Comparison of Sampling and Detection Methods for Chinook Salmon and Steelhead Naturally Infected with Myxobolus cerebralis

Abstract

Myxobolus cerebralis (Mc) is a myxozoan parasite causing whirling disease in hatchery- and natural-origin salmonids. To minimize spread of this parasite and the incidence of its associated disease, fish health professionals routinely screen fish for Mc before stocking or moving the fish to Mc-free waters. Sample collection for Mc traditionally entails lethal sampling of cranial tissue followed by pepsin-trypsin digest (PTD) and screening of the sample for mature myxobolid myxospores (PTD method); however, nonlethal sampling methods would be advantageous in some circumstances, such as when dealing with rare or otherwise valuable fish. Accordingly, we compared Mc detections in cranial cartilage by using the PTD method with PCR assays of fin biopsies collected from juvenile Chinook Salmon Oncorhynchus tshawytscha and adult steelhead O. mykiss. Cranial samples were also analyzed using PCR methods for comparative purposes. Results indicated that Mc could be detected by PCR in fin clips, but the results generated by this approach differed significantly from those associated with PTD- and/or PCR-based analysis of cranial cartilage samples. Polymerase chain reaction-based analysis-of individual head samples and head digest pools in both species as well as fins in steelhead-yielded more positive detections than PTD analysis alone. The PCR-based analysis of head and fin tissues yielded different Mc detection rates in both species, but the nature of the detection disparity varied depending on the species and/or life stage of the fish. We conclude that for lethal cranial samples, neither PTD nor PCR should be used alone, but using these techniques in concert may provide the most complete and accurate estimation of Mc presence in a group of salmonids. If imperiled or highly valuable fish are in question, nonlethal fin samples may be used to generate some information regarding Mc status, with the understanding that parasite DNA detections do not necessarily signify mature infections or disease.

A Comparison of Nonlethal and Destructive Methods for Broad‐Based Infectious Agent Screening of Chinook Salmon Using High‐Throughput qPCR

“…Previous studies that have characterized the utility of using one or more nonlethally sampled tissues or diagnostic approaches (e.g., molecular methods, cell culture) for infectious agent screening have found inherent variability in results among tissues, especially if the nonprimary tissues of agents are evaluated (Chiaramonte et al. ). The broad‐based screening approach that we applied seeks to comprise an array of infectious agents, thereby forcing compromise in the tissues included in the analysis.…”

“…; Chiaramonte et al. ). Considering multiple infections, cumulative variation in results among host tissues could substantially bias survey conclusions regarding coinfection status and virulence potential.…”

qPCR-based environmental monitoring of the whirling disease-causing parasite Myxobolus cerebralis and phylogenetic analysis of the tubificid hosts in Alberta, Canada

“…Most current testing programs for M. cerebralis rely on detecting the parasite in fish tissues, which often requires lethal testing of fish to detect the relevant stages of parasite development (Chiaramonte et al 2018). Only early infections, less than 60 d, can be detected by non-lethal sampling such as caudal fin clips (American Fisheries Society−Fish Health Section 2006, Skirpstunas et al 2006).…”

“…Waiting until the parasite has established in the fish population leaves preventative measures lagging behind an already entrenched population-level infection. This was noted in many of the US states combating M. cerebralis invasion, where the parasite was only de tected once established in fish populations (Chiaramonte et al 2018). It also leaves a testing gap for locations without a year-round fish population.…”