Myxobolus insidiosus sp. n., a Myxosporidian from the Musculature of Oncorhynchus tshawytscha (Walbaum)

Abstract

Spring chinook salmon fingerlings from the Oregon State Fish Commission hatcheries on the South Santiam River in Linn County and the McKenzie River in Lane County were found to be infected with Myxobolus insidiosus sp. n. Cysts and vegetative stages were found in the body musculature. Free spores were found in the gill capillaries, kidney, liver, and spleen. Some infected fish exhibited slightly swollen circular areas upon the body, which are devoid of scales, as well as frayed fins and tails. In August 1959 the senior author observed a rather serious deterioration of the general health of spring chinook salmon fingerlings being reared at the Oregon State Fish Commission hatchery on the South Santiam River in Linn County. Histological examination of tissues from these fish revealed the presence of a previously undescribed myxosporidian of the genus Myxobolus. This parasite was also observed in spring chinook salmon fingerlings at the O.S.F.C. hatchery on the McKenzie River in Lane County, Oregon. MATERIALS AND METHODS Spring chinook salmon fingerlings weighing about 25 fish to the pound and between 99 and 150 mm in length from the O.S.F.C. hatcheries on the South Santiam River in Linn County, and the Willamette and McKenzie Rivers in Lane County, were examined both grossly and microscopically for evidence of myxosporidian parasitization. Tissues that were to be sectioned were fixed in Bouin's fixative, washed, and stored in 70% alcohol. Ten-micron paraffin sections were cut and stained with Heidenhain's iron hematoxylin and eosin. Smear preparations of infected musculature were fixed in Schaudinn's fluid and stained with Giemsa stain. Similar preparations fixed in alcohol-formol were stained with Bauer's modified Feulgen reaction for glycogen. Fresh spores were subjected Received for publication 1 January 1963. * Published with the approval of the Oregon State University Monographs Committee. Research Paper No. 456, Department of Zoology, School of Science. t Present address: Central Oregon College, Department of Biology, Bend. to Lugol's iodine for further demonstration of the iodinophilous vacuole. Extrusion of the polar filaments was under the influence of NaOH. Airdried smears of infected muscle tissue stained with Ziehl-Neelson's carbol fuchsin and Loffler's methylene blue provided permanent preparations of spores with extruded polar filaments. -Measurements were made of fresh spores as well as Giemsa-stained material. Between 50 and 100 spores were measured for each biometric entry. Photomicrographs were taken with a 35mm Leica camera mounted on a Leitz microscope. Drawings were made with the aid of a Leitz drawing lens. All measurements are in microns unless otherwise stated. Vegetative form: A few trophozoites were observed (Fig. 2). They are oval in cross section with an average size of 33 by 70. Many nuclei and sporonts are observed within the trophozoite. Trophozoite nuclei as well as some sporont nuclei contain a prominent but eccentrically placed nucleolus or endosome. Trophozoites with actively dividing sporont nuclei also contain structures presumably nuclear, which are circular to irregular in outline and have a great affinity for hematoxylin. The cysts are spindle shaped (Fig. 3) with aver-