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.

Ostertagia ostertagi (Brown Stomach Worm)

Ostertagia ostertagi is the most important and most pathogenic nematode of cattle in temperate North America. Adults (6-9 mm) reside in the abomasum. The prepatent period is approximately 3 weeks. Ingested L3 larvae enter abomasal gastric glands, undergo two molts, and emerge as adults after 17-21 days.

Type I vs Type II Ostertagiasis

Pathogenesis

Emergence of larvae from gastric glands causes extensive damage leading to: (1) Chronic gastritis with thickened, hyperplastic, non-functional mucosa; (2) Elevated abomasal pH (normal approximately 2.0 rising to near 7.0); (3) Decreased pepsin and HCl production; (4) Protein-losing enteropathy with hypoalbuminemia.

The classic gross lesion is the "Moroccan leather" or "crazy paving" appearance of the abomasal mucosa, characterized by raised hyperplastic nodules (2-3 mm) with central orifices representing parasitized gastric glands.

Haemonchus placei (Barber's Pole Worm)

Haemonchus placei is the primary cattle species (H. contortus predominantly affects small ruminants but can cross-infect). This is an extremely hematophagous (blood-feeding) parasite residing in the abomasum. Adult females measure 20-30 mm with the characteristic "barber's pole" appearance due to white ovaries coiled around blood-filled intestine. The prepatent period is 18-21 days.

Clinical Signs and Pathogenesis

Each worm removes approximately 0.05 mL blood/day. Heavy infections (greater than 5,000 worms) can cause loss of 250 mL blood daily. Clinical signs include: severe anemia (pale mucous membranes), bottle jaw (submandibular edema from hypoproteinemia), weakness, unthriftiness, and sudden death. Importantly, diarrhea is NOT typical unless coinfected with other trichostrongyles.

Cooperia Species (Small Intestinal Worms)

Cooperia oncophora, C. punctata, and C. pectinata are common small intestinal parasites. C. oncophora is now the most prevalent parasite in US cow-calf operations. Adults are small (5-8 mm), red, coiled worms found in the first 3-6 meters of duodenum. The prepatent period is approximately 3 weeks.

Cooperia species are NOT hematophagous; however, C. punctata and C. pectinata cause severe enteritis with anorexia, emaciation, and duodenal petechiae. C. oncophora produces milder disease with poor productivity and weight loss. Significant infections are typically restricted to animals less than 3 years old.

Liver Flukes (Trematodes)

Fasciola hepatica (Common Liver Fluke)

Fasciola hepatica is the most important liver fluke worldwide. Adults are large (30 x 13 mm), leaf-shaped, and reside in bile ducts. The life cycle requires an aquatic snail intermediate host (Galba truncatula in Europe, Lymnaea species in North America). The prepatent period is 10-12 weeks.

Life Cycle

Eggs pass in feces and embryonate in freshwater over approximately 2 weeks. Miracidia hatch and penetrate snails, developing through sporocyst and redia stages. Cercariae emerge from snails, encyst as metacercariae on aquatic vegetation. Cattle ingest metacercariae while grazing. Immature flukes excyst in the duodenum, penetrate the intestinal wall, cross the peritoneal cavity, and penetrate the liver capsule. They migrate through liver parenchyma for 6-8 weeks, causing traumatic hepatitis, before entering bile ducts where they mature.

Pathogenesis and Clinical Signs

In cattle, chronic fascioliasis is most common (acute disease is more typical in sheep). Clinical signs are often subtle: reduced weight gains, poor feed conversion, decreased milk production (5-15% reduction reported), and reproductive losses (increased services per conception, extended calving intervals). Bottle jaw may develop. Liver damage leads to condemnation at slaughter (over 25% of cattle livers condemned in some regions).

Diagnosis

Fecal sedimentation (NOT flotation) is used for fluke egg detection. Eggs are large (130-150 x 60-90 micrometers), operculated, and yellowish-brown. However, sensitivity is limited because: (1) prepatent period is long, (2) egg shedding is intermittent, (3) negative results do not rule out immature fluke infections. ELISA tests for antibody detection (individual or bulk tank milk) are available in some countries.

Lungworms

Dictyocaulus viviparus (Bovine Lungworm)

Dictyocaulus viviparus causes parasitic bronchitis ("husk" or "hoose"). Adults are white, thread-like worms (up to 8 cm females, 5.5 cm males) residing in the trachea and bronchi. The life cycle is direct. Unlike GI nematodes, L1 larvae (not eggs) are passed in feces. Development to infective L3 takes only 5 days under favorable conditions. The prepatent period is 3-4 weeks.

Pathogenesis

Ingested L3 larvae penetrate the intestinal wall, migrate to mesenteric lymph nodes, molt to L4, and reach the lungs via lymphatics and blood. In the lungs, larvae cause bronchiolitis and bronchitis. Adult worms in airways cause mucus production, bronchial obstruction, and secondary bacterial pneumonia. Gross lesions include atelectasis, emphysema, and consolidation of caudal lung lobes.

Clinical Signs

First season grazers (calves) are most susceptible. Clinical signs include: persistent dry, non-productive "husky" cough (initially after exercise, then at rest), tachypnea, dyspnea, mouth breathing, extended head and neck, weight loss, and reduced milk yield. Heavy infections can cause death within 1-2 days of onset. Note: Only 200-300 worms can cause clinical disease (compare to over 40,000 Ostertagia needed).

Diagnosis

The Baermann technique is used to detect L1 larvae in feces. Fresh samples are essential as larvae die rapidly at room temperature (80% within 48 hours). Important limitations: larvae are not detectable until approximately 25 days post-infection (patent phase), and severe disease can occur BEFORE patent infection. A blood ELISA for antibody detection is available in some countries.

Diagnostic Methods Summary

Anthelmintic Therapy

Three major anthelmintic classes are used in cattle: Benzimidazoles (white drenches), Macrocyclic Lactones (avermectins/milbemycins), and Imidazothiazoles (levamisole). Understanding spectrum of activity and withdrawal times is essential.

Strategic Deworming Programs

Key timing recommendations: (1) At housing (fall): Treat to eliminate hypobiotic larvae and reduce spring pasture contamination; (2) At turnout (spring): Treat if pastures were contaminated the previous year; (3) Periparturient cows: Treat at or near calving when immune suppression increases susceptibility.