The myxozoan parasite Ceratomyxa shasta is a significant pathogen of juvenile salmonids in the Pacific Northwest of North America and is limiting recovery of Chinook (Oncorhynchus tshawytscha) and coho (O. kisutch) salmon populations in the Klamath River. We conducted a 5-year monitoring program that comprised concurrent sentinel fish exposures and water sampling across 212 river kilometers of the Klamath River. We used percent mortality and degree-days to death to measure disease severity in fish. We analyzed water samples using quantitative PCR and Sanger sequencing, to determine total parasite density and relative abundance of C. shasta genotypes, which differ in their pathogenicity to salmonids. We detected the parasite throughout the study zone, but parasite density and genetic composition fluctuated spatially and temporally. Chinook and coho mortality increased with density of their specific parasite genotype, but mortality-density thresholds and time to death differed. A lethality threshold of 40% mortality was reached with 10 spores liter ؊1 for Chinook but only 5 spores liter ؊1 for coho. Parasite density did not affect degree-days to death for Chinook but was negatively correlated for coho, and there was wider variation among coho individuals. These differences likely reflect the different life histories and genetic heterogeneity of the salmon populations. Direct quantification of the density of host-specific parasite genotypes in water samples offers a management tool for predicting host population-level impacts.

Water temperature influences almost every biological and physiological process of salmon, including disease resistance. In the Klamath River (California), current thermal conditions are considered sub-optimal for juvenile salmon. In addition to borderline temperatures, these fish must contend with the myxozoan parasite Ceratomyxa shasta , a significant cause of juvenile salmonid mortality in this system. This paper presents 2 studies, conducted from 2007 to 2010, that examine thermal effects on C. shasta -induced mortality in native Klamath River Chinook ( Oncorhynchus tshawytscha ) and coho ( Oncorhynchus kisutch ) salmon. In each study, fish were exposed to C. shasta in the Klamath River for 72 hr and then reared in the laboratory under temperature-controlled conditions. The first study analyzed data collected from a multi-year monitoring project to asses the influence of elevated temperatures on parasite-induced mortality during the spring/summer migration period. The second study compared disease progression in both species at 4 temperatures (13, 15, 18, and 21 C) representative of spring/summer migration conditions. Both studies demonstrated that elevated water temperatures consistently resulted in higher mortality and faster mean days to death. However, analysis of data from the multi-year monitoring showed that the magnitude of this effect varied among years and was more closely associated with parasite density than with temperature. Also, there was a difference in the timing of peak mortality between species; Chinook incurred high mortalities in 2008 and 2009, whereas coho was greatest in 2007 and 2008. As neither temperature nor parasite density can be easily manipulated, management strategies should focus on disrupting the overlap of this parasite and its obligate hosts to improve emigration success and survival of juvenile salmon in the Klamath River.

The myxozoan parasite Ceratomyxa shasta is a significant pathogen of juvenile Chinook salmon Oncorhynchus tshawytscha in the Klamath River, California, USA. This parasite requires 2 hosts to complete its life cycle: a freshwater polychaete (Manayunkia speciosa) and a salmonid. The complex life cycle and large geographic area where infection occurs make it difficult to monitor and manage ceratomyxosis. We present a model for ceratomyxosis-induced mortality in O. tshawytscha, from which parameters important to the persistence of C. shasta are identified. We also experimentally quantify specific parameters from the model and identify a mortality threshold (a critical parameter), by naturally exposing native O. tshawytscha to C. shasta in the Klamath River. The average percent mortality that resulted from these experimental challenges ranged from 2.5 to 98.5% over an exposure dose of 4.4 to 612 × 10 6 parasites. This experiment identified a non-linear mortality threshold of 7.7 ± 2.1 × 10 4 actinospores fish -1 for Chinook salmon from the Iron Gate Hatchery on the Klamath River. Below this threshold no mortality occurred and above it mortality increased dramatically, thus providing a target by which to reduce parasitism in emigrating juvenile O. tshawytscha. KEY WORDS: Ceratomyxa shasta · Epidemiological model · Mortality threshold · Chinook salmon · Disease ecology · Myxozoan · Macroparasite Resale or republication not permitted without written consent of the publisherDis Aquat Org 93: [63][64][65][66][67][68][69][70] 2010 value of 1, and below this threshold a pathogen cannot maintain itself within the population; however, when R o > 1 the pathogen will spread throughout a naïve population (Macdonald 1952, Reno 1998. Thus, identifying R o for a particular pathogen provides information on how severe a disease outbreak is likely to be and what kind of management effort may be required to reduce it. These models have been successfully developed for infectious diseases of humans, the first being the population dynamic model of malaria developed by Ross (Ross 1911, Smith et al. 2007). Epidemiological models have been applied to diseases occurring in wildlife populations; however, they tend to be biased towards large-bodied and easily observable hosts (Dobson & Foufopoulos 2001). One model commonly used for examining the effect of pathogens on cultured fish has been the Susceptible-Infected-Recovered (SIR) model. Although this model works well for directly transmitted pathogens such as infectious haematopoietic necrosis virus (IHNV) and the bacterium Aeromonas salmonicida (Ögüt 2003), for aquatic pathogens that require multiple hosts their application becomes increasingly complex, involving multiplicative SIR models. Thus, SIR models are useful only if the appropriate information on the various components is available (Reno 1998).In this paper we introduce a mathematical model for the Ceratomyxa shasta life cycle, adapted from the Ross (1911) malaria population dynamic model, to identify parameters that nee...