- About Us
- Outreach & Engagement
- Publications & Videos
- Sea Grant People
- Blogs and Social Media
Most parasites and their hosts live in a balance within their environment; however a disease outbreak can occur when either the parasite, host, or environment, are perturbed. Myxozoan parasites are associated with a wide variety of cultured and wild fish populations. Most myxozoans are relatively benign to their vertebrate host; however some cause dramatic population level effects on both cultured and wild fish populations. These parasites have a complex life cycle involving a vertebrate host (fish), an invertebrate host (annelid), and two spore stages (actinospore and myxospore). Interactions between these parasites and their hosts can be strongly influenced by environmental factors, most notably by water temperature and water velocity. Given the complex life cycle of myxozoan parasites and the lack of any chemical treatments or preventatives, controlling infections and disease caused by these parasites is challenging, especially for wild populations.
The myxozoan Ceratomyxa shasta is endemic to many of the major rivers of the Pacific Northwest and infects all species of Pacific salmon. In the Klamath River, CA, USA, C. shasta infection is associated with decreased returns of adult Chinook salmon (Oncorhynchus tshawytscha). The goals of this dissertation were to 1) quantify the effect that elevated water temperature has on C. shasta-induced disease severity and mortality rate for both Chinook and coho (O. kitsch) salmon, 2) identify transmission patterns and quantify transmission rates of the actinospore stage to the salmon host, 3) develop an epidemiological model of this host-parasite life cycle and assess the sensitivity of specific parameters that may act as suitable management strategies, and 4) utilize a mixture cure model, an alternative survival analysis method, to quantify the effects water temperature and discharge on the total and rate of C. shasta-induced mortality of both Chinook and coho salmon.
I found that, similar to disease progression naïve salmon species (i.e. from waters where C. shasta is absent), elevated water temperature increases the rate and overall mortality for salmon species from river systems where the parasite is endemic. Elevated water temperatures also increase the transmission rate of the actinospore stage to the salmon host. The transmission rate of the actinospore stage to the salmon host was inversely related to water velocity, and I identified a potential velocity threshold of ~0.3m/sec, above which transmission was greatly reduced. From the epidemiological model I sensitivity analyses and identified that reduction of the myxospore transmission rate from the adult salmon to the polychaete host during the winter may be the most effective management action to reduce C. shasta-related disease in the Klamath River. This action could potentially be achieved by increasing discharge during the winter to minimize contact between the polychaete host and myxospore stage. Lastly, I applied the mixture cure models to quantify how the daily survival rates of Chinook and coho salmon change over time after the fish become infected with C. shasta. Although varied in approach, the output from both of the models presented in this dissertation can be used to guide management and conservation actions for fish populations affected by myxozoan parasites.