Internal Parasites

Part of Veterinary Medicine

Understanding, identifying, and managing the internal worm and fluke burdens that reduce livestock productivity and cause death.

Why This Matters

Internal parasites are a nearly universal burden on pastured livestock. Every herd of cattle, sheep, goats, or horses grazing on pasture carries a worm burden. The question is not whether they have worms, but whether that burden is controlled within productive limits. Uncontrolled, internal parasites cause chronic wasting, anemia, hypoproteinemia (protein deficiency), reduced milk and meat production, and death — particularly in young animals, pregnant females, and animals under nutritional stress.

The most important conceptual shift for sustainable parasite management is understanding that zero worms is not the goal and is not achievable without destroying the productivity of your pastures. The goal is maintaining worm burdens below the threshold where they cause significant production losses, while avoiding the aggressive over-treatment that drives anthelmintic resistance. Resistance to all major classes of pharmaceutical dewormers is now widespread globally. Where pharmaceutical dewormers are available, they must be used strategically. Where they are unavailable, alternative approaches — managed grazing, botanical anthelmintics, nutritional support, genetic selection — become the primary tools.

Major Parasite Species by Host

Understanding which parasites affect which species guides appropriate management:

Small ruminants (sheep, goats):

• Haemonchus contortus (barber pole worm): The most important parasite worldwide in warm climates. Blood-sucking abomasal worm; a single adult female produces thousands of eggs daily. Heavy infestations cause life-threatening anemia. The FAMACHA system (conjunctiva color scoring) was developed specifically for Haemonchus.
• Trichostrongylus colubriformis: Damages the small intestinal mucosa, causing diarrhea and protein loss.
• Teladorsagia circumcincta: Important in cooler climates; abomasal worm causing weight loss.
• Nematodirus battus: Affects primarily young lambs; causes explosive watery diarrhea, often fatal.

Cattle:

• Ostertagia ostertagi: The most important cattle roundworm in temperate zones; damages abomasal glands, causes severe protein loss and diarrhea.
• Cooperia species: Small intestinal worms; reduce growth rates.
• Fasciola hepatica (liver fluke): Not a roundworm but a flatworm infesting the bile ducts of the liver. Causes significant liver damage, anemia, submandibular edema (“bottle jaw”), and death. Requires specific flukicide drugs; most roundworm dewormers have no activity against flukes.

Horses:

• Strongylus vulgaris (large strongyle): Historically the most pathogenic horse parasite; larvae migrate through the cranial mesenteric artery, causing colic.
• Cyathostomins (small strongyles): Now the dominant concern; massive larval emergence from the gut wall in spring causes severe diarrhea and weight loss.
• Parascaris equorum: Large roundworm important in foals; causes impaction and respiratory disease.
• Oxyuris equi (pinworm): Causes intense perianal itching; significant welfare issue.
• Gasterophilus (bots): Fly larvae that attach to the stomach wall; more irritant than life-threatening.

Pigs:

• Ascaris suum: Large roundworm; larvae migrate through liver and lungs, causing respiratory signs (“thumps”) in young pigs. Adults reduce growth efficiency.
• Oesophagostomum: Nodular worm; larvae form nodules in intestinal wall.
• Trichuris suis (whipworm): Cecal parasitism; causes bloody diarrhea.

Life Cycles and Management Implications

Understanding parasite life cycles reveals where management interventions can be most effective.

Basic direct life cycle (most roundworms): Adult worm in host gut → eggs passed in feces → eggs hatch to L1 larvae on pasture → develop to L2 → L3 (infective stage, survives on pasture for weeks to months) → ingested by grazing host → develop to L4 and adult in host gut. The infective L3 larvae concentrate near the soil surface and on plant material at grass height — animals grazing close to the ground ingest the highest numbers.

Management implications:

• Larvae cannot survive indefinitely; pasture rested for 6–8 weeks has significantly lower larval burden than continuously grazed pasture.
• Young animals are exposed to far higher burdens on contaminated pasture than if moved to clean pasture.
• Peak contamination follows peak egg shedding — typically in spring and early summer when conditions favor larval development.
• Larvae cannot survive freezing temperatures; winter reduces pasture contamination in temperate zones.

Liver fluke (indirect life cycle): Adult fluke in liver → eggs in feces → miracidium → requires specific mud snail intermediate host (Galba truncatula) → snail sheds cercariae → metacercariae encyst on pasture → ingested. Control requires either treating the host or controlling the snail habitat (draining wet areas, lime application to wet ground). This is why standard dewormers don’t work against flukes.

FAMACHA Scoring System

The FAMACHA system provides a practical, low-technology method for identifying which sheep and goats are most severely affected by Haemonchus contortus and need targeted treatment.

Lower eyelid mucous membrane color is assessed on a 1–5 scale:

• 1: Red/pink — healthy, no anemia
• 2: Pink — acceptable, no treatment needed
• 3: Pale pink — borderline, may treat based on risk factors
• 4: Pale — treat
• 5: White — severe anemia, immediate treatment required

Score each animal monthly at minimum, more frequently during peak risk seasons. Treat only animals scoring 3, 4, or 5. Record which animals score poorly repeatedly — these are candidates for culling or at minimum should not be selected for breeding.

FAMACHA training requires seeing certified laminated color charts to calibrate eye assessment. If you have access to these materials before grid-down, preserve them.

Fecal Egg Counting

Fecal egg counting (FEC) quantifies parasite egg output, allowing more precise treatment targeting than FAMACHA alone. It requires only a microscope and basic laboratory materials.

Modified McMaster technique (simplified):

1. Weigh 3 grams of fresh feces
2. Mix with 45 ml saturated salt solution (salt dissolved in water until no more dissolves)
3. Strain through fine mesh or cheesecloth
4. Fill a standard McMaster counting chamber (if available) or improvise a counting slide
5. Count eggs under low magnification (40-100x)
6. Calculate eggs per gram (EPG) using the dilution factor

Interpretation: The threshold for treatment varies by species and parasite, but as a guide for small ruminants: under 500 EPG: low burden; 500–1000 EPG: moderate; over 1000 EPG: high, treat. Note that FAMACHA is more important than FEC for Haemonchus because anemia, not egg count, is the life-threatening outcome.

Reducing Dependence on Chemical Dewormers

Whether pharmaceutical dewormers are available or not, building a parasite management program that minimizes dependence on them is essential:

Managed rotational grazing: Move animals frequently, rest pastures for 6–8 weeks. “Clean” pastures (ungrazed by the same species for at least one season) dramatically reduce infective larval burden.

Mixed species grazing: Cattle and sheep/goats share relatively few parasite species. Alternating pasture use between species breaks parasite life cycles effectively.

Nutritional support: Animals with adequate protein and energy intake have significantly better parasite immune responses. Protein supplementation of thin animals during high-risk periods substantially reduces clinical disease.

Breeding for resistance: The genetic component of parasite resistance is substantial and heritable. Over time, selecting from animals that maintain good condition and FAMACHA scores without treatment creates a more resistant herd.

Botanical supplements: See the dedicated herbal dewormers article for evidence-based plant-based options.