Grazing Management for Internal Parasites

This set covers the life cycle of internal parasites (nematodes), environmental factors affecting their development, and grazing management strategies like Smart Grazing to control infection in livestock.

What are the two primary factors contributing to pasture contamination with internal parasites?

Paddocks being contaminated with worm eggs and the biological constraints of larvae moving from the pasture into the animal.

What is the L5 stage of the nematode life cycle?

The adult stage where worms live inside the animal's gut and reproduce by laying eggs.

How long does it typically take for worm eggs to hatch into L1 and L2 larvae under standard conditions?

Around 10 days, though it can be as fast as 4 days in warmer weather.

At what temperature does the hatching process of worm eggs significantly slow down?

Below $10^\circ\text{C}$.

What happens to worm egg hatching once environmental temperatures exceed $35^\circ\text{C}$ in very dry conditions?

The ability of the eggs to hatch is inhibited.

Why is the L3 stage referred to as the 'infective larvae'?

Because L3 larvae wriggle out of the dung and move onto the pasture grass where they are available for ingestion by grazing animals.

What is 'anhydrobiosis' in the context of L3 larvae?

A state of suspended animation where larvae desiccate during extreme heat above $40^\circ\text{C}$, allowing them to be spread by wind and later reanimate when rehydrated.

What is the primary metabolic difference between L1/L2 larvae and L3 larvae?

L1 and L2 larvae eat faeces in the dung, whereas L3 larvae do not eat and must rely on stored energy reserves until they enter a host's digestive tract.

How does season affect the survival duration of L3 larvae on pasture without a host?

They typically die within 3 months in warmer summer conditions but can last up to 6 months in winter due to a slowed metabolism.

Where do L3 larvae tend to gathered within the pasture sword structure?

At the base of the stems where conditions are cooler, moister, and protected from sunlight.

How can 'auto-infection' be prevented in a grazing system?

By moving animals off a clean paddock in less than the 4-day minimum hatching period to keep ahead of the worm hatching cycle.

What is the minimum daily temperature required for Barber's pole worm ($Haemonchus\,contortus$) to hatch?

$18^\circ\text{C}$.

Which worm species is noted for being more tolerant of cold weather than the Barber's pole worm?

$Trichostrongylus$, also known as the black scour worm.

What is the 'pre-patent period' for most roundworm species?

The minimum time period of $18$ to $21$ days between an animal becoming infected with L3 larvae and the time the mature worms begin spreading eggs back onto the pasture.

What are the three main categories of grazing management strategies for parasite control?

Preventative strategies (avoiding exposure), evasive strategies (moving animals before eggs hatch), and dilution strategies (reducing the ingestion rate of L3 larvae).

Why is it important to have 'refugia' during drenching?

To maintain a proportion of the worm population that is not exposed to the drench, which slows down the development of antibiotic resistance by preserving non-resistant genetics.

What is the 'Smart Grazing' strategy for cleaning up a pasture?

Using resistant, recently drenched stock (like wathers or cattle) at high stocking densities for a short period ($<21$ days) to 'vacuum' up L3 larvae without depositing new eggs.

Which livestock species can be used to clean pastures because their immune systems kill sheep worm larvae?

Cattle, because the internal parasites that infect sheep generally cannot reach adulthood or reproduce inside them.

How does heavy rainfall or moisture impact the transition from L1/L2 to L3 larvae?

Sufficient moisture (from rain or dew) is required for eggs to hatch and for larvae to move out of the dung and up the stems of pasture grass.

What are the most vulnerable and least vulnerable age groups of sheep regarding worm infection?

Weaners are the most vulnerable with low immunity, while mature weathers and ewes are the least vulnerable and more resistant.