Post-Drench Faecal Worm Egg Counts

A post-drench faecal worm egg count (WEC) is conducted 14 days after drenching to count the number of worm eggs remaining in the faeces post drench. It provides an indication of whether or not worms have developed resistance to a drench.

If cattle are treated with an effective drench, very few eggs are expected to be present in the faeces after treatment. Taking a faecal sample for a WEC will therefore provide some information on drench efficacy. Identifying the worm species present (which can be done using a larval culture or DNA test) can also assist in the interpretation of your WEC results, as different worm species lay different numbers of eggs.

Timing is vital. The interval between treating with an anthelmintic, and taking faecal samples for WECs is important. It needs to be long enough for dead worms to be out of the animals’ intestines, but short enough that new infections have not had time to develop. Three factors need to be considered:

1. The pre-patent period: This is the time it takes from the initial parasite infection until worm eggs can be detected in the faeces. For most roundworms this is 14 to 21 days. However, some small intestinal worms (Cooperia species) and inhibited small brown stomach worms (Ostertagia ostertagi) may lay eggs early and could appear in the faeces after only 12 days.
2. The passage period: This is the time it takes for the contents of the gastrointestinal tract to pass through the animal from mouth to anus. This varies in cattle, depending on the nature of the diet, but is generally less than seven days.
3. Suppression of egg-laying: Worms that survive anthelmintic treatment can temporarily stop laying eggs for 10 or more days.

The timing of faecal collection post treatment must be shorter than the pre-patent period, greater than the passage period and after egg-laying suppression has stopped. For these reasons, collection of post-treatment faecal samples for egg counts generally occurs after a period of 14 days.

What equipment is required?

One WEC kit (of the type supplied or recommended by your testing laboratory)

How is a post-drench check conducted?

1. Drench the mob.

• Any drench (or combination) suitable to treat the worm burden carried may be tested.

2. Conduct a worm egg count (WEC)14 days after drenching.

• 14 days after drenching (i.e. on the same weekday 2 weeks later), collecting fresh faecal samples from the paddock (individual samples) and conduct a WEC.
• Ideally, a  larval culture or DNA test should be requested with this WEC.

3. Assess the results of the ‘after drench WEC’.

• 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 DrenchCheck or DrenchTest (WECRT) 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 ‘after drench WEC’ results and to decide suitable drenches for future use.

Interpreting your post-drench WEC results

High worm egg count 14 days after treatment could mean:

• Resistance can be considered as a potential cause.
• Treatment failure
• Very high starting WEC (consider a DrenchCheck next time you treat).
• Rapid re-infestation with small intestinal worms (Cooperia species, determined from the larval culture or DNA test) is also a possibility.

Low or zero worm egg count 14 days after treatment could mean:

• The anthelmintic works and there is little or no resistance in the worm population.
• The starting WEC was already low prior to treatment (consider a DrenchCheck next time you treat).