Lecture 12- Gastrointestinal Nematode Parasites of Domestic Production Animals (Sheep & Goats)

Why are goats particularly vulnerable to parasitic infections?

Goats evolved as browsers in arid and mountainous environments, where parasite burdens were low. They never developed strong immunity to worms like cattle and sheep.

How does the natural habitat of goats affect their parasite susceptibility?

Goats prefer browsing rather than grazing, avoiding worm-contaminated grass.

They evolved in dry, arid regions with low parasite burdens.

In humid subtropical climates, they lack natural resistance to heavy worm infestations.

Why is Haemonchus contortus a major concern for small ruminants?

It is a highly pathogenic blood-feeding parasite, causing severe anemia and hypoproteinemia, which can be fatal.

What is the life cycle of Haemonchus contortus?

Direct life cycle with prolific egg production (thousands per day).

Eggs passed in feces, larvae develop and hatch on pasture.

Animals become infected while grazing.

Prepatent period: ~ 21 days.

How do environmental factors affect Haemonchus contortus transmission?

Optimal transmission: Late Spring & Summer (warm & humid).

Poor development below 48°F (cold slows larval development).

Dry pastures & desiccation are lethal to larvae.

Hard freeze (Nov/Dec) kills larvae but hypobiotic larvae overwinter inside hosts to seed pastures in the next season.

Where do Haemonchus contortus adults reside in the host?

Abomasum of small ruminants.

What is the distinctive morphological feature of female Haemonchus contortus?

"Barber Pole" appearance due to a blood-filled gut spirally wrapped around white ovaries.

What is the size range of Haemonchus contortus worms?

Males: 10-20 mm

Females: 18-30 mm

How does Haemonchus contortus feed, and what is its characteristic structure?

It is a blood-feeding parasite with a single tooth in its small buccal capsule.

What are the hallmark clinical signs of Haemonchus contortus infection?

Severe anemia (PCV < 15%)

Pale mucous membranes

Hypoproteinemia → leads to submandibular edema ("Bottle Jaw")

How do sheep mitigate parasite disease risk?

Predominantly grazing animals → higher risk when grazing rough rangeland.

Develop acquired immunity to regulate parasite numbers.

How do goats mitigate parasite disease risk?

Preferential browsers (avoid worm-infested grass).

Can detoxify ingested plants (e.g., tannins may inhibit parasites).

Lack strong acquired immunity against parasites.

What are key strategies to reduce pasture contamination and prevent reinfection?

1. Avoid overstocking/overgrazing (limit to ~5 sheep/goats per acre).

2. Consider host factors: age & susceptibility, risk of acquiring large parasite burdens.

3. Use effective anthelmintics and monitor herd drug resistance.

4. Cross-species grazing: horses & ruminants don't share parasites.

5. Incorporate mixed forage in pastures for parasite-inhibitory effects.

How is a Fecal Egg Count Reduction Test (FECRT) performed?

1. Fecal egg counts (FEC) on 10% of herd (or at least 6 animals).

2. Exclude animals with FEC < 100 EPG from testing.

3. Treat remaining animals with a preferred deworming drug.

4. Repeat FEC after 7-10 days.

5. Calculate efficacy

6. Dewormers with FECR < 90% should be discontinued.

7. Widespread multi-drug resistance: FECR of ~75-80% is common.

what is the equation for calculating efficacy?

[(Pre-Tx EPG − Post-Tx EPG​)/Pre-Tx EPG] ×100

How can FEC results be applied in parasite control?

Used with FAMACHA scoring for Selective Deworming.

Helps prevent hazardous reinfection from contaminated pastures.

Identifies genetically resistant animals for breeding programs.

What are the limitations of using FEC results?

Cannot predict individual worm burdens in clinically affected animals.

Should not be used alone to make treatment decisions.

What is the FAMACHA scoring rubric?

Qualitative system for assessing anemia in sheep and goats.

Correlates with hematocrit levels and disease risk.

Used to determine which animals need anthelmintic treatment.

Why is Selective Deworming beneficial?

Reduces pasture contamination while using fewer dewormers.

Preserves genetic diversity in parasite populations.

Maximizes refugia (portion of parasite population not exposed to drugs).

Less expensive than whole-herd treatment.

Allows animals to develop natural immunity.

What are the consequences of excessive deworming?

Strong drug selection pressure leads to widespread resistance.

Once resistance is high, treatment failure occurs.

Economic loss, animal death, and emotional distress for producers.

How can FEC be used to select resistant livestock?

Identifies animals that reject parasitic challenge and suffer less disease.

Some breeds and individuals within breeds are naturally resistant.

Worm resistance is heritable (~0.25) and can guide breeding decisions.

What strategies reduce reliance on dewormers?

Provide "safe" pastures (low parasite risk):

1. Hay aftermath, crop stubble, ungrazed pastures since mid-summer.

2. Cross-species grazing with horses or mixed ruminants.

3. Reserve safest pastures for young animals at highest risk.