Others, such as adrenaline, divert energy from production. It is caused by hormonal changes which occur in ewes just before birth of lambs and into lactation. In these ewes more of the larvae ingested develop to adults, larvae emerge from arrest and egg production per female worm increases. As a result faecal egg production may double leading to increased contamination of pasture and increased availability of infective larvae to young lambs. Arrested development -occurs when the larva stops development in the lining of the gastrointestinal tract.
Arrest usually occurs during unfavourable conditions.
For example, during dry or very cold weather eggs produced by adult worms and passed in faeces will not survive. Arrested worms wait for conditions to improve before resuming development to maturity and laying eggs. The triggers to become arrested are hostile host immunity or exposure of larvae to a cold winter on pasture. The triggers to resume development are not fully understood. PPR may be one, feeding on fresh, spring pasture growth another.