Bovine Internal Parasites Study Guide

Overview and Clinical Importance

Internal parasitism represents one of the most economically significant disease complexes affecting cattle worldwide. Gastrointestinal nematodes (GIN), liver flukes, and lungworms cause billions of dollars in annual losses through reduced weight gains, decreased milk production, compromised feed conversion efficiency, and death. Understanding parasite life cycles, pathophysiology, clinical presentations, and treatment strategies is essential for the NAVLE and clinical practice.

Cattle can be infected by three major helminth classes: nematodes (roundworms), trematodes (flukes), and cestodes (tapeworms). Roundworms are considered the most economically devastating, with Ostertagia ostertagi recognized as the single most important nematode parasite of cattle in North America.

Gastrointestinal Nematodes

Gastrointestinal nematodes follow a direct life cycle without intermediate hosts. Adult worms in the GI tract produce eggs that pass in feces. Under favorable conditions (warmth, moisture), eggs hatch and develop through three larval stages (L1, L2, L3). The L3 larvae are infective and are ingested with herbage. Larvae then develop to L4 and adults in the GI tract.