Skeletal Abnormalities Associated with Parasites in Fishes from the Willamette Valley

A high prevalence of skeletal deformities has been observed in various fishes from the Willamette River for many years (see Markle, D. F. et al. 2002. Northwest Naturalist 83: 7-14). Surveys have consistently shown that precaudal skeletal lesions are more prevalent near Portland (i.e., Newberg Pool), leading to the hypothesis that toxicant exposure may be the cause. The Oregon Watershed Enhancement Board has funded a project at OSU (L. Curtis, PI; K.A. Anderson, J.J. Jenkins, M.L. Kent and D.F. Markle, Co-PIs) to investigate the etiology of these lesions (see http://oregonstate.edu/dept/ncs/newsarch/2003/Jan03/newberg.htm)

Examination of many affected chiselmouth chub, northern pike minnow and peamouth chub collected from Newberg Pool using histology and in whole mounts of cleared fish showed that most lesions are associated with infections by a metacercariae of a digenean trematode, which we identified as Apophallus sp., probably A. donicus. We have also recreated the condition in the laboratory by infecting parasite-free (lab-reared) fathead minnows to the cercariae of the parasite collected from snails from Newberg Pool. Myxozoans (Mxyobolus sp.) were also found associated with the lesions, but to a lesser extent than the worm.

Our investigation, therefore, indicates that the metacercaria is at least partially responsible for the high prevalence of skeletal deformities that have been seen for many years in cyprinid fishes from certain regions of the Willamette River. A question remains: given that that the parasite is probably a major cause of skeletal lesions in these fish, why are the lesions more prevalent near Portland?.

PHOTOS- Whole mount, cleared fish, histo, experimental infection

Metacercariae of Apophallus associated with spinal curvature (white arrow), and near anal fin (black arrow). Chiselmouth chub. Whole mount, cleared and stained with alcian blue and alizarin red S.

Histological section showing two metacercariae associated with lesions in the vertebrae of a northern pike minnow.

Two metacercariae of Apophallus in experimentally infected fathead minnow.

A myxozoan similar to Myxobolus cyprini was also associated with some skeletal lesions in northern pike minnow.