Mortality threshold for juvenile Chinook salmon Oncorhynchus tshawytscha in an epidemiological model of Ceratomyxa shasta

Abstract

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...

Klamath River Thermal Refuge Provides Juvenile Salmon Reduced Exposure to the Parasite Ceratonova shasta

“…For each exposure, 150 of each species were placed into separate live cages at a depth of approximately 1 m ( Table 1). The live cages consisted of 0.28-× 1-m PVC pipe with 0.64-cm mesh covering both ends, and they were oriented parallel to the current and anchored to the bottom with stakes (Ray et al 2010). Every 6 h, three 1-L water samples and water velocity measurements were collected immediately upstream from the live cages.…”

“…Infection in the polychaete host culminates in the release of actinospores into the water column, where they encounter salmon. The outcome of infection is primarily influenced by temperature (Udey et al 1975;Ray et al 2012) and exposure dose (Ray et al 2010;Hallett et al 2012), but is also affected by host susceptibility (Zinn et al 1977;Bartholomew 1998) and parasite genetics (Atkinson and Bartholomew 2010). Elevated water temperatures cause increased and faster mortality in infected fish (e.g.…”

Density of the Waterborne Parasite Ceratomyxa shasta and Its Biological Effects on Salmon

“…However, quantification of waterborne C. shasta gives a measure of instantaneous parasite density in the river rather than the actual infectious dose that fish receive, which is a product of density (genotype specific, number of spores liter Ϫ1 ), water flow (liters min Ϫ1 ), and length of exposure (min). Lethal infectious doses are estimated to be 1 spore for rainbow trout (7,8) and ϳ75,000 spores for KR fall Chinook (24); the lethal dose for coho is unknown. Because water sampling analyzes a fraction of the volume that fish are exposed to, the pathogen may be missed at low spore densities (Ͻ1 spore per 3 liters), at which fish may still become infected.…”

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

“…Data from sentinel ('caged') fish and water samples collected since 2006 were used to parameterize parasite distribution and density, and the severity of infection in fish at the study site (Ray et al 2010;Hallett et al 2012). Data from out-migrating juvenile salmon were used to parameterize prevalence of infection in juveniles (True et al 2013).…”