Immunity is essential to productive pastoral farming. Development and maintenance of this immunity costs energy and requires constant low level exposure to worms.

Immunity in both cattle and sheep is normally fully developed by 18-20 months of age. Stock are most at risk of production loss and being a source of pasture contamination in their first year of life.

Factors that may enhance or depress immunity include:

• age – 2-tooths are at greater risk than MA ewes
• species – cattle are generally superior to sheep
• pregnancy status – risk of immunity depression increases with fecundity
• feed intake – increased feeding, especially protein, enhances the immune response
• body condition
• weather
• other diseases (e.g. Johne’s disease)
• breed choice or selection within a breed
• worm challenge.

Do not consider these factors in isolation. An underfed single-bearing ewe in poor condition may be at greater risk than a well fed twin-bearing ewe, and massive pasture contamination can overwhelm even very good levels of immunity. If sheep selected for enhanced immunity to parasites are drenched the same as non-selected sheep, drench resistance may develop faster!