The intramuscular phase of development of Kudoa thyrsites, the myxosporean associated with post-mortem myoliquefaction, or 'soft flesh syndrome', is described using histological preparations of the musculature of seawater netpen-reared Atlantic salmon Salmo salar. Hatchery-reared Atlantic salmon were naturally exposed to the infective stage whlle held in the expenmental seawater netpens of the Pacific Biological Station in Nanaimo, British Columbia, Canada. In fish exposed during the summer of 1995, K. thyrsites infections were first detected in the somatic musculature at 13 wk postexposure (p.e.) using only light microscopy. In the 1997 exposure, infections were first detected at 6 wk p.e. using a PCR test and at 9 wk p.e. using light microscopy. The earliest stage detected by histology was a small plasmodium containing 4 nuclei. No host response was observed that was directly related to the presence of intact plasmodia within muscle fibers. However, a response was associated with ruptured plasmodia, which was characterized by chronic, multlfocal inflammation between the muscle fibers.
Efficacy of the fumagillin analog TNP-470 forNucleospora salmonis and Loma salmonae infections in chinook salmon Oncorh ynch us tsha wytscha 'Department of Fisheries and Oceans, Pacific Biological Station, Nanaimo, British Columbia V9R 5K6, Canada *~e p a r t m e n t of Pathology, Albert Einstein College of Medicine, Bronx, New York 10461, USA ABSTRACT: Oral treatment with fumagillin is effective for controlling various inicrosporean and myxosporean infections in fish. We tested a synthetic analog of fumagillin, TNP-470 (Takeda Chemical Industries), for its efficacy against 2 microsporean pathogens of salmon: Loma salmonae and Nucleospora salmonis. Chinook salmon Oncorhynchus tsharvjrtscha were experimentally infected with elther L. salmonae (per OS) or N. salmonis (intrapentoneal, i.p , injection) and held in fresh water at 15'C. Flsh were then divided into 3 replicate groups. untreated or treated orally at 1.0 mg or at 0.1 mg drug k g ' flsh d-'. With L. salmonae, the high dose flsh had 0.32 xenomas mm-' of gill tissue compared to controls at 24.5 xenomas per mm2. With N. salmon~s infections, untreated fish exhibited 100'Yo infection, showed prominent clinical signs (e.g. renal swelling, anaemia), and high mortality. In contrast, fish treated at 1.0 mg kg-' showed no clinical signs, and 16% of those treated at 0.1 mg kg-' showed only m~l d gross pathological changes. With the treated groups, over 50i% of the fish exhibited extremely light infections, even with high dose treatments, but no mortalitles were attributed to N. salmonis infections. Uninfected fish treated at 1.0 mg drug kg-' fish d-' for 5 wk appeared clinically normal and showed no reduction in growth. However, about half of these fish exhibited atrophy of the renal interstitial hematopoietic tissue.
Ulcers in Atlantic menhaden Brevoortia tyrannus (Latrobe) (Clupeidae), observed along the USA east coast, have been attributed to diverse etiologies including bacterial, fungal and, recently, harmful algal blooms. To understand the early pathogenesis of these lesions, we examined juvenile Atlantic menhaden collected during their seasonal presence in Chesapeake Bay tributaries from April to October 1999 and from March to August 2000. We conducted histopathological examinations of young-of-the-year fish from the Pocomoke River tributary, which has a history of fish mortalities and high lesion prevalence. Kudoa clupeidae (Myxozoa: Myxosporea) spores were present in the muscles of fish collected in both years. Of the fish assessed by histology in April, 5 to 14% were infected, while in May 90 to 96% were infected. Infection rates remained high during the summer. Mature spores were primarily located within myomeres and caused little or no observable pathological changes. Ultrastructure showed spores with capsulogenic cells bearing filamentous projections, and a basal crescentic nucleus with mottled nucleoplasm containing cleaved, condensed chromatin. Also, a highly invasive plasmodial stage of a myxozoan was found in the lesions of juvenile Atlantic menhaden. The plasmodia were observed in fish collected between May and July, with the maximum occurrence in late June 1999 and late May 2000. Plasmodia penetrated and surrounded muscle bundles, causing grossly observable raised lesions in 73% of all fish infected with this invasive stage. Plasmodia were also detected in the visceral organs, branchial arches, and interocular muscles of some fish. Some of the invasive extrasporogonic plasmodial lesions were associated with ulcers and chronic inflammatory infiltrates. The plasmodial stage appeared to slough out of the tissue with subsequent evidence of wound healing. Ultrastructure showed plasmodia with an elaborate irregular surface, divided into distinct ectoplasm and endoplasm; the latter contained numerous spherical vegetative nuclei, secondary generative cells, and occasional cell doublets. Our ultrastructural studies indicate that the plasmodial organisms, which are important in the etiology of the skin lesions, are myxozoans, and they may represent early stages of K. clupeidae.
KEY WORDS: Atlantic menhaden · Brevoortia tyrannus · Kudoa clupeidae · Myxosporea · Plasmodia · LesionsResale or republication not permitted without written consent of the publisher
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