Drench monitoring - why bother?

Drench monitoring - why bother?
By Lucy Scott, Veterinarian, Anexa Vets Raglan

A faecal egg count (FEC) is a simple way to tell if your animals are carrying parasites and to what level. While most farmers subscribe to a ‘drench every 28 days’ system, the reality is you could be drenching more than necessary. It’s also a great way to tell if your drench is working correctly.

A pre-drench FEC can tell you if they need a drench or if this can wait another week, reducing costs and labour.

The cut off level depends on a few things - age of animal and stocktype, as well as the parasite type and weather.

The cut off for subclinical disease is lower than that of clinical disease, meaning the egg counts can be high when the animals show no sign of disease. If you are seeing no sign of parasitism in your lambs or calves, a FEC can tell you if there are subclinical effects.

Keep in mind that during Barber’s Pole season (for lambs) and during periods of warm wet weather, a low FEC can be deceptive (due to shorter lifecycle length of the worms) and stock can have a higher burden of immature larvae that don’t produce eggs.

Also, as animals age their immune system will shut down the egg production by the adults, also producing a lower fecal egg output.

A FEC can also be used 10 days post drench to see if there are eggs being produced. If eggs are being produced, this may indicate:
  • that the drench was not fully effective and some worms are still present (and laying eggs)
  • That the FEC was extremely high before the test and the residual egg count at ten days is not unexpected.
  • That the animals were not drenched correctly; either the dose rate was insufficient, the administration was not correct or not all animals were drenched.


A drench check is a useful tool to use over the season to check the effectiveness of the drenching process.

If you are concerned about possible drench resistance then further tests, called Faecal Egg Reduction Tests (FECRT) can be carried out to determine if this is an issue.



Date Added: Monday, 4th May 2020


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