Spatial and temporal variation of whirling disease risk in Montana spring creeks and rivers

Spring creeks are important spawning and rearing areas for wild trout, but the stable flows, cool temperatures, and high nutrient levels that characterize these unique habitats may also make them highly susceptible to establishment and proliferation of the whirling disease pathogen Myxobolus cerebralis. We evaluated the spatial and temporal dynamics in whirling disease risk by using sentinel rainbow trout Oncorhynchus mykiss fry in nine different spring creeks and their conjoining rivers or reservoirs in Montana over a 20-month period. Whirling disease risk was high in five of the seven pathogen-positive spring creek study sites; at these sites, prevalence levels exceeded 90% and over 50% of sentinel fry had moderate to high infection severity scores. Spring creeks generally had higher disease prevalence and severity than paired river or reservoir sites. Fine sediment levels varied widely among springs creeks with high and low whirling disease risk, and we found no significant association between fine sediment level and infection severity. The low risk measured for some spring creeks was likely attributable to the pathogen invasion being in its early stages rather than to environmental characteristics limiting the severity of infection. High whirling disease risk occurred over a wide range of temperatures at spring creek sites (4.5-13°C) and river sites (1.7-12.5°C). There was an unusual seasonal cycle of infection in spring creeks, with peak infection levels occurring from late fall to early spring and declining to near zero in late spring to early fall. The low infection risk during spring suggests that spring-spawning trout would be at a low risk of infection, even in spring creeks with otherwise high disease severity. In contrast, fry of fall-spawning trout may be much more susceptible to infection in spring creek environments.

The effects of Myxobolus cerebralis on Apache and Gila trout in laboratory exposures

Whirling disease has been implicated in salmonid population declines in several western states. To determine the risk of a species or strain of salmonid to whirling disease it is critical to establish its relative susceptibility to Myxobolus cerebralis infection. Gila trout Oncorhynchus gilae and Apache trout Oncorhynchus gilae apache were exposed to various doses of M. cerebralis triactinomyxons (TAMs) in laboratory experiments. In trials conducted in consecutive years, fish were exposed to TAMs in doses ranging from 25 to 2,000/fish at ages ranging from 66 to 201 d posthatch (dph). All fish were held for 900 degree-days, and then the infection intensity of each fish was determined by histological examination. In 2002, 98% of the Gila trout died during exposure or within 48 h postexposure. Seventy-four percent of the Apache trout died before the end of the 90-d study period. Those that survived the entire study period had an average histological score of more than 4.0. In subsequent trials, the TAM dosage was decreased to 25-1,000/fish. Also, the age of the fish was increased from 66-72 dph to 89-201 dph. The survival rate increased from 16.0% to 49.1%. Average histological grades ranged from 1.6 to 4.8. Based on this data, it can be concluded that both Apache and Gila trout are highly susceptible to M. cerebralis in laboratory trials.

Epizootiology of Myxobolus cerebralis, the causative agent of salmonid whirling disease in the Rock Creek drainage of west-central Montana: 2004-2008

Whirling disease, caused by the myxozoan parasite Myxobolus cerebralis , remains a serious health threat to salmonid fish in the western United States. A previously published study on the epizootiology of whirling disease in the Rock Creek watershed of west-central Montana, conducted from 1998 to 2003, showed that the intensity of M. cerebralis infections in sentinel trout increased significantly throughout the drainage and that the range of M. cerebralis had expanded considerably. In addition, the parasite had apparently caused a dramatic decline in rainbow trout densities, but the brown trout population numbers had increased. This earlier study was continued from 2004 to 2008 and the results are reported here. It now appears that the disease intensity may have peaked in 2006 and is on the decline in this watershed. The decline cannot be directly attributed to a change in the prevalence of M. cerebralis-infected Tubifex tubifex, as these numbers remained statistically the same from 1998 to 2008. Similarly, changes in water temperature and water flow do not account for the decrease in disease intensity. However, it is possible that wild rainbow trout are developing resistance to the parasite, a phenomenon recently documented to be occurring in the Willow Creek Reservoir of southwest Montana.

Rapid natural selection for resistance to an introduced parasite of rainbow trout

Introduced species and infectious diseases both independently pose challenges for the preservation of existing biodiversity. However, native species or disease hosts are by no means ‘unarmed’ when faced with novel environmental challenges, provided that adequate adaptive genetic variation exists to mount effective evolutionary responses. In this study, we examined the consequences of the recently introduced parasite and causative agent of whirling disease (Myxobolus cerebralis) in a wild rainbow trout (Oncorhynchus mykiss) population from Harrison Lake, Montana (USA). Consistent with the parasite’s age-specific effects, juvenile rainbow trout recruitment into Harrison Lake was substantially reduced following parasite detection in 1995. However, experimental data suggest that natural selection has rapidly reduced whirling disease susceptibility within the population over time. The rapid observed temporal change in resistance patterns argues that the standing genetic variation for parasite resistance facilitated this process. Our findings ultimately underscore the importance of preserving genetic diversity to ensure that species of economic importance or of conservation concern have maximal chances for persistence in future changing environments.

Epizootiology of Myxobolus cerebralis, the causative agent of salmonid whirling disease in the Rock Creek drainage of west-central Montana

Whirling disease, caused by the myxozoan parasite Myxobolus cerebralis, remains a health threat to salmonid fish in the western United States. Although various aspects of this host-parasite system have been studied, investigations examining the overall epizootiology of whirling disease in an ecosystem are lacking. Therefore, in June 1998, studies were initiated in the Rock Creek watershed of west-central Montana and continued through 2003 to assess the intensity of infection in trout using sentinel cages stationed throughout the drainage. Additional studies determined the percentage of the annelid worm, Tubifex tubifex, releasing M. cerebralis at various localities in Rock Creek and whether there was a seasonal or daily periodicity in the release of the triactinomyxon stage of the parasite from T. tubifex. Lastly, habitat and water quality parameters, and the effects of habitat restoration on transmission of M. cerebralis, were assessed. Overall, the intensity of M. cerebralis infections in sentinel trout increased significantly throughout the drainage between June of 1998 and 2003, with the biggest jump occurring between 1998 and 1999. In addition, the range of M. cerebralis expanded considerably over the period of study. There was no strict correlation between habitat condition and the occurrence of the parasite; fish became heavily infected in optimal and marginal habitats. However, fish exposed at a locality that had the lowest habitat ranking consistently had the highest intensity of infection. The parasite has apparently caused a dramatic decline in rainbow trout densities, but the brown trout population numbers have increased, and the overall fish density remains high. Although a major habitat restoration project did not seem to have an effect on decreasing disease intensity, this was not surprising because the restored area was located just downstream from a "hotspot" of infected T. tubifex.

Risk of Myxobolus cerebralis infection to rainbow trout in the Madison River, Montana, USA

Myxobolus cerebralis, the parasite that causes salmonid whirling disease, has had detrimental effects on several salmonid populations in the Intermountain West, including the rainbow trout in the Madison River, Montana, USA. The goal of this study was to examine relationships among characteristics of the environment, Tubifex tubifex (the alternate host) populations, and rainbow trout whirling disease risk in the Madison River. Environmental characteristics were measured in side channels of the Madison River, and differences were described with a principal components analysis. The density of T. tubifex, the prevalence of infection in T. tubifex, and the density of infected T. tubifex were determined for the side channels using benthic core samples and examination of live tubificids for infection. The site-specific contribution to whirling disease risk in the side channels was determined using in situ exposures of sentinel rainbow trout. Regression analyses were used to determine correlations among these characteristics. Side channels differed in site-specific contribution to rainbow trout whirling disease risk, which was positively correlated to the density of infected T. tubifex. Side channels with fine sediments and lower water temperatures made greater site-specific contribution to whirling disease risk and had higher densities of infected T. tubifex than side channels with coarser sediments and higher temperatures. The ability to characterize areas of high whirling disease risk is essential for improving our understanding of the dynamics of M. cerebralis such that appropriate management strategies can be implemented. In addition, this study provides a model of how the disease ecology of complex aquatic parasites can be examined when the influential processes operate on different spatial scales.

Myxobolus cerebralis infection in rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) exposed under natural stream conditions

From early April into mid-June 1977, sequential groups of juvenile rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) were each exposed for 10 days to the parasite Myxobolus cerebralis by immersion in a stream inhabited by infected wild trout. Following incubation in a M. cerebralis-free facility, trout were subsequently killed, and heads and gill arches were examined by routine histologic methods. A grading scale to quantify lesion severity was developed and applied. Percentage infected, lesion severity scores, effects of water temperature and flow rates on percentage infected and lesion severity scores, and resulting pathology were determined for each species at each exposure period. The percentage of rainbow trout infected with M. cerebralis was significantly higher than the percentage of brown trout infected for each exposure period. The percentages of rainbow trout infected in exposure periods later in the calendar year were significantly higher than those in earlier periods. The percentages of brown trout infected were not significantly different among exposure periods. Overall average lesion severity scores were significantly higher in rainbow than in brown trout. Lesion severity scores in rainbow trout increased over time (a positive correlation with exposure period). Lesion severity scores were not significantly different for brown trout among exposure periods. A significant correlation existed between water temperature and percentage of rainbow trout infected; a significant correlation also existed between water temperature and lesion severity scores in rainbow trout. Similar correlations did not exist for percentage of brown trout infected or accompanying lesion severity scores. In rainbow trout, ventral calvarium was the most common site of M. cerebralis replication, followed by gill arches. In brown trout, lesions were virtually confined to gill arches. Early lesions consisted of foci of cartilage necrosis with small numbers of M. cerebralis developmental stages. More advanced lesions consisted of multifocal areas of cartilage necrosis with numerous M. cerebralis developmental stages and/or mature myxospores bordered and/or infiltrated by mono- and multinuclear leukocytes. Lesions in brown trout were smaller and had fewer associated leukocytes and M. cerebralis developmental stages and/or mature myxospores. Higher infection rates, lesion severity scores, and differences in lesion location in rainbow versus brown trout explain in part why numbers of rainbow but not brown trout have fallen in western rivers inhabited with M. cerebralis-infected trout.