Abstract
Seasonal occurrence of Myxobolus artus infection in common carp Cyprinus carpio was monitored in a fry pond (Tokyo Metropolis) for one and a half year. Formation of pseudocysts in the skeletal muscle was detected from August, and the prevalence of infection reached a maximum of 10% in September. After maturation of pseudocysts, diseased fish released spores into the water at a maximum level of about 3×106 spores/ day/fish in October. Spore release decreased in late autumn and winter, increased in spring and exhausted in the following summer in 1-year old recovered fish. Mortality of diseased fish was negligible in September. However, chronic mortality with a typical sign of anemic gills was found during the spore releasing period. During a high level period of spore discharge, hematological indices (Ht, Hb, and RBC) in diseased fish significantly decreased, and the percentage of immature erythrocytes increased. Histopathological examination showed that numerous phagocytosed spores caused local occlusion and destruction of gill capillaries and exfoliation of gill epithelium, suggesting blood loss from the damaged gills. It was suggested that chronic hemorrhagic anemia was associated with spore discharge from carp heavily infected with M. artus.
