Parasitism–Nutrition Interactions in Grazing Ruminants: Comprehensive Study Notes

Administrative & Logistical Announcements

Next Monday = field-trip day → no class.
Compulsory homework posted on the LEARN site: Ray Kaplan webinar on parasite control, refugia & anthelmintic resistance (recorded during the Boehringer-Ingelheim COVID webinar series).
Wednesday of next week = in-depth lecture on refugia + combination drenches.

Why Parasitism Matters (Beyond “Interesting Parasites”)

Parasitology literature is often parasite-centric; production animal science must remain host-centric.
We only worry about organisms that negatively affect productivity, welfare or survival – by definition those are the parasites.
Key metric: impact on the host’s nutrient economy and productivity.

Two Primary Pathways of Production Loss

Reduced feed intake (anorexia)
- Accounts for $60\% \to 90\%$ of the observed growth or production loss.
- Triggered by pro-inflammatory cytokines (IL-1, IL-6, TNF-α) acting on the ventromedial hypothalamus – the same mechanism that makes humans “lose their appetite” when sick.
Increased nutrient demand for immunity & tissue repair
- Immune activation is energy-expensive and even more protein-expensive (large demand for sulfur-containing amino acids e.g. cysteine).^*

^* Protein scarcity already limits growth in young ruminants; adding an immune tax makes the deficit critical.

Species Differences & Case Study: Haemonchus contortus (Barber’s-pole)

Temperate parasites (e.g. Trichostrongylus, Teladorsagia) rarely kill outright; killing the host is maladaptive for their lifecycle.
Haemonchus is an exception:
- Blood-feeder ⇒ acute anemia.
- High fecundity & rapid life-cycle ⇒ small initial burdens can become lethal quickly.
- Animals frequently die before immune or nutritional costs fully manifest.

Pathophysiology: “Leaky Gut” & Endogenous Losses

Mucosal browsers damage epithelial tight junctions and/or acid-producing cells (abomasal species).
Consequence: leakage of plasma proteins into gut lumen → partial re-absorption downstream but sizeable endogenous N loss in faeces.
Classic radiotracer studies ((^{51}\mathrm{Cr})) show losses ≈ $5\ \text{g N day}^{-1}$ in lambs ⇒ $\frac{5\,\text{g N}}{6.25}=30\,\text{g true protein}\Rightarrow\frac{30}{0.60}\approx50\,\text{g MP day}^{-1}$ extra requirement.

Terminology: Resistance vs Resilience vs Anthelmintic Resistance

Host Resistance = ability to limit parasite burden (immunity).
Host Resilience = ability to maintain performance despite burden.
Anthelmintic Resistance = heritable drug insensitivity in the parasite – totally different concept.

Nutrient Partitioning Rules

Limited metabolisable energy (ME) & metabolisable protein (MP) enter the system.
Priorities depend on physiological state:
- Young/naïve animals allocate to immunity > growth.
- Peri-parturient ewes reroute nutrients to lactation & offspring → relaxation of immunity (Peri-parturient Rise, PPR).
If supply meets demand, animal can be simultaneously resistant & growing (= “nutritional resilience”).

Protein vs Energy: What the Experiments Show

1 – Protein Turn-over Study (1980s–90s)

Infected vs pair-fed controls (same intake) – protein synthesis rose sharply in
- Stomach & small intestine.
- Liver (+50 % of maintenance).
- Net result: less protein left for muscle & bone.

2 – Casein vs Glucose Infusion (Bound et al.)

Abomasal infusion for 6 & 12 weeks.
12-wk outcomes:
- Casein: ↓ worm burden, growth rate = uninfected controls.
- Glucose (isocaloric): no worm-burden change.

3 – “Vaccination-Challenge” + Protein Supplement (Scottish study)

Casein-supplemented lambs developed stronger effector response (mast-cell protease, globule leukocytes) & fewer worms.

4 – Peri-parturient Ewes (Donaldson et al.)

Fish-meal (rumen-bypass protein) lowered ewe worm counts irrespective of litter size.
Supplemental energy had negligible effect.

Key Thresholds

Young lambs: need ≈ $50\,\text{g MP day}^{-1}$ extra.
Peri-parturient ewes: MP intake > $300\,\text{g day}^{-1}$ to materially reduce egg output.

How Body Condition & Targeted Supplementation Interact

Low BCS (≤ 2) ewes often have higher EPG; causal web:
- Low intake → less faeces → higher EPG concentration.
Targeted protein supplementation to poor-condition ewes lowered EPG by ≈50 % during first month of lactation (R. Tambernen PhD).
Epidemiological impact was variable – climate-driven larval survival often overrides a mere 50 % cut.

Weight Landmarks for Developing Immunity

Assume pasture provides $\approx80\,\text{g MP kg DM}^{-1}$ and lamb eats $3.5\%$ LW in DM.
Lamb must weigh ≈ $45\,\text{kg}$ before habitual surplus MP ≈ $50\,\text{g day}^{-1}$ appears.
Calves: endogenous losses ≈ $90\,\text{g MP day}^{-1}$ → need ≈ $200\,\text{kg}$ LW before immune costs are affordable.
Implication: finishing stock are generally slaughtered before they develop full immunity → control must rely on management & drugs, not natural resistance.

Forage, Secondary Compounds & Intake Rate

Condensed tannin pastures (e.g. sericea lespedeza) can have direct anthelmintic effect.
Other forages (chicory, plantain, lucerne, red clover) help indirectly:
- More erect habit ↓ micro-climate suitability for larvae.
- Higher digestibility + faster rumen turnover → animals eat ≈30 % more DM day⁻¹.
- Therefore surplus MP is reached earlier (see recalculated intercept at mid-30-kg lamb LW when intake = $4.5\%$ LW).

Grazing Behaviour, Sward Structure & Larval Exposure

Larvae concentrate near faecal pats & in the lower sward.
Naïve lambs graze indiscriminately; partially immune but clinically affected lambs show faecal avoidance; fully immune adults relax avoidance.
High grazing pressure, tight residuals or forced proximity to dung ⇒ higher larval intake.

Fecal Egg Counts (FEC): Beware of Dilution/Concentration Effects

FEC = $\frac{\text{eggs}}{\text{g faeces}}$ .
Factors altering faecal DM or volume (digestibility, growth rate, water intake) distort numerator perception.
Worked example ((1{,}000{,}000) eggs day⁻¹):
- Diet 70 % digestible → $\approx575\,\text{EPG}$ .
- Diet 90 % digestible → $\approx1{,}700\,\text{EPG}$ (same worm burden!).
Slow-growing lamb excretes less faeces → artificially elevated EPG.

Research Trend Timeline (Very Approx.)

1990s – nutrition & supplementation studies (protein focus).
Late 1990s – fungal biological control (e.g. Duddingtonia).
2000s-2010s – refugia theory, selective treatment, combination drenches.

Strategic Control Philosophy

Nutrition is the trump card: provide sufficient MP to cover immune costs → resilience + resistance.
Anthelmintics are the joker: invaluable but should be played sparingly to preserve efficacy (resistance risk).
Combining good feeding, smart grazing and refugia dramatically lowers drench reliance.

Ethical & Practical Implications

Balancing refugia (to protect drug susceptibility) vs contamination (to protect the host) requires integrated nutrition-grazing-drench strategy.
Better nutrition improves welfare (less clinical disease) and productivity (growth, wool, milk) while indirectly slowing anthelmintic resistance evolution.

Key Numerical Summary (Quick Reference)

Parameter	Value / Range	Context
Production loss via anorexia	$60\%-90\%$	All grazing ruminants
Endogenous N loss (lamb)	$5\,\text{g N day}^{-1}$	$\approx50\,\text{g MP}$ extra needed
Extra MP need (calf)	$90\,\text{g MP day}^{-1}$	Based on tracer work
MP threshold for ewe PPR control	>300\,\text{g MP day}^{-1}
Lamb LW for natural immunity (@3.5 % BW intake)	$\approx45\,\text{kg}$
Lamb LW with high-intake forage (@4.5 % BW)	Mid- $30\,\text{kg}$

Final Take-Home Messages

Feed the animal first – a well-nourished host can pay the immune “tax” without sacrificing growth.
Protein is king for immunity; extra energy alone does little.
Understand FEC maths – concentration ≠ burden.
Alternate forages & higher intakes pull forward the age/weight at which immunity manifests.
Use anthelmintics judiciously; nutrition, grazing management & refugia form the sustainable core of parasite control.

Next Monday is designated as a field-trip day, therefore there will be no class. Compulsory homework has been posted on the LEARN site, which includes the Ray Kaplan webinar on parasite control, refugia, and anthelmintic resistance, recorded during the Boehringer-Ingelheim COVID webinar series. Wednesday of next week will feature an in-depth lecture on refugia and combination drenches.

Parasitology literature often focuses on the parasite itself, but in production animal science, the focus must remain host-centric. Concerns are limited to organisms that negatively affect productivity, welfare, or survival, which are by definition, parasites. The key metric for evaluating impact is the effect on the host’s nutrient economy and productivity.

There are two primary pathways through which production loss occurs. The first is reduced feed intake, or anorexia, which accounts for $60\%$ to $90\%$ of the observed growth or production loss. This is triggered by pro-inflammatory cytokines such as IL-1, IL-6, and TNF-α, which act on the ventromedial hypothalamus, a mechanism similar to what causes humans to lose their appetite when sick. The second pathway is increased nutrient demand for immunity and tissue repair; immune activation is both energy-expensive and especially protein-expensive, given the large demand for sulfur-containing amino acids like cysteine. Protein scarcity is already a limiting factor for growth in young ruminants, and the additional immune tax can make this deficit critical.

Species differences are notable, as seen in the case study of Haemonchus contortus, also known as Barber’s-pole worm. Temperate parasites like Trichostrongylus and Teladorsagia rarely cause outright death, as killing the host is maladaptive for their lifecycle. However, Haemonchus is an exception because it is a blood-feeder, leading to acute anemia. Its high fecundity and rapid life-cycle mean that small initial burdens can quickly become lethal, and animals frequently die before the immune or nutritional costs fully manifest.

Pathophysiology often involves a “Leaky Gut” and endogenous losses. Mucosal browsers damage epithelial tight junctions or acid-producing cells, particularly in abomasal species. The consequence is the leakage of plasma proteins into the gut lumen, which, although partially re-absorbed downstream, results in a sizeable endogenous N loss in feces. Classic radiotracer studies using $(^{51}\mathrm{Cr})$ reveal losses of approximately $5\ \text{g N day}^{-1}$ in lambs, which translates to about $30\ \text{g}$ of true protein and an extra requirement of roughly $50\ \text{g MP day}^{-1}$ .

Understanding terminology is crucial, distinguishing between Host Resistance, Host Resilience, and Anthelmintic Resistance. Host Resistance refers to the ability to limit parasite burden through immunity. Host Resilience is the ability to maintain performance despite a parasite burden. Anthelmintic Resistance, however, is a completely different concept, referring to the heritable drug insensitivity in the parasite.

Nutrient partitioning rules dictate how limited metabolisable energy (ME) and metabolisable protein (MP) are allocated within the system. Priorities depend on the physiological state of the animal: young or naïve animals typically prioritize immunity over growth, while peri-parturient ewes reroute nutrients to lactation and offspring, leading to a relaxation of immunity known as the Peri-parturient Rise (PPR). If the supply meets the demand, an animal can be simultaneously resistant and growing, which is termed “nutritional resilience.”

Experimental findings highlight the importance of protein versus energy. A protein turn-over study conducted from the 1980s to the 1990s compared infected animals with pair-fed controls (having the same intake), showing that protein synthesis rose sharply in the stomach, small intestine, and liver (by $+50\%$ of maintenance). The net result was less protein remaining for muscle and bone. In a casein versus glucose infusion study by Bound et al., abomasal infusions were given for 6 and 12 weeks. After 12 weeks, the casein group showed a decreased worm burden and growth rate equivalent to uninfected controls, whereas the glucose (isocaloric) group showed no change in worm burden. A “Vaccination-Challenge” study combined with protein supplement in Scotland revealed that casein-supplemented lambs developed a stronger effector response, characterized by increased mast-cell protease and globule leukocytes, and consequently had fewer worms. Research on peri-parturient ewes by Donaldson et al. found that fish-meal, a rumen-bypass protein, lowered ewe worm counts irrespective of litter size, while supplemental energy had negligible effect. Key thresholds identified from these studies suggest that young lambs need approximately $50\ \text{g MP day}^{-1}$ extra, and peri-parturient ewes require MP intake of more than $300\ \text{g day}^{-1}$ to materially reduce egg output.

Body condition and targeted supplementation interact in complex ways. Ewes with low Body Condition Score ( $\le 2$ ) often exhibit higher Egg Per Gram (EPG) counts, a causal web wherein low intake leads to less feces and thus higher EPG concentration. Targeted protein supplementation administered to poor-condition ewes was shown by R. Tambernen’s PhD work to lower EPG by approximately $50\%$ during the first month of lactation. However, the epidemiological impact of this intervention was variable, as climate-driven larval survival often overrides a mere $50\%$ cut.

Weight landmarks are important for developing immunity. Assuming pasture provides approximately $80\ \text{g MP kg DM}^{-1}$ and a lamb eats $3.5\%$ of its live weight in dry matter, a lamb must weigh approximately $45\ \text{kg}$ before a habitual surplus of MP, around $50\ \text{g day}^{-1}$ , appears. Calves experience endogenous losses of about $90\ \text{g MP day}^{-1}$ , meaning they need to reach approximately $200\ \text{kg}$ live weight before immune costs become affordable. This implies that finishing stock are generally slaughtered before they develop full immunity, necessitating reliance on management and drugs rather than natural resistance for control.

Forage, secondary compounds, and intake rate also play a role. Pastures containing condensed tannin, such as sericea lespedeza, can have a direct anthelmintic effect. Other forages, including chicory, plantain, lucerne, and red clover, help indirectly. Their more erect growth habit reduces micro-climate suitability for larvae, and their higher digestibility combined with faster rumen turnover means animals eat approximately $30\%$ more dry matter per day. Consequently, the surplus MP is reached earlier, as seen in the recalculated intercept at mid- $30\text{-kg}$ lamb live weight when intake reaches $4.5\%$ of live weight.

Grazing behavior, sward structure, and larval exposure are intertwined factors. Larvae tend to concentrate near fecal pats and in the lower sward. Naïve lambs graze indiscriminately, while partially immune but clinically affected lambs exhibit fecal avoidance, a behavior that relaxes in fully immune adults. High grazing pressure, tight residuals, or forced proximity to dung all result in higher larval intake.

When evaluating Fecal Egg Counts (FEC), which is expressed as $\frac{\text{eggs}}{\text{g faeces}}$ , it is important to be aware of dilution and concentration effects. Factors that alter fecal dry matter or volume, such as digestibility, growth rate, and water intake, can distort the perception of the numerator. For example, if an animal excretes $\text{1,000,000}$ eggs per day, a diet that is $70\%$ digestible would result in approximately $575\ \text{EPG}$ , whereas a diet that is $90\%$ digestible would yield about $1,700\ \text{EPG}$ for the same worm burden. A slow-growing lamb excretes less feces, leading to an artificially elevated EPG.

The research trend timeline, very approximate, indicates that the 1990s focused on nutrition and supplementation studies, with a strong emphasis on protein. The late 1990s saw interest in fungal biological control, specifically Duddingtonia. From the 2000s to the 2010s, research shifted towards refugia theory, selective treatment, and combination drenches.

The strategic control philosophy can be summarized by two key ideas: nutrition is the trump card, meaning providing sufficient MP to cover immune costs leads to both resilience and resistance. Anthelmintics, while invaluable, act as the joker and should be used sparingly to preserve their efficacy due to resistance risk. Combining good feeding, smart grazing, and refugia significantly lowers reliance on drenches.

Ethical and practical implications involve balancing refugia, which protects drug susceptibility, with contamination, which protects the host, requiring an integrated nutrition-grazing-drench strategy. Better nutrition improves welfare, reducing clinical disease, and productivity, enhancing growth, wool, and milk, while also indirectly slowing the evolution of anthelmintic resistance.

Key numerical summaries provide a quick reference for various parameters. Production loss due to anorexia in all grazing ruminants ranges from $60\%$ to $90\%$ . Endogenous Nitrogen loss in lambs is about $5\ \text{g N day}^{-1}$ , which implies an extra need of approximately $50\ \text{g MP}$ . Calves require an extra $90\ \text{g MP day}^{-1}$ based on tracer work. For ewe Peri-parturient Rise control, the MP threshold is greater than $300\ \text{g MP day}^{-1}$ . A lamb reaches natural immunity at approximately $45\ \text{kg}$ live weight with $3.5\%$ Body Weight intake. However, with high-intake forage at $4.5\%$ Body Weight intake, a lamb can reach immunity at mid- $30\text{-kg}$ live weight.

The final take-home messages emphasize several crucial points. Firstly, the animal must be fed first, as a well-nourished host can afford the immune “tax” without sacrificing growth. Secondly, protein is paramount for immunity; extra energy alone offers little benefit. Thirdly, it is important to understand FEC mathematics, recognizing that concentration does not equate to burden. Fourthly, alternate forages and higher intakes accelerate the age and weight at which immunity manifests. Lastly, anthelmintics should be used judiciously; nutrition, grazing management, and refugia form the sustainable core of parasite control.