Myxobolus Cerebralis - Pathology

Pathology

M. cerebralis infections have been reported from a wide range of salmonid species: eight species of "Atlantic" salmonids, Salmo; four species of "Pacific" salmonids, Oncorhynchus; four species of char, Salvelinus; the grayling, Thymallus thymallus; and the huchen, Hucho hucho. M. cerebralis causes damage to its fish hosts through attachment of triactinomyxon spores and the migrations of various stages through tissues and along nerves, as well as by digesting cartilage. The fish's tail may darken, but aside from lesions on cartilage, internal organs generally appear healthy. Other symptoms include skeletal deformities and "whirling" behavior (tail-chasing) in young fish, which was thought to have been caused by a loss of equilibrium, but is actually caused by damage to the spinal cord and lower brain stem. Experiments have shown that fish can kill Myxobolus in their skin (possibly using antibodies), but that the fish do not attack the parasites once they have migrated to the central nervous system. This response varies from species to species.

In T. tubifex, the release of triactinomyxon spores from the intestinal wall damages the worm’s mucosa; this may happen thousands of times in a single worm, and is believed to impair nutrient absorption. Also, fish infected with worms have lower body mass and may be discolored. Spores are released from the worm almost exclusively when the temperature is between 10°C and 15°C, so fish in warmer or cooler waters are less likely to be infected, and infection rates vary seasonally.