Abstract

Whirling disease of salmonid fish is caused by the myxosporean parasite, Myxobolus cerebralis. Clinical manifestations of whirling disease include skeletal deformities, aberrant behavior such as "tail-chasing", and hyperpigmentation (blacktail). Histologically, whirling disease is characterized by different stages of M. cerebralis spores associated with degenerative and inflammatory changes throughout the central nervous system and in the cranial cartilaginous tissue. Little is currently known of potential physiological effects, including host bioenergetics and immune responses, of whirling disease in salmonids.

Eighty fingerling Erwin strain rainbow trout (Oncorhynchus mykiss) were exposed to triactinomyxon spores (infective stage) of M. cerebralis at 9 x 10³ spores/fish, and a second control group of eighty fingerlings was similarly sham-exposed. Fish were maintained in flow-through tanks with running spring water and were fed a commercial pelleted trout diet at 2.5% body weight/day for the duration of the experiment. Fish were randomly removed from control and treatment tanks at intervals from 2 hours to 24 weeks post-exposure for histological sampling. At 24 weeks post-exposure, all remaining fish in each of the two groups were sampled; total body weight (g) and length (mm) were determined and heparinized blood samples were collected. Hematological parameters including hematocrit, plasma protein concentration, total leukocyte count, and differential leukocyte counts were determined for all fish.

The presence of mature M. cerebralis spores within the cranial tissues of all fish sampled from the treatment group at 24 weeks post-exposure was confirmed histologically. Histological specimens showed other changes associated with mature and immature spores in tissues, including degenerative and inflammatory lesions. Both total body weight and total length were significantly lower in the treatment group (p<0.05), representing a potential effect of M. cerebralis infection on growth of the host. Decreased growth of fish with whirling disease is most likely due to shifts in bioenergetics including decreased caloric intake and increased caloric expenditure. It is also possible that host bioenergetics could be affected by the degenerative and inflammatory changes in cranial tissues impeding function of neuroendocrine pathways. Of the hematological parameters examined, only total leukocyte count and small lymphocyte count were different in infected compared to control fish (p<0.05). Both types of blood counts were lower in the infected fish, and small lymphocytes consistently represented a high percentage of the total leukocyte count. Although the underlying nature of the lymphopenia is not known (i.e., stress leukogram or direct response to the pathogen), it does suggest a leukocytic response associated with M. cerebralis infection in rainbow trout.