COINFECTION OF BAY SCALLOPS ARGOPECTEN-IRRADIANS WITH PERKINSUS-KARLSSONI (APICOMPLEXA, PERKINSEA) AND AN UNIDENTIFIED COCCIDIAN PARASITE

A comparison of the coinfection of bay scallops with Perkinsus karlsso ni and an as yet unidentified coccidian is described. P. karlssoni occ urred in a range of host tissues including mantle, digestive gland, in testine, gill and gonad. Host tissue response elicited against P karls soni ranged from focal encapsulation by haemocytes to formation of abs cesses filled with ceroid-containing cells. The coccidian was restrict ed to the kidney in light infections and occurred within the renal cel ls and the lumen of the renal tubules. In heavy infections the coccidi an also occurred in the mantle, digestive gland, intestine, gill and g onad. There was no apparent host response elicited against the coccidi an parasite regardless of the tissue in which it occurred or whether t he parasite was intra- or intercellular. In dual infections, dense hae mocytic encapsulation arid infiltration by ceroid-containing cells mas ked the causative organism. Llltrastructural examination of lesions fr om concurrent infections revealed the presence of both parasites. Indi vidual P karlssoni infections were not found without a host response a nd the coccidian occurred with no apparent host response. It was there fore dssumed that the response elicited in dual infections was caused predominantly by P karlssoni.