Morphological and molecular confirmation of Myxobolus cerebralis myxospores infecting wild‑caught and cultured trout in North Carolina (SE USA)

Myxobolus cerebralis Causes Presporogonic Mortality in Juvenile Mountain Whitefish

Recent range expansions of whirling disease impelled us to understand the impacts of its causative agent, the myxozoan parasite Myxobolus cerebralis, on lesser-studied fish hosts. Mountain Whitefish Prosopium williamsoni overlap broadly with M. cerebralis across the western United States and Canada, and populations have experienced widespread declines since the 1990s. To evaluate effects of the parasite on Mountain Whitefish, we revisit formerly unpublished work of the Colorado Division of Wildlife (now Colorado Parks and Wildlife), comparing infection in age-matched Mountain Whitefish, Rainbow Trout Oncorhynchus mykiss, and Brown Trout Salmo trutta. To complement the original report, we reanalyze mortality data and include additional SEM imagery. Infection of M. cerebralis in juvenile Mountain Whitefish was characterized by a brief but heavy period of mortality in the first 2 weeks after exposure, with limited pathology. This clinical effect is unique among the known salmonid hosts of M. cerebralis.

Detection of Myxobolus cerebralis (Bivalvulida: Myxobolidae) in two non-Tubifex tubifex oligochaetes in the southeastern USA

Myxobolus cerebralis (Hofer, 1903), the etiological agent of salmonid whirling disease, reportedly matures in only the oligochaete 'Tubifex tubifex'. The concept of 'T. tubifex' is problematic because it is renowned as a species complex (or having 'strains'), and many sequences ascribed to this taxon in GenBank are misidentified or indicate several cryptic species. These facts cast doubt on the long-held notion that M. cerebralis is strictly host-specific to the single definitive host, T. tubifex. Herein, as part of an ongoing regional whirling disease monitoring project, oligochaetes (452 specimens) were collected from 31 riverine sites in western North Carolina (August through September 2015) and screened for infection by M. cerebralis. The species-specific nested PCR for M. cerebralis was positive for 8 oligochaete specimens from the French Broad River Basin (Mill Creek and Watauga River) and New River Basin (Big Horse Creek). We individually barcoded these M. cerebralis-positive oligochaete specimens using cytochrome oxidase 1 (CO1) primers and then conducted a Bayesian inference phylogenetic analysis. We identified 2 oligochaete genotypes: one sister to a clade comprising Limnodrilus udekemianus (Haplotaxida: Naididae) and another sister to Limnodrilus hoffmeisteri. This is the first detection of M. cerebralis from an oligochaete in the SE USA and the first detection of M. cerebralis from an oligochaete other than T. tubifex. These results suggest that other non-T. tubifex definitive hosts can harbor the pathogen and should be considered in the context of fish hatchery biosecurity and monitoring wild trout streams for M. cerebralis and whirling disease in the southeastern USA.

Standard Weight Equation for Brook Trout in Southern Appalachian Mountains Streams

Brook Trout Salvelinus fontinalis in southern Appalachian Mountains streams of the United States occur at the southernmost portion of their native range, and occupy small, isolated, and low-productivity headwater streams. The existing standard weight (Ws) equation is applicable only to Brook Trout > 120 mm total length (TL), but many individuals in the region are smaller than this minimum size threshold due to their habitat characteristics. Here, we developed a new Ws equation for Brook Trout in southern Appalachian Mountains streams using length–weight data on 72,502 individuals. The weighted quadratic empirical-percentile method minimized length-related bias in relative weight compared to the regression-line-percentile and weighted linear empirical-percentile methods. The proposed Ws equation was: log10W = −3.364 + 1.378 × log10L + 0.397 × (log10L)2, where W was weight (g) and L was TL (mm). The new equation characterized body condition of Brook Trout in southern Appalachian Mountains streams more accurately than the existing equation.

A new species of Myxobolus Bütschli, 1882 (Bivalvulida: Myxobolidae) infecting stratum spongiosum of the imperiled sicklefin redhorse, Moxostoma sp. (Cypriniformes: Catostomidae) from the Little Tennessee River, North Carolina, USA

The sicklefin redhorse, Moxostoma sp. (Cypriniformes: Catostomidae), is an innominate imperiled catostomid endemic to the Hiwassee and Little Tennessee river basins, which has been restricted to a few tributaries of these systems by impoundments. During collections to propagate sicklefin redhorse for reintroduction, a myxozoan, described herein, was observed infecting sicklefin redhorse in the Little Tennessee River Basin, North Carolina. Myxobolus naylori Ksepka et Bullard sp. n. infects the stratum spongiosum covering the scales of sicklefin redhorse. Myxospores of the new species differ from all congeners by the combination of having a mucous envelope, intercapsular process, and sutural markings as well as lacking an iodinophilic vacuole in the sporoplasm. A phylogenetic analysis of the 18S rDNA gene recovered the new species in a polytomy with Myxobolus marumotoi Li et Sato, 2014 and a clade comprised of species of Myxobolus Bütschli, 1882; Thelohanellus Kudo, 1933, and Dicauda Hoffman et Walker, 1973. Histological sections of infected sicklefin redhorse skin revealed myxospores within a plasmodium in the stratum spongiosum dorsal to scales, encapsulated in collagen fibres, and associated with focal erosion of scales directly beneath the plasmodium; in some instances, the scale was perforated by the plasmodium. The specificity of the new species to sicklefin redhorse may make it a useful biological tag to differentiate sicklefin redhorse from morphologically similar species. The new species is the first parasite reported from sicklefin redhorse, a species of concern to the United States Fish and Wildlife Service. No species of Myxobolus has been reported from species of Moxostoma in the Southeast United States. As it was observed that Myxobolus minutus Rosser, Griffin, Quiniou, Alberson, Woodyard, Mischker, Greenway, Wise et Pote, 2016 is a primary junior homonym of Myxobolus minutus Nemeczek, 1911, we propose the replacement name Myxobolus diminutus (Rosser, Griffin, Quiniou, Alberson, Woodyard, Mischker, Greenway, Wise et Pote, 2016).

An updated geographic distribution of Myxobolus cerebralis (Hofer, 1903) (Bivalvulida: Myxobolidae) and the first diagnosed case of whirling disease in wild-caught trout in the south-eastern United States

Myxobolus cerebralis (Bivalvulida: Myxobolidae), the aetiological agent of salmonid whirling disease, was detected in 2 river basins of North Carolina during 2015, which initiated the largest spatial-temporal monitoring project for the disease ever conducted within the south-eastern United States (focused mainly in eastern Tennessee and western North Carolina). A total of 2072 rainbow trout Oncorhynchus mykiss, 1,004 brown trout Salmo trutta and 468 brook trout Salvelinus fontinalis were screened from 113 localities within 7 river basins during June 2017 through October 2019. Infections were detected by pepsin-trypsin digest, microscopy and the species-specific nested polymerase chain reaction (PCR) in 19 localities across 6 river basins. Myxospore morphology was indistinguishable from the published literature. In 2019, five rainbow trout that symptomatic for whirling disease (sloping neurocranium and lordosis) were captured and processed for histopathology. Myxospores were detected in the calvarial cartilage of two deformed trout with associated erosion of the cartilage consistent with reported whirling disease lesions. This is the first report of M. cerebralis in Tennessee and the first histologically confirmed cases of whirling disease in southern Appalachian (south-eastern United States) rivers and streams and expands the distribution of M. cerebralis throughout western North Carolina and eastern Tennessee.