A chronic mortality in subadult Atlantic salmon Salmo salar reared in California was caused by systemic infections with a Dermocystidium-like species. Parasitic cells occurred primarily at the periphery of granulomas in the kidney, liver, spleen and testes. Actively replicating vegetative stages 2 to 7 pm in diameter were also found withln melanomacrophages of the kidney. These stages contained a dense granular cytoplasm with osmiophilic inclusions, a nucleus with an indishnct nucleolus and were surrounded by thick, Periodic-Acld-Schiff-positive cell walls. The parasite induces a similar disease and has structural affinities to Dern~ocystidium spp. described as causes of systemic infections in salmonids in Europe.
Myxobolus cerebralis, the causative agent of whirling disease, has become widely established in wild California salmonid populations since its initial discovery in Monterey County in 1965. Most significant is the occurrence of the parasite in the ''blue ribbon'' trout waters of the Owens Valley basin of the eastern Sierra. From the Lahontan basin on the north to the Owens Valley basin 320 km to the south, the parasite has become well established. In spite of the presence of the parasite, streams of the eastern Sierra are considered by many to support high quality trout populations, attracting thousands of anglers annually to the region. Empirical observations suggest that fish populations are healthy in the Owens Valley drainage and in the M. cerebralis-positive waters of the Lahontan and Pacific drainages. These observations are supported by population data comparing populations of rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta on Sagehen Creek and the lower Truckee River in the Lahontan basin, and rainbow trout populations in M. cerebralis-positive and -negative sections of the Carmel River on the central California coast near Monterey. The chronological appearance and distribution of M. cerebralis strongly implicates dispersal of live or processed state and commercially produced fish as a major factor in the spread of the parasite in California. Infected anadromous stocks have not appeared to spread detectable levels of M. cerebralis into numerous coastal waters and waters entering directly into San Francisco Bay. A severe epizootic of M. cerebralis at the Mt. Whitney State Fish Hatchery in the spring of 1995 confirmed the virulent potential of M. cerebralis in California. Spores of M. cerebralis can no longer be detected in wild populations at three locations since elimination of the source of infection in those waters.
A microsporidian parasite, Microsporidium rhabdophilia n. sp. is described from rodlet cells of salmonid flshes. Chinook salmon, Oncorhynchus tshawytscha, coho salmon, Oncorhynchus kisutch, steelhead rainbow trout, Salmo gairdnerii gairdnerii, and various strains of domesticated rainbow trout were found infected. In living wet mount phase-contrast observations of various tissues, 16 mature spores could be seen infecting the nucleus of rodlet cells. The number of spores infecting each nucleus generally numbered 16 and spores were not found in any cell or organelle other than the nucleus ofthe rodlet cell. All observations were limited to California hatchery-reared fish.
Three myxosporeans were encountered in the cranial tissues of a California population of rainbow trout Oncorhynchus mykiss examined for the presence of Myxobolus cerebralis, the causative agent of whirling disease. Typical spores of M. cerebralis and a previously undescribed species ofMyxobolus were found in the cranial tissues prepared by the pepsin HCl-trypsin digestion method. Henneguya zschokkei was also detected in digest preparations of cranial tissues, but was more numerous when branchial cartilage was included in the preparations. Microscopic examinations of tissues of individual rainbow trout showed occasional infections with both myxobolid species. Myxobolus cerebralis trophozoites and spores were found in the cranial and gill cartilage, and Myxobolus sp. was found in the brain and spinal cord. Henneguya zschokkei was also found within granulomas in the connective tissues below the gill arch. Both M. cerebralis and H. zschokkei were associated with a chronic inflammatory response in their respective tissues. In contrast, the Myxobolus sp. spores were found in pockets within the nervous tissues with no detectable host response. The spore measurements, calculated from fresh digests of infected tissues for the three myxosporeans (N -20), for length x width x thickness in micrometers (SD) were 11.7 (0.6) without tails and 42.6 (5.2) with tails x 7.7 (0.8) x 7.0 (0.1) for H. zschokkei, 9.9 (0.4) x 8.4 (0.1) x 6.5 (0.3) for M. cerebralis, and 12.7 (0.7) x 10.5 (1.0) x 9.5 (0.8) for Myxobolus sp. Examined under scanning electron microscopy, the latter two species were morphologically similar although distinctive in size.
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