Newly Discovered Intermediate Hosts for an Eel Swimbladder Nematode,
an article summary by Brad Gorzek, BIOL 490 Fall 2014

Haenen & Van Banning (1990) investigated whether or not Anguillicola crassus infects seven fish species that tend to be prey of the European eel (Anguilla anguilla). Since 1985, scientists had noted that the eel swimbladder nematode was infecting nearly all of the European eel populations spread throughout the inland waters of the Netherlands. This parasitic nematode came to the west from Asia through trading, where it had originally fed on the Japanese Eel (Anguilla japonica). The infection leads to lesions in the swimbladder of the eel which cause inflammation, susceptibility to diseases, and sometimes death. Prior experiments showed that the intermediate hosts for the nematodes were copepods and crustaceans, but no evidence was found to suggest fish could be intermediate hosts as well. Haenen & Van Banning (1990) collected individuals if each of the seven prey-fish species with a net, dissected them, and examined them under a light microscope. The numbers of nematodes in each stage of development (there are five from egg to adult) were recorded, as well as the effects on the fish. The study showed that nematodes infect the swimbladder of Osmerus eperlanus, Gymnocephalus cernuus, Perca fluviatilis, Stizostedion lucioperca, and Gasterosteus aculeatus. They do not infect Rutilus rutilus or Abramis brama for reasons unknown. There was no evidence that the nematodes injured these fish species as they do in freshwater eels. These results suggest that A. anguilla may feed on these intermediate hosts and become infected themselves, though more research would be useful to confirm this new mode of infection.

Haenen O.L.M. & P. Van Banning. 1990. Detection of larvae of Anguillicola crassus (an eel swimbladder nematode) in freshwater fish species. Aquaculture 87: 103-109.