High parasite challenge after drought breaks

by Ben Allot, March 2018

The first 2-3 weeks after a drought breaks can be a very high parasite challenge period.  Stock, particularly young animals, should be observed closely for signs of worm burdens and aggressive, early drenching decisions made.

Once hatched and developed, the infective parasite larval stages move out of faecal material and migrate onto pasture and into the upper layer of the soil.  They require moisture to break out of faeces and to move up the pasture to a height where they are grazed.

During the prolonged hot, dry conditions of a drought, animals with worm burdens continue to pass faeces with parasite eggs.  After these eggs hatch, the larvae get ‘trapped’ in faecal material and the upper layer of soil as there is not enough moisture for them to move onto pasture.  While the drought persists, this ‘bank’ of trapped larvae builds up.

After rain a number of things happen that can contribute to a ‘wave’ of parasites:

  1. Rain provides the moisture to release larvae ‘trapped’ in faeces and soil. The ‘bank’ of parasites can now all freely move up to grazing height onto pasture
  2. Dry, drought preserved pasture rapidly rots, resulting in a rapid decline in pasture quantity and quality available. This often places animals under significant nutritional stress which can exacerbate the effect of parasite challenge.
  3. As fresh green pick grows, this feed encourages animals to graze low, close to the soil, where a large number of parasite larvae are now moving up to grazing height.

There is a lag period of three weeks between parasite larvae being eaten and that parasite maturing in the gut to lay eggs that pass in faeces.  This lag period can mean that young animals can develop severe clinical signs of parasitism, even sudden death, while at the same time having low faecal egg counts.

A drought breaking is a fantastic relief, but the moisture can bring with it a few challenges that take a month or so to start to work through.  With recent rain breaking drought in several regions recently, we hope this serves as a timely reminder to observe young stock closely and approach your trusted animal health advisor for quality parasite management advice.