Myxobolus neurophilus: morphologic, histopathologic and molecular characterization.

Archived tissues from affected yellow perch Perca flavescens, as well as fresh submissions of juvenile yellow perch, walleye, fathead minnows, golden shiners and smallmouth bass cultured in the same pond or from a shared water source were examined for the presence of Myxobolus neurophilus. Archived tissues were sectioned and stained with hematoxylin and eosin or with Giemsa, revealing myxozoan spores consistent with M. neurophilus. The myxospores were found beneath the ependymal lining of the central canal of the brain or free within the stratum periventriculare, with minimal or no inflammation. Unstained and stained (Wright Giemsa or Lugol's iodine) touch impressions of the brains from fresh submissions of all 5 fish species revealed similar myxozoan spores only in the brains of yellow perch. These were Giemsa-positive, with no iodinophilous vacuoles evident. Portions of the affected brains were fixed in neutral buffered 10% formalin and sectioned for histology. Pseudocysts containing myxospores were only evident in sections of the brains and spinal cords of yellow perch. Mild mononuclear meningoencephalitis was present when myxospores appeared outside of the pseudocysts. Brains fixed in 5% gluteraldehyde for scanning electron microscopic examination revealed pyriform myxospores with a smooth capsular surface. Sequencing and phylogenetic analysis of the 18S small subunit ribosomal DNA gene placed the organism within the family Myxobolidae, with no direct matches to sequences available via GenBank. Aquatic annelids from sediment obtained from the affected pond were negative for actinospores.