How to check if your worm dose is effective

Given the widespread issue of anthelmintic resistance and the need for good worm control it is important to check drench efficacy and to use a wormer that kills all the worms. Orla Keane, Teagasc Researcher at Grange, Co. Meath, describes how this can be done step by step here

At this time of year, growing lambs are susceptible to stomach worms. All lambs are born with no immunity to these worms but build up immunity over time. The post-weaning period is a high-risk period and failure to control worms at this stage will result in poor thrive and lambs taking longer to finish. A variety of management steps can be taken to reduce the worm challenge to susceptible lambs but inevitably, wormers (anthelmintics) are used to control these parasites.

The five classes of wormers

Despite the large number of wormers on the market, they all fall into one of 5 classes;

• the white wormers (benzimidazoles),
• the yellow wormers (levamisole),
• the clear wormers (macrocyclic lactones),
• orange wormer (amino-acetonitrile derivative) or
• purple wormer (spiroindole).

Anthelmintic resistance

These wormers have been very effective in the past at killing worms; however, their extensive use has resulted in the emergence of worms that are resistant to the wormers, a phenomenon known as anthelmintic resistance.

All wormers that belong to the same class kill worms in the same way, therefore if worms develop resistance to one wormer in a class then all wormers in that class are usually also ineffective at killing the worms. Therefore, when resistance develops the options for worm control narrow.

Anthelmintic resistance is now common on Irish sheep farms. A recent study found that the worms on 100% of Irish lowland sheep farms tested were resistant to white wormers, on 17% of farms the worms were resistant to yellow wormers and on 61% of farms the worms were resistant to clear wormers. Therefore, anthelmintic resistance is a major challenge for lamb producers.

The best wormer to use to control stomach worms in lambs is the one that works on your farm. The only way to know if a wormer is working as it should is to test whether the worms are killed after the wormer is used. 

The Drench Test

The simplest and most cost effective test to check wormer efficacy is the ‘drench test’. This test involves measuring the number of worm eggs in dung before and after dosing to ensure that all worms are killed.

• The first step in a drench test is to gather approximately 20 lambs from the group to be tested, mark them and leave them in a clean pen for 1 or 2 hours to defecate.
• Collect a sample, a teaspoonful, from at least 10 separate faecal deposits and treat the lambs with the wormer to be tested at the recommended dose rate.
• Make sure to weigh the lambs and calibrate the dosing gun to ensure that the dose rate is correct.
• Then send the faecal samples to a laboratory for testing. Send the samples as soon as possible after collection, preferably the same day.

In the laboratory, the 10 faecal samples will be combined and the number of worm eggs in the composite sample will be determined. This indicates how heavily parasitized the lambs were when they were treated.

The second test. After a set number of days, the process is repeated.

• The same marked lambs are placed in a clean pen and left for 1-2 hours to defecate.
• Samples are again collected from at least 10 different faecal deposits and submitted to a laboratory for testing.
• This second test should be 7 days after the first test if a yellow wormer was used to dose the lambs and 14 days after the first test if a white or clear wormer was used to dose the lambs.

In the laboratory the number of worm eggs in a composite sample will again be determined.

In order to determine drench efficacy, the reduction in egg count after dosing must be calculated. If there were 600 eggs per gram (epg) of faeces in the pre-dosing sample and 200 epg in the post-dosing sample then the reduction is 600 - 200 = 400 epg and the percentage reduction is (400/600) x 100 = 67%.

To be considered fully effective a dose should reduce the egg count by 95% or more. In many cases this will mean reducing the egg count after treatment to 0. Therefore, in the example above the drench is not fully effective.

One thing to note is that if the pre-drenching sample has a very low egg count then there are very few worms in the lambs and efficacy cannot be reliably determined. An egg count of 200 epg pre-dosing would be the minimum required to check drench efficacy.

Given the widespread issue of anthelmintic resistance and the need for good worm control it is important to check drench efficacy as outlined above and to use a wormer that kills all the worms.

For further information see Anthelmintic Resistance or speak to your adviser or veterinary practitioner.

The Teagasc Sheep Specialists, Researchers and Teagasc Advisors issue an article on a topic of interest to sheep farmers on Tuesdays here on Teagasc Daily.  Find more on Teagasc Sheep here  Teagasc provides a Local Advisory and Education service to farmers. Find your local Teagasc office here