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

Abstract

We used microscopy and molecular biology to provide the first documentation of infections of Myxobolus cerebralis (Myxozoa: Myxobolidae), the etiological agent of whirling disease, in trout (Salmonidae) from North Carolina (USA) river basins. A total of 1085 rainbow trout Oncorhynchus mykiss, 696 brown trout Salmo trutta, and 319 brook trout Salvelinus fontinalis from 43 localities across 9 river basins were screened. Myxospores were observed microscopically in pepsin-trypsin digested heads of rainbow and brown trout from the Watauga River Basin. Those infections were confirmed using the prescribed nested polymerase chain reaction (PCR; 18S rDNA), which also detected infections in rainbow, brown, and brook trout from the French Broad River Basin and the Yadkin Pee-Dee River Basin. Myxospores were 9.0-10.0 µm (mean ± SD = 9.6 ± 0.4; N = 119) long, 8.0-10.0 µm (8.8 ± 0.6; 104) wide, and 6.0-7.5 µm (6.9 ± 0.5; 15) thick and had polar capsules 4.0-6.0 µm (5.0 ± 0.5; 104) long, 2.5-3.5 µm (3.1 ± 0.3; 104) wide, and with 5 or 6 polar filament coils. Myxospores from these hosts and rivers were morphologically indistinguishable and molecularly identical, indicating conspecificity, and the resulting 18S rDNA and ITS-1 sequences derived from these myxospores were 99.5-100% and 99.3-99.8% similar, respectively, to published GenBank sequences ascribed to M. cerebralis. This report comprises the first taxonomic circumscription and molecular confirmation of M. cerebralis in the southeastern USA south of Virginia.

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

“…We detected M. cerebralis in 6 river basins (Catawba, French Broad, Hiwassee, New, Savannah and Upper Tennessee) in North Carolina and Tennessee including the first detection of M. cerebralis in Tennessee and positives from three sites that were negative (Linville River, North Fork Ivy Creek and Caney Fork) in Ruiz et al. (2017). As fish were pooled per AFS‐FHS Blue Book, prevalence cannot be determined; however, infections were more commonly detected in pellets sourced from rainbow (3.5%) and brown (3.6%) trout.…”

“…Myxobolus cerebralis positive sites presented in Ruiz et al. (2017) were limited to 2 river basins (French Broad and Yadkin Pee‐Dee river basins) in western North Carolina. We detected M. cerebralis in 6 river basins (Catawba, French Broad, Hiwassee, New, Savannah and Upper Tennessee) in North Carolina and Tennessee including the first detection of M. cerebralis in Tennessee and positives from three sites that were negative (Linville River, North Fork Ivy Creek and Caney Fork) in Ruiz et al.…”

“…(2017). Our myxospores, sourced from PTD pellets, were indistinguishable from those of Lom and Hoffman (1971) and Ruiz et al (2017) and measured 7–10 (8.4 ± 0.9; n = 20) long, 7–10 (8.85 ± 1.1; n = 20) wide 6–8 (6.8 ± 0.8; n = 5) thick, with a mucous envelope on the posterior margin of the spore; with two polar capsules 5–6 µm (5.4 ± 0.5; n = 40) long, 3–4 µm (3.5 ± 0.5; n = 40) wide, with 5–6 polar filament coils (Figures 6–9). This constitutes the first morphological identification of M. cerebralis in Tennessee.…”

“…Hoffman (1970) suggested this parasite is likely endemic to brown trout Salmo trutta Linnaeus, 1758 (Salmoniformes: Salmonidae), which are endemic to rivers in Europe, western Asian and north Africa and are resistant to developing clinical signs of whirling disease when infected with M. cerebralis . The translocation of salmonids, particularly the global dissemination of brook trout Salvelinus fontinalis (Mitchill, 1814) (Salmoniformes: Salmonidae), brown trout and rainbow trout, has led to M. cerebralis infecting a number of salmonid species and being documented in 26 countries and 24 states in the United States (Bartholomew & Reno, 2002; El‐Matbouli, Fischer‐ Scherl, & Hoffmann, 1992; Halliday, 1976; Ruiz et al., 2017; Sarker et al., 2015). Whirling disease is one of the most economically and ecologically devastating diseases of salmonids, harming both wild and cultured stocks, and is of particular concern for stocks of brook and rainbow trout, both of which have been demonstrated to be more susceptible to developing clinical signs of whirling disease than brown trout (Bartholomew & Reno, 2002; Halliday, 1976; Nehring & Walker, 1996; Thompson, Nehring, Bowden, & Wygant, 1999).…”

“…Whirling disease is one of the most economically and ecologically devastating diseases of salmonids, harming both wild and cultured stocks, and is of particular concern for stocks of brook and rainbow trout, both of which have been demonstrated to be more susceptible to developing clinical signs of whirling disease than brown trout (Bartholomew & Reno, 2002; Halliday, 1976; Nehring & Walker, 1996; Thompson, Nehring, Bowden, & Wygant, 1999). Despite being one of the most well‐studied parasites of freshwater fishes, M. cerebralis was not documented in the south‐eastern United States, south of Virginia, until 2015; however, wild salmonids still had not been diagnosed with whirling disease in the south‐eastern United States (Ruiz et al., 2017).…”