04/22/2026
Great info for those new to sheep and goats!
Barber Pole Worm in Sheep & Goats — ARTICLE 5
Resistance vs Resilience — Why That Distinction Matters
By now, you’ve seen the pattern.
The parasite is present in most systems.
It cycles continuously.
And at certain points, the balance shifts.
But under the same conditions… not all animals respond the same way.
Some struggle.
Some don’t.
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This Is Where the Conversation Usually Goes
You’ll often hear:
“Breed for parasite resistance.”
That sounds straightforward.
But in practice, what people are seeing—and what they’re selecting for—is often something different.
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Two Very Different Concepts
Resistance
Resistance is the ability to:
- prevent infection
- limit the number of worms that establish
A resistant animal:
- carries fewer parasites
- sheds fewer eggs
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Resilience
Resilience is the ability to:
- tolerate the parasite
- maintain condition despite infection
A resilient animal may:
- carry a parasite burden
- still appear healthy
- continue to perform
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One limits the parasite.
The other tolerates it.
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What Resilience Actually Looks Like
With a parasite like Haemonchus contortus, which feeds on blood, resilience often shows up as:
- better recovery from blood loss
- maintenance of red blood cell levels
- ability to hold weight and continue eating
- delayed or reduced visible signs like anemia or edema
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Resilience isn’t fewer worms—it’s a better response to them.
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Why This Gets Confused
In real-world systems, these two can look very similar.
An animal that:
- maintains weight
- raises lambs
- has a good FAMACHA score
…is often labeled as “resistant.”
But that animal may still be:
- carrying parasites
- shedding eggs
- contributing to pasture contamination
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What looks like resistance is often resilience.
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The Resilience Trap
This is where it becomes important.
Some of the best-looking animals on a farm:
- always seem to “do fine”
- don’t show obvious signs
- rarely get pulled for treatment
But when you actually check them…
They’re often the ones:
- carrying a parasite load
- shedding eggs into the environment
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The best-looking animal in the pasture is sometimes the one contaminating it the most.
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Why That Happens
Resilient animals:
- don’t show problems early
- don’t trigger concern
- don’t get checked or tested
So they stay in the system…
while contributing to parasite pressure.
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A Note on Males (Important Nuance)
You may notice this more in males.
Not necessarily because they are inherently more resilient…
But because they are not under the same physiologic stress as females in late gestation and early lactation.
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They don’t hit the tipping point as easily—so the problem stays hidden longer.
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Resilience Is System-Dependent
You’ll often hear:
“My animals are parasite resistant.”
But in many cases, what that really means is:
“My animals are performing well under the conditions I raise them in.”
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That performance is influenced by:
- parasite pressure
- nutrition
- environment
- management
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Resilience doesn’t exist in isolation… it exists within a system.
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Animals that perform well in one system may not perform the same way in another.
Not because the animal changed…
but because the pressures around it did.
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What looks like resistance is often resilience, and that resilience is often specific to the system it developed in.
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This Connects Back to Everything We’ve Covered
- The parasite is already present
- The system is always cycling
- The animal’s ability to control it shifts over time
So when you evaluate animals, you’re not just asking:
“Do they have worms?”
You’re asking:
“How do they respond to the pressure—and what are they contributing to the system?”
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System-Level Takeaway
Resistance and resilience are not the same thing.
And understanding the difference helps you:
- interpret what you’re seeing
- avoid misclassifying animals
- make better selection decisions
- manage parasite pressure more effectively
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Why This Matters
Because without this distinction:
- good performers can be misunderstood
- high shedders can go unnoticed
- system-level pressure can remain elevated
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Next Article
If animals can look good while still carrying parasites, the next question is:
How do you actually evaluate that in a practical setting?
In the next article, we’ll look at tools like FAMACHA and what they do, and don’t, tell you.
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Good livestock management isn’t about always having the right answer — it’s about learning how to think when the answer isn’t obvious yet.
