When a drench resistance test is done, samples are checked 10–14 days after drenching. Why then, and not sooner or later?

Whether you test the efficacy of a number of drenches with a Faecal Egg Count Reduction Test (FECRT; DrenchTest) or a simple before and after test when the mob receives a particular drench (DrenchCheck), the post-drench faecal samples for worm egg counts are advised to be collected 10–14 days after the drench, with 14 days as the preference.

For an anthelmintic to be fully effective, no worms should survive treatment. Even if all worms are killed, it generally takes two or three days after drenching for all of the existing worm eggs to disappear from the faeces, so in the first instance sampling must wait for those eggs to clear.

But when resistant worms survive the treatment, their egg production can be temporarily suppressed. In these cases, no eggs in the faeces does not mean no worms in the animal. If an allowance is not made for the temporary egg suppression in the test, the results may be misleading: a drench might appear more effective than it actually is, underestimating anthelmintic resistance.

The egg suppression period varies with the drench group (3 days for Levamisole (LEV), 8 days for Benzimidazoles (BZs) and 14–17 days for Macrocyclic lactones (MLs)). This is why in some literature it is advised to take the second sample at the following time intervals after treatment: Levamisole 3–7 days, Benzimidazoles 8–10 days, Macrocyclic lactones 14–17 days.

However, after drenching, the sheep continue to graze and become infected with worm larvae from the pasture. It’s critical that samples are collected before any of these larvae can mature, mate and lay their own eggs. The time taken for adult worms to reach full egg production after developing from newly-acquired infective larvae is variable, but the first eggs can appear from 18 days post-treatment for the major genera (Haemonchus, Teladorsagia, Trichostrongylus), and as early as 16 days for Cooperia.

To ensure the test applies only to the worm population that was already within the animal at the time of drenching, ParaBoss recommends 14 days as a compromise between the time required for reasonable certainty that any temporary suppression of egg production of female worms is reduced to an acceptable minimum, and the time before newly-acquired infective third stage larvae (L3) may have developed to egg-laying adults.

Given the importance of the macrocyclic lactones, it is essential that FECRTs do not underestimate resistance to abamectin, and especially moxidectin. Faecal samples should, therefore be collected as close as possible to 14 days after treatment with an ML. Where it is certain that the specified interval will be adhered to, up to 16 days would be a reasonable, and perhaps better, maximum period, however, waiting this long risks getting no samples if unforeseen circumstances (e.g. wet weather) arise. It is recommended that the general statement for post-treatment sampling is: “14 days standard, and 10 days the absolute minimum”.