LIGHT AND ELECTRON-MICROSCOPIC STUDIES ON THE CHRONOLOGICAL DEVELOPMENT OF MYXOBOLUS-CEREBRALIS TO THE ACTINOSPOREAN STAGE IN TUBIFEX-TUBIFEX

Whirling disease caused by Myxobolus cerebralis has become the most wi dely known disease of salmonids in the 1990s. In the last 5 years we h ave studied many aspects regarding the host-pathogen relationship of t his parasite. The parasite's histozoic development causes significant damage to cartilage and induces CNS symptoms by pressure on the brain and spinal cord. Myxobolus cerebralis has a two-host life-cycle involv ing a salmonid fish and a tubificid oligochaete. Two different stages of sporogony occur, one in each host. Early developmental stages in th e fish can be found multiplying in the epidermis and peripheral and ce ntral nervous systems. The presporogenic stages then migrate to verteb ral and cranial cartilages, where the first sporogonic phase occurs. M ature M. cerebralis spores found in fish cartilage are infectious for T. tubifex when ingested by the oligochaete after destruction of the i nfected fish. In the gut lumen of the tubificid, the spores extrude th eir polar capsules and attach to the gut epithelium by polar filaments . The shell valves then open along the suture line and the sporoplasm penetrates between the gut epithelial cells. The binucleate sporoplasm multiplies by schizogony, producing many one-cell stages which begin gamogonic development. As a result of the multiplication process, the intercellular space of the epithelial cells in more than 10 neighbouri ng worm segments may become infected. At this time (60-90 days p.i.), pansporocysts with eight zygotes start the sporogonic phase. The final stage of this development is a pansporocyst containing eight folded t riactinomyxon spores. Shortly afterwards, the spores are liberated int o the gut lumen. The spores reach the water either by egestion or foll owing the death of the infected tubificids. Infected tubificids can re lease triactinomyxons for at least 1 year. The ultrastructure of all f our phases, schizogony, gametogony, gametogamy and sporogony, is demon strated and discussed. (C) 1998 Australian Society for Parasitology. P ublished by Elsevier Science Ltd.