Grazing Management for Internal Parasites

Biological Lifecycle and Mechanisms of Internal Parasite Infection

• General Principles: The management of internal parasites in livestock (particularly small ruminants/small women) involves understanding the biological constraints of larvae and their movement from the paddock to the animal.

• Host Immune Response:

  • Inside the animal (the host), worms reach adulthood at the $L5$ stage.

  • The animal's immune system can actively fight infections by engulfing worms with macrophages and white blood cells.

  • Immune responses can kill off, expel, or suppress the reproductive ability of the worms.

• Larval Stages and Development:

  • There are up to five different larval stages.

  • Egg Stage: Adult worms reproduction occurs in the gut; eggs are passed onto the pasture via faeces.

  • Dumb Stage (Stages $L1$ and $L2$): Larvae grow inside the dung from eggs into mature larvae. They feed on the faeces during this time.

  • Infective Stage ($L3$): Once critical maturation is reached, larvae wriggle out of the dung and onto the grass. They are termed "infective larvae" because they are available for ingestion by grazing animals.

  • Anatomy of $L3$: These larvae possess an external sheath that protects them from environmental extremes like heat, cold, and dry weather.

• The Infective Process:

  • Larvae move randomly through the pasture, utilizing moisture from dew or rain to travel down or up the stems.

  • They gather densely at the base of the sword structure (lower grass levels) where it is cooler and moister, protected from sunlight.

  • Some $L3$ larvae can burrow into the upper layer of the soil as conditions become warmer or drier.

Environmental Factors Affecting Contamination

• Factor A: Hatching Time:

  • The transition from egg to $L3$ infective larvae typically takes $4\text{--}10\,\text{days}$.

  • The minimum time required for most species is $4\,\text{days}$.

• Factor B & C: Climatic Conditions:

  • Warmer temperatures accelerate metabolic processes, speeding up development.

  • Temperature Thresholds:

    • Below $10\,^{\circ}\text{C}$: The speed of hatching and development slows down significantly.

    • Above $35\,^{\circ}\text{C}$ (dry): The ability of eggs to hatch is inhibited.

    • Below $15\,^{\circ}\text{C}$: Many species stop progressing from the $L2$ to $L3$ stage.

  • Species Variability:

    • Marcus / Barbara Poland / Christina Barbara / Harvest Poland (Barber's Pole): Requires a minimum overnight daily temperature of $18\,^{\circ}\text{C}$ to hatch efficiently.

    • Trump is stronger until the size (Trichostrongylus) / Black skirt: These are more tolerant of cold weather and can continue causing problems into autumn and winter.

  • Moisture Requirements: Sufficient rainfall or soil moisture is necessary for the hatching of eggs. Very dry conditions act as a limiting factor.

• Factor D: Viability and Energy Reserves:

  • $L3$ larvae do not eat. They survive entirely on stored energy reserves while waiting for a host.

  • Metabolism and Survival:

    • In summer/warm conditions: Metabolism is faster. They typically die off or expend energy within $3\,\text{months}$.

    • In winter/cold conditions: Metabolism slows down. They can survive on the pasture waiting for a host for $6\,\text{months}$ and occasionally longer than a year.

  • Anhydro Virus (Anhydrobiosis): In temperatures over $40\,^{\circ}\text{C}$, larvae can enter a state of suspended animation/desiccation. They can be spread by wind in this state and reanimate/rehydrate when moisture returns.

  • Mortality Rate: If maximum temperatures exceed $40\,^{\circ}\text{C}$, $90\,\%$ of Marcus $L3$ larvae can die within $1\text{--}2\,\text{weeks}$.

• Factor E: Pre-patent Period:

  • This is the time between an animal ingesting $L3$ larvae and those larvae maturing to produce the next generation of eggs.

  • For most roundworm species, the minimum pre-patent period is $18\text{--}21\,\text{days}$.

Grazing Management and Control Strategies

• Integrated Pest Management (IPM): While anti-romantic/chemical drenching is the primary control, managing pastures reduces reliance on chemicals and slows the development of antibiotic resistance.

• Preventative Strategies:

  • Keeping animals on clean pastures to avoid exposure.

  • Quarantine Drenching: Treating new animals to prevent importing antibiotic-resistant worms.

  • Pasture Renovation: Receding, ploughing, cultivating, or planting annual forage crops disrupts the lifecycle by breaking up faeces and burying larvae.

  • Hay and Silage Making: Moving and drying the pasture effectively removes and kills larvae.

  • Species Rotation: Utilizing the fact that sheep worms do not infect cattle and vice versa (though goats carry both).

• Evasive Strategies:

  • Moving animals off a contaminated pasture before eggs have time to hatch into infective $L3$ stages (i.e., within the $4\text{--}10\,\text{day}$ window).

  • Avoiding "auto infection," where an animal is reinfected by its own previous generation of worms.

• Dilution Strategies:

  • Low Stocking Rates: Reducing the rate at which animals ingest larvae by minimizing grazing pressure near the base of stems.

  • Mixing Ages and Species: Grazing resistant animals (mature weathers, ewes, or cattle) with vulnerable animals (weaners/lambs).

  • Host Immunity: Adult weathers have higher immunity; if they ingest $L3$, their immune system kills the worms before they reach maturity.

Smart Grazing Systems

• Overview: A combination of preparing clean paddocks and utilizing high-intensity rotational grazing to maximize larvae removal with minimal reinfection.

• The Preparation Phase:

  • Select resistant stock (e.g., weathers or cattle).

  • Apply an effective drench to ensure the livestock themselves are not shedding eggs into the pasture.

• The Cleaning Phase:

  • Stock the paddock at roughly $3\,\text{times}$ the normal stocking rate for a short period.

  • The goal is to act like a "vacuum cleaner," where resistant stock eat the majority of the existing $L3$ larvae in the sword.

  • Duration: The grazing period must be shorter than the pre-patent period ($18\text{--}21\,\text{days}$) to ensure any larvae eaten do not survive to shed new eggs.

• Post-Grazing Management:

  • Once stock are moved (within $21\,\text{days}$), the paddock is de-stocked and spelled to allow regrowth.

  • By grazing down to a residual of $2\text{--}3\,\text{cm}$ (around $700\,\text{kg}$ of dry matter), any remaining eggs or larvae are exposed to UV light, wind, and drying, which increases mortality.

• Regional Timing:

  • Winter Rainfall Zones: Prepare wieners/weaners for the autumn break using summer drenches.

  • Summer Rainfall Zones (e.g., New England): Clean pastures in late summer or autumn before winter temperatures drop below $18\,^{\circ}\text{C}$, extending the period of pasture cleanliness.

Specific Species and Resistance Patterns

• Barber’s Pole (Marcus/Haemonchus contortus): High risk in warm, humid conditions ($18\,^{\circ}\text{C}+$). Rapid life cycle but susceptible to cold and extreme heat over $40\,^{\circ}\text{C}$.

• Black Scour / Trump is stronger until the size (Trichostrongylus): More cold-tolerant, continuing lifecycle even in cool temperate winters.

• Liver Fluke: Differs from roundworms by requiring an intermediary host, specifically a prototype snail.