‘Before and after’ testing (DrenchCheck)

A DrenchCheck provides a simple, fast and low cost indication of possible drench resistance. It uses two faecal worm egg counts (WECs); the first test measures worm egg numbers ‘before’ drenching and the other test measures worm egg numbers 14 days ‘after‘ drenching. The ‘before and after’ WEC scores are then compared to see how well the drench works at reducing worm numbers.

As with the post-drench WEC, fresh faeces for testing can be collected in the paddock, animals do not need to be yarded.

What equipment is required?

Two WEC kits (of the type supplied or recommended by your testing laboratory).

How is a DrenchCheck conducted?

1. Conduct a routine faecal worm egg count (WEC).

  • Use the results of a routine WEC where fresh dung samples were collected in the paddock.
  • Ideally, a larval culture or DNA test should be requested with this WEC.
  • If the mob requires drenching, this test will be compared to a WEC done 14 days after drenching.
  • This first WEC will be called the ‘before drenching WEC’.
  • If the mob does not require drenching, wait until a later WEC indicates drenching is required.

2. Drench the mob.

  • Drench the mob within a week of receiving results from the ‘before drenching WEC’.
  • Any drench (or combination) suitable to treat the worm burden carried may be tested.

3. Conduct a WEC 14 days after drenching.

  • 14 days after drenching (i.e. on the same weekday 2 weeks later), conduct another WEC (individual samples), collecting fresh faecal samples from the paddock (see ‘Post-drench faecal worm egg counts’.)
  • Request a larval culture or DNA test regardless of the egg count result.
  • Request that the laboratory calculate a DrenchCheck using the results from the ‘WEC after drenching’ and the ‘WEC before drenching’ (provide them with the laboratory submission number for the first test).
  • Results will be provided as for a WEC as well as a DrenchCheck result if this has been requested.
  • If a mid-length or long-acting drench was used, further WECs should also be done (ask your testing laboratory for these times).

4. Assess the results of the DrenchCheck

  • If the worm egg count results of the ‘after drench WEC’ at day 14 are neither zero, nor very low, then you should seriously consider more thoroughly investigating the effectiveness of this drench with a DrenchTest (WECRT) or at least continue using the DrenchCheck method if the drench is used again.
  • The larval culture or DNA test results from the ‘after drench WEC’ can also indicate which worm types have lived through the drench and are likely to be drench-resistant.
  • If there are worm eggs remaining at the ‘after drench WEC’, 14 days after treatment, further anthelminthic treatment may be required.
  • Seek professional advice to interpret DrenchCheck results and to decide suitable drenches for future use.

An example of how drench efficacy is calculated

A mob of cattle with mean ‘before drench WEC’ of 500 eggs per gram (epg) and a Day 14 mean ‘after drench WEC’ of 100 epg will have an estimated drench efficacy of 80%, calculated using the mathematical formula shown below:

‘Percent Reduction’ or ‘efficacy’ = R

R = [1- (Average ‘after drench WEC’÷ Average ‘before drench WEC’)] x 100

R = [1-(100/500)] x 100

R = [1-0.2] x 100

R = [0.8] x 100

R = 80%

However, depending on the different worm species present at the time of drenching, this ‘overall efficacy’ may indicate 0% efficacy against one worm type and 100% efficacy against another worm type. Only by doing a larval culture or DNA test before and after drenching can the efficacy against different worm species be detected.

Limitations of the DrenchCheck test

The reliability of this test depends on:

  • The number of animals tested. Current standards recommend 15 animals (and no fewer than 6) be sampled at each time point so that an average WEC can be calculated. This accounts for animal to animal variability in WEC scores within groups of cattle.
  • Sensitivity of the faecal worm egg count method. If the before drench egg count is very low, the test may not be valid, as it does not allow for a reasonable comparison of the two counts.