Whirling Disease in California: A Review of Its History, Distribution, and Impacts, 1965–1997

Abstract

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.