Importance of refugia

 
 

‘Refugia’ refers to a population of parasites in the environment (pasture) not exposed to anthelmintic (drench) treatments. These worms will not have drench resistance, and so by remaining in the environment they provide a population of worms ‘susceptible’ to our drenches.

 

Why is it important?

This process ensures that there are some non-resistant/ drench-susceptible worms available in the environment to reproduce with those worms that have been exposed to drench. The aim being to ensure that the number of drench-susceptible larvae significantly outnumber the drench-resistant population. When worms from these two populations breed within an animal, the genes for drench-resistance will be diluted, this process will result in dilution of the genes within the population.

How can it be achieved in practice?

  • Not drenching all animals in a mob every time

    Selecting which animals remain un-drenched is important – you can either randomly select animals to not treat (i.e. those that are heaviest or in highest body condition) or target the animals to treat by either a high FEC (performed in-clinic), low live weight gain or poor milk production.

    If you are using liveweight (and/or liveweight gain) to select which animals not to drench we would suggest leaving a maximum of 10% of the mob undrenched. This recommendation is based on NZ trial work performed by Beef + Lamb New Zealand.

    If your are not using any production measures such as liveweight to select which animals not to drench we would suggest leaving a maximum of 5% of the mob undrenched.

    Drenching intervals should be kept to 28 days or more. However if you are extending the interval to greater than 28 days we would highly recommend that you monitor stock closely for signs of a worm burden and to avoid going beyond this interval during ideal climatic conditions for worm numbers to build up.

  • Returning animals to the same (infected) pasture for a week or so after drenching, before they go onto ‘clean pasture’ also promotes refugia. As described above, this process promotes ‘dilution’ of any resistant worms that remain on the pasture.

  • Drenching intervals should be kept to 28 days or more.

Although leaving un-drenched worms in the environment, adding to pasture contamination, may seem counter-intuitive at first, and could cause a mild decrease in productivity initially, drench-resistance is a big and long-term cost for a farm.