Myxobolus cerebralis is the myxozoan parasite responsible for causing whirling disease in salmonid fish. Although the parasite was first described nearly 100 yr ago, it received relatively little attention until the discovery of its 2-host life cycle in the mid 1980s. This was the first, complete, myxozoan life cycle to be described, and it was greeted with some skepticism because it united 2 stages of M. cerebralis that were previously classified in 2 separate taxa. In the last decade, there has been a renewed interest in this parasite because whirling disease has been implicated in the decline of wild trout populations in several western states in the United States. Subsequent research efforts have dramatically increased the understanding of the biology of M. cerebralis and the numerous factors that affect the severity of whirling disease in salmonid hosts. These efforts also have provided a great deal of new information concerning interactions between M. cerebralis and its aquatic oligochaete host Tubifex tubifex. This review examines the current state of M. cerebralis in relation to 3 categories: the life cycle, the salmonid hosts, and the oligochaete host.
The objective of this study was to quantify and determine the periodicity in the release of the triactinomyxon (TAM) stage of Myxobolus cerebralis, the causative agent of salmonid whirling disease, by its aquatic oligochaete host Tubifex tubifex. For this, 24 individual T. tubifex (infected as a group at 15 C) were examined daily for the release of M. cerebralis TAMs, and the number of waterborne TAMs released by each worm was quantified. The duration of the infection in these worms was also monitored using a polymerase chain reaction (PCR) diagnostic test. TAMs were first released 74 days postexposure (PE) and continued to be released until 132 days PE. During this period, each worm released on average, 1.5 x 10(3) waterborne TAMs 12 times; however, no pattern or periodicity was noted. The results of the PCR diagnostic tests conducted at 5, 7, 9, and 15 mo PE were positive, and the persistent infection was confirmed at 606 days PE (approximately 20 mo) when the remaining worms began releasing TAMs again. Similar results were observed in naturally infected T. tubifex, indicating that these worms remain infected for the duration of their natural lifespan and are capable of shedding viable TAMs, in temporally separate periods. These findings open the possibility of a seasonal periodicity in TAM release by T. tubifex.
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