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The histopathology present in Etheostoma caeruleum naturally infected with the acanthocephalans, Acanthocephalus dirus and Pomphorhynchus bulbocolli, was studied and described. The major difference in parasite-induced histopathology was related to the shallow penetration of A. dirus proboscides in the intestinal wall as compared to the penetration of P. bulbocolli. The host responded to the presence of the parasites with connective tissue hyperplasia and with an infiltration of leukocytes, predominantly lymphocytes and eosinophilic granulocytes. The response was greater to P. bulbocolli than to A. dirus. The complete penetration of the intestinal wall by the proboscis, bulb, and neck of P. bulbocolli elicited an intense host response. This resulted in the formation of a tunnel around the neck and capsule around the bulb and proboscis of the parasite. Damage to the epithelial lining of the intestine occurred in areas of contact with the trunk of the parasites. Concurrent infections were common, but there did not appear to be a synergistic effect as a result of the presence of both parasites.

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