Julie D. Alexander

Host and Parasite Populations After a Ten Year Flood: Manayunkia speciosa and Ceratonova (syn Ceratomyxa) shasta in the Klamath River

Abstract Manayunkia speciosa is the obligate invertebrate host of Ceratonova (syn Ceratomyxa) shasta (Myxozoa), the parasite that causes ceratomyxosis (enteronecrosis) in salmon and trout. High peak discharge has been correlated with reduced ceratomyxosis in salmon hosts but how it may influence parasite dynamics in the invertebrate host is unknown. We sampled M. speciosa populations from three sections of the Klamath River, California, in spring and summer months in 2006, a water year that was characterized by i) winter and spring discharge equivalent in magnitude to a 10 year flood and ii) low risk of ceratomyxosis for salmon. M. speciosa were observed in all river sections and months but densities were highest in the upper section, where peak discharge was lowest. Populations were dominated by immature individuals in all river sections and months, followed by progeny and mature individuals. Prevalence of C. shasta infection was 0.25% (SE 0.09) in June; 0.13% (SE 0.09) in July; 0% (SE 0) in August, and 0.03% (SE 0.02) in September. This study provides baseline data on the density and size structure of M. speciosa populations and the prevalence of C. shasta infection following a high magnitude flood event. We discuss the results in the context of using flow manipulation as a tool for managing ceratomyxosis.

Effect of Nanophyetus salmincola and Bacterial Co-Infection on Mortality of Juvenile Chinook Salmon

The freshwater trematode Nanophyetus salmincola has been demonstrated to impair salmonid immune function and resistance to the marine pathogen Vibrio anguillarum, potentially resulting in ocean mortality. We examined whether infection by the parasite N. salmincola similarly increases mortality of juvenile Chinook Salmon Oncorhynchus tshawytscha when they are exposed to the freshwater pathogens Flavobacterium columnare or Aeromonas salmonicida, two bacteria that juvenile salmonids might encounter during their migration to the marine environment. We used a two-part experimental design where juvenile Chinook Salmon were first infected with N. salmincola through cohabitation with infected freshwater snails, Juga spp., and then challenged with either F. columnare or A. salmonicida. Cumulative percent mortality from F. columnare infection was higher in N. salmincola-parasitized fish than in nonparasitized fish. In contrast, cumulative percent mortality from A. salmonicida infection did not differ between N. salmincola-parasitized and nonparasitized groups. No mortalities were observed in the N. salmincola-parasitized-only and control groups from either challenge. Our study demonstrates that a relatively high mean intensity (>200 metacercariae per posterior kidney) of encysted N. salmincola metacercariae can alter the outcomes of bacterial infection in juvenile Chinook Salmon, which might have implications for disease in wild fish populations.

Context-specific parasitism in Tubifex tubifex in geothermally influenced stream reaches in Yellowstone National Park

Abstract Parasites can regulate host abundance and influence the composition and structure of communities. However, host–parasite interactions might be context-specific if environmental conditions can alter the outcome of parasitism and disease. An understanding of how host–parasite interactions might change in different contexts will be useful for predicting and managing disease against a background of anthropogenic environmental change. We examined the ecology of Myxobolus cerebralis, the parasite that causes whirling disease in salmonids, and its obligate host, Tubifex tubifex, in geothermally variable stream reaches in Yellowstone National Park. We identified reaches in 4 categories of geothermal influence, which were characterized by variable substrates, temperatures, specific conductivities, and pH. In each reach, we measured aspects of host ecology (abundance, relative abundance, size, and genotype of T. tubifex), parasite ecology (infection prevalence in T. tubifex and abundance of M. cerebralis-infected T. tubifex), and risk to fish of contracting whirling disease. Tubifex tubifex abundance was high all in reaches characterized by geothermal influence, whereas abundance of M. cerebralis-infected T. tubifex was high only in reaches characterized by intermediate geothermal influence. We suggest that habitat had a contextual effect on parasitism in the oligochaete host. Abundance of infected hosts appeared to depend on host abundance in all reach types except those with high geothermal influence, where abundance of infected hosts depended on environmental factors.

Integrating models to predict distribution of the invertebrate host of myxosporean parasites

Manayunkia speciosa, a freshwater polychaete, is the invertebrate host of myxosporean parasites that negatively affect salmonid populations in the Pacific Northwest of the USA. Factors that drive the distribution of M. speciosa are not well understood, which constrains our understanding of disease dynamics and the development of management solutions. We described the distribution of M. speciosa at 3 sites on the Klamath River, California, based on 2-dimensional hydraulic models (2DHMs) and a generalized linear mixed model (GLMM). 2DHMs were built to explain hydraulic variation at each site and used to stratify biological sampling effort along depth–velocity gradients and by substrate class. We assessed the presence/absence of M. speciosa at 362 georeferenced locations in July 2012 and built GLMMs to describe relationships between hydraulic and substrate variables and the distribution of M. speciosa. The best-fitting GLMMs demonstrated that M. speciosa distributions were associated with depth–velocity conditions and substrate size during base discharge (area under the receiver operating characteristic curve [AUC] = 0.88) and at peak discharge (AUC = 0.86). We evaluated the GLMMs with an independent data set collected in July 2013 (n = 280) and found that the top models predicted the distribution of M. speciosa with a high degree of accuracy (AUC = 0.90). These results support the conclusion that the summer distribution of M. speciosa is related to observed hydraulic and substrate conditions during base discharge (summer) and modeled hydraulic and substrate conditions during peak discharge (late winter to early spring). These results may have implications for the use of flow manipulation as a disease management tool. These results also illustrate the importance of examining species distribution data in the context of temporally disconnected environmental factors and demonstrate how models can fulfill this need.

Annelid-Myxosporean Interactions

This chapter provides an overview of the diversity of annelids parasitized by myxosporeans in both marine and freshwater systems and reviews the interactions between myxosporeans and their annelid hosts during the so-called actinospore phase of the myxosporean life cycle. Both host and environmental factors can influence infection. Our chapter examines these influences from the initial infection of annelids through to the release of spores infectious to fish. Topics covered include how myxosoporean infections may relate to the trophic ecology of annelid hosts and what factors may influence dispersal and increase encounter rates of infectious spores with hosts. We also review the little that is known about annelid-myxosporean interactions, focusing on patterns of host specificity, host susceptibility and host immune defenses. Finally, we examine development within and effects on annelid hosts. To illustrate these interactions we draw primarily on examples from the relatively well-studied freshwater salmon parasites Myxobolus cerebralis and Ceratonova shasta, and interactions with their respective annelid hosts.

Modeling the Effects of Climate Change on Disease Severity: A Case Study of Ceratonova (syn Ceratomyxa) shasta in the Klamath River

Shifts in future temperature and precipitation patterns will have profound effects on host-parasite interactions and the dynamics of disease in freshwater systems. The aims of this chapter are to present an overview of myxozoan disease dynamics in the context of climate change, and to illustrate how these might be predicted over the next several decades by developing a case study of disease dynamics of Ceratonova (syn Ceratomyxa) shasta in the Klamath River, California USA. Our case study introduces a model ensemble for predicting changes in disease dynamics under different climate scenarios (warm/dry, moderate/median, and cool/wet) from 2020 to 2060. The ensemble uses Global Circulation Models (GCMs) and basin scaled models for the Klamath River to generate predictions for future water temperature and river discharge. The environmental data are used as inputs for a predictive model and a degree day model to simulate effects of climate change on polychaete host populations and on C. shasta spore viability, respectively. Outputs from these models were then used to parameterize an epidemiological model to predict changes in disease dynamics under each climate scenario. The epidemiological model outputs were measured against baselines established using real data for low (2006), high (2008) and intermediate (2011) disease risk years. In general, the epidemiological model predicts that except for infrequent high discharge years, C. shasta dynamics will be similar to the high disease risk baseline (2008). This suggests that the recovery and management of Klamath River salmon will continue to be impacted by C. shasta.