The myxozoan parasite, Ceratomyxa shasta, is the most significant pathogen of juvenile Chinook salmon (Oncorhynchus tshawytscha) in the Klamath River, CA, USA. This parasite requires two hosts - a freshwater polychaete (Manayunkia speciosa) and a salmonid - to complete its life cycle. The complex life cycle and large geographic area where infection occurs make monitoring and managing the disease, ceratomyxosis, difficult. Epidemiological models are helpful tools to examine complex disease systems as they serve to identify parameters and rank their relative importance. A system of equations is used to derive the basic reproductive number (Ro) of the parasite. In this paper we present a model for ceratomyxosis induced mortality in Chinook salmon. The field experiments described herein quantify the mortality threshold (a critical parameter in the model), by exposing native Chinook salmon to C. shasta in the Klamath River. The average percent mortality that resulted from this challenge ranged from 2.5% to 98.5% over an exposure dose of 4.4 - 612 x 106 parasites. This study identified a non-linear mortality threshold for Iron Gate Hatchery (IGH) Chinook salmon that ranged from 5.6 - 9.9 x 104 total parasites. Below this threshold no mortality occurs, yet above it mortality dramatically increases. This threshold provides a target to reduce parasitism in emigrating juvenile Chinook salmon.
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