Myxobolus cerebralis possesses unique phenotypic and genotypic characteristics when compared with other histozoic parasites from the phylum Myxozoa. The parasite infects the cartilage and thereby induces a serious and potentially lethal disease in salmonid fish. Comparisons of the small subunit ribosomal DNA (ssu rDNA) sequences of M. cerebralis to other myxozoans demonstrate that the parasite has evolved separately from other Myxobolus spp. that may appear in cartilage or nervous tissues of the fish host. Myxobolus cerebralis has a complex life cycle involving two hosts and numerous developmental stages that may divide by mitosis, endogeny, or plasmotomy, and, at one stage, by meiosis. In the salmonid host, the parasite undergoes extensive migration from initial sites of attachment to the epidermis, through the nervous system, to reach cartilage, the site where sporogenesis occurs. During this migration, parasite numbers may increase by replication. Sporogenesis is initiated by autogamy, a process typical of pansporoblastic myxosporean development that involves the union of the one cell (pericyte) with another (sporogonic). Following this union, the sporogonic cell will give rise to all subsequent cells that differentiate into the lenticular shaped spore with a diameter of approximately 10 μm. This spore or myxospore is an environmentally resistant stage characterized by two hardened valves surrounding two polar capsules with coiled filaments and a binucleate sporoplasm cell. In the fish, these spores are found only in cartilage where they reside until released from fish that die or are consumed by other fish or fish-eating animals (e.g., birds). Spores reaching the aquatic sediments can be ingested and hatch in susceptible oligochaete hosts. The released sporoplasm invades and then resides between cells of the intestinal mucosa. In contrast to the parasite in the fish host, the parasite in the oligochaete undergoes the entire developmental cycle in this location. This developmental cycle involves merogony, gametogamy or the formation of haploid gametes, and sporogony. The actinosporean spores, formed at the culmination of this development, are released into the lumen of the intestine, prior to discharging into the aquatic environment. The mechanisms underlying the complex development of M. cerebralis, and its interactions with both hosts, are poorly understood. Recent advances, however, are providing insights into the factors that mediate certain phases of the infection. In this review, we consider known and recently obtained information on the taxonomy, development, and life cycle of the parasite.
|Original language||English (US)|
|Number of pages||9|
|Journal||American Fisheries Society Symposium|
|State||Published - Dec 1 2002|
ASJC Scopus subject areas
- Aquatic Science