastating to wild trout in North America. The disease is considered a major threat to survival of wild rainbow trout in the intermountain west of the United States. Difficulties in containing the spread and potentially eliminating the pathogen are tied to features of a complex life cycle involving two hosts, the salmonid fish and an aquatic oligochaete. Details of the morphologic development of the parasite have been described in each host but only now are we beginning to appreciate the breadth of interactions between these developmental forms and the sequential responses of the host. Fundamental mechanisms of the recognition and attachment of the parasite to the hosts, how host immunity is evaded and the unknown influences of environmental factors all contribute to a rather poor understanding of the biology of the parasite. Although the biology and ecology of the salmonid host are better known than for the oligochaete host, our knowledge is inadequate to interpret their complex interactions with the parasite. This uncertainty precludes the development of effective management activities designed to enhance the viability and productivity of wild trout populations in M. cerebralis‐ positive river systems. Improving our understanding of the hosts, the parasite and the environmental factors determining their interaction should provide for more focused and effective control methods for containing the spread and devastating effects whirling disease is causing to our wild trout populations.

Susceptibility of Selected Inland Salmonids to Experimentally Induced Infections with Myxobolus cerebralis, the Causative Agent of Whirling Disease

Abstract Laboratory exposures to the infectious stages (triactinomyxons) of Myxobolus cerebralis demonstrated a range of susceptibility to whirling disease among four species of inland salmonids. Replicate groups of each species were exposed to two concentrations of triactinomyxons, a low dose (100–200 per fish) and a high dose (1,000–2,000 per fish). Exposed fish were evaluated for clinical signs, for severity of microscopic lesions at 35 d, 2 and 5 months, and for spore concentrations in the head cartilage at 5 months. A standard strain of rainbow trout Oncorhynchus mykiss matched for age served as a susceptible species control. Rainbow trout, westslope cutthroat trout O. clarki lewisi, Yellowstone cutthroat trout O. clarki bouvieri, and bull trout Salvelinus confluentus were susceptible to M. cerebralis infections. Clinical signs, including radical swimming (“whirling”) and black tails, were observed at 7 weeks postexposure among rainbow and cutthroat trout challenged at 3 weeks of age. Clinical signs ...

Characterizing aquatic health using salmonid mortality, physiology, and biomass estimates in streams with elevated concentrations of arsenic, cadmium, copper, lead, and zinc in the Boulder River watershed, Montana

Abstract Abandoned tailings and mine adits are located throughout the Boulder River watershed in Montana. In this watershed, all species of fish are absent from some tributary reaches near mine sources; however, populations of brook trout Salvelinus fontitalis, rainbow trout Oncorhynchus mykiss, and cut-throat trout O. clarki are found further downstream. Multiple methods must be used to investigate the effects of metals released by past mining activity because the effects on aquatic life may range in severity, depending on the proximity of mine sources. Therefore, we used three types of effects—those on fish population levels (as measured by survival), those on biomass and density, and those at the level of the individual (as measured by increases in metallothionein, products of lipid peroxidation, and increases in concentrations of tissue metals)—to assess the aquatic health of the Boulder River watershed. Elevated concentrations of Cd, Cu, and Zn in the water column were associated with increased morta...

Susceptibility of Three Species of Anadromous Salmonids to Experimentally Induced Infections with Myxobolus cerebralis, the Causative Agent of Whirling Disease

Abstract The susceptibility of three species of anadromous salmonids to whirling disease was examined after their experimental exposures to the infectious stages of Myxobolus cerebralis. Chinook salmon Oncorhynchus tshawytscha exposed as alevins were very susceptible to infection; the appearance of clinical signs, prevalence of infection, severity of microscopic lesions, and spore counts at 130 d postexposure are similar to those of age-matched rainbow trout O. mykiss exposed at the same dose. In contrast, coho salmon O. kisutch demonstrated no clinical signs of infection and had a lower prevalence of infection and spore numbers than did the exposed rainbow trout. A comparison of two strains of steelhead (anadromous rainbow trout), one from an enzootic site (San Lorenzo River) where M. cerebralis has been present for the past 35 years and a second from a site where the parasite is not found (Dry Creek), showed them both to be highly susceptible to experimental infections with M. cerebralis. These controll...

Effects of freezing, drying, ultraviolet irradiation, chlorine, and quaternary ammonium treatments on the infectivity of myxospores of Myxobolus cerebralis for Tubifex tubifex.

The effects of freezing, drying, ultraviolet irradiation (UV), chlorine, and a quaternary ammonium compound on the infectivity of the myxospore stage of Myxobolus cerebralis (the causative agent of whirling disease) for Tubifex tubifex were examined in a series of laboratory trials. Freezing at either -20 degrees C or -80 degrees C for a period of 7 d or 2 months eliminated infectivity as assessed by the absence of production of the actinospore stage (triactinomyxons [TAMs]) from T. tubifex cultures inoculated with treated myxospores over a 4-5-month period. Myxospores retained infectivity when held in well water at 5 degrees C or 22 degrees C for 7 d and when held at 4 degrees C or 10 degrees C d for 2 months. In contrast, no TAMs were produced from T. tubifex cultures inoculated with myxospores held at 20 degrees C for 2 months. Drying of myxospores eliminated any evidence of infectivity for T. tubifex. Doses of UV from 40 to 480 mJ/cm2 were all effective for inactivating myxospores of M. cerebralis, although a few TAMs were detected in one replicate T. tubifex culture at 240 mJ/cm2 and in one replicate culture at 480 mJ/cm2. Treatments of myxospores with chlorine bleach at active concentrations of at least 500 mg/L for 15 min largely inactivated myxospore infectivity for T. tubifex. Likewise, there was no evidence of TAMs produced by T. tubifex inoculated with myxospores treated with alkyl dimethyl benzyl ammonium chloride (ADBAC) at 1,500 mg/L for 10 min. Treatments of myxospores with 1,000-mg/L ADBAC for 10 min reduced TAM production in T. tubifex cultures sevenfold relative to that in cultures inoculated with an equal number of untreated myxospores. These results indicate that myxospores of M. cerebralis demonstrate a selective rather than broad resistance to selected physical and chemical treatments, and this selective resistance is consistent with conditions that myxospores are likely to experience in nature.

Cellular inflammatory response of rainbow trout to the protozoan parasite that causes proliferative kidney disease.

Abstract The cellular inflammatory response of rainbow trout Oncorhynchus mykiss (formerly Salmo gairdneri) to the myxozoan parasite PKX that causes proliferative kidney disease was investigated. The response was studied from 3 to 20 weeks after the fish were injected with infected kidney homogenate. Kidney samples were examined by light and electron microscopy. In contrast to most myxosporeans, PKX provoked a severe host response. Parasites were found in peritubular capillaries and sinusoids 3 weeks postinjection. The initial response to PKX was hemopoietic hyperplasia followed by a marked granulomatous nephritis that was resolved by termination of the study at 20 weeks postinjection. The macrophage was the predominant cell type involved in the inflammatory response to PKX. We presume that the macrophage effectively interrupts the development of PKX and eliminates the parasite from the host.

Influence of in‐stream diel concentration cycles of dissolved trace metals on acute toxicity to one‐year‐old cutthroat trout (Oncorhynchus clarki lewisi)

Extrapolating results of laboratory bioassays to streams is difficult, because conditions such as temperature and dissolved metal concentrations can change substantially on diel time scales. Field bioassays conducted for 96 h in two mining-affected streams compared the survival of hatchery-raised, metal-naïve westslope cutthroat trout (Oncorhynchus clarki lewisi) exposed to dissolved (0.1-microm filtration) metal concentrations that either exhibited the diel variation observed in streams or were controlled at a constant value. Cadmium and Zn concentrations in these streams increased each night by as much as 61 and 125%, respectively, and decreased a corresponding amount the next day, whereas Cu did not display a diel concentration cycle. In High Ore Creek (40 km south of Helena, MT, USA), survival (33%) after exposure to natural diel-fluctuating Zn concentrations (range, 214-634 microg/L; mean, 428 microg/L) was significantly (p = 0.008) higher than survival (14%) after exposure to a controlled, constant Zn concentration (422 microg/L). Similarly, in Dry Fork Belt Creek (70 km southeast of Great Falls, MT, USA), survival (75%) after exposure to diel-fluctuating Zn concentrations (range, 266-522 microg/L; mean, 399 microg/L) was significantly (p = 0.022) higher than survival (50%) in the constant-concentration treatment (392 microg/L). Survival likely was greater in these diel treatments, both because the periods of lower metal concentrations provided some relief for the fish and because toxicity during periods of higher metal concentrations was lessened by the simultaneous occurrence each night of lower water temperatures, which reduce the rate of metal uptake. Based on the present study, current water-quality criteria appear to be protective for streams with diel concentration cycles of Zn (and, perhaps, Cd) for the hydrologic conditions tested.

Salmonids Resistant to Ceratomyxa shasta Are Susceptible to Experimentally Induced Infections with Myxobolus cerebralis

Abstract Two strains of rainbow trout Oncorhynchus mykiss and one strain of steelhead (anadromous rainbow trout) previously shown to demonstrate resistance to the myxosporean pathogen Ceratomyxa shasta were found to be susceptible to experimentally induced infections with the myxosporean Myxobolus cerebralis. Laboratory exposures to waterborne infectious stages (triactinomyxons) of M. cerebralis of both strains of rainbow trout and the steelhead resulted in clinical signs of whirling disease, including the characteristic tail chasing and black tail, approximately 42–49 d after exposure at a water temperature of 15°C. At 5 months postexposure, the severity of microscopic lesions and spore concentrations in the head cartilage of all salmonids resistant to C. shasta were evaluated. The Iron Gate steelhead and the Pit River rainbow trout that were resistant to C. shasta had lesion scores and spore counts similar to those of control rainbow trout known to be susceptible to C. shasta. Evidence of some resistanc...

Iridovirus Infections among Missouri River Sturgeon: Initial Characterization, Transmission, and Evidence for Establishment of a Carrier State

Iridovirus infections of the integument were associated with disease and mortality among hatchery-reared populations of juvenile pallid sturgeon Scaphirhynchus albus and shovelnose sturgeon S. platorynchus from the Missouri River. Virus-infected cells in the integument of fins and body were greatly enlarged, possessed pleomorphic and eccentric nuclei, and exhibited an amphophilic to eosinophilic staining of the cytoplasm in hematoxylin-and-eosin-stained sections. Virus particles found in the host cell cytoplasm were composed of an outer hexagonal capsid measuring 254 nm in diameter and surrounding a dense nucleoid. Despite numerous attempts, the virus could not be propagated on routine cell lines used in fish viral diagnostics or from established cell lines from white sturgeon Acipenser transmontanus, pallid sturgeon, or shovelnose sturgeon. Bath exposures of healthy juvenile pallid sturgeon to a crude extract or a 0.45-microm-filtered extract from the fins of infected fish resulted in transmission of the virus and mortality. At water temperatures of 15 degrees C, the first deaths occurred at approximately 1 month; mortality peaked between 50 and 60 d postexposure, after which surviving fish recovered. Presence of the virus was confirmed among dead and moribund pallid sturgeon by both histology and detection of viral DNA by polymerase chain reaction methods. Feeding of infected tissues and cohabitation with virus-infected shovelnose sturgeon also resulted in successful virus transmission to juvenile pallid sturgeon. Virus infections among experimentally exposed pallid sturgeon that recovered from clinical episodes persisted for at least 8.5 months, and these apparently healthy fish transmitted the virus and disease to juvenile pallid sturgeon by cohabitation. The newly described Missouri River sturgeon iridovirus (MRSIV) as found in pallid sturgeon and shovelnose sturgeon shares many properties with a group of iridoviruses associated with serious skin and gill infections in several species of sturgeon.

Proliferative Kidney Disease in Feral Cutthroat Trout from a Remote Montana Reservoir: A First Case

Abstract Proliferative kidney disease (PKD) caused by the unclassified myxosporean referred to as PKX was found to be associated with mortality of feral cutthroat trout Oncorhynchus clarki in a remote Montana reservoir during August and September of 1990 and 1991. Affected fish had swollen and mottled kidneys, pale livers, and ascites. Microscopic examination of stained tissue sections revealed PKX in the gill, liver, and kidney. This report of PKD is the first in Montana and in feral cutthroat trout. This epizootic also raises questions regarding our knowledge of geographic distribution and potential impact of PKD on feral trout populations.