Migration Route of Invasive Juvenile Philonema oncorhynchi (Nematoda: Philometridae) in Young Salmon

Abstract

The infective juveniles of the dracunculoid nematode Philonema oncorhynchi were known to reach their site of development in the wall of the swim bladder of salmon within 18 hr. To establish the route which they follow, a small, parasite-free, fingerling sockeye salmon was fed copepods containing the infective third stage of the worm. Seventeen hours later the fish was fixed, decalcified, and sectioned at 8–10 μ. The distribution of the invasive juvenile worms in the serial transverse sections indicates that they penetrate the gut wall, mainly in the region of the pyloric caeca. The majority of juveniles reach the swim bladder by moving through the coelomic cavity; a smaller number appears to cross in the mesentery and associated tissues such as the pancreas. No evidence was found suggesting that the vascular system is used as a pathway.