How to check if wormers are working on your farm

As livestock graze pastures they are constantly picking up gastrointestinal nematodes, also known as stomach worms. Orla Keane, Research Officer in Infection Biology, tells us how to deal with them.

These parasites cause disease, including scour and ill-thrift. Sheep develop immunity to stomach worms over time and usually have good immunity by 1 year of age; therefore, these worms are primarily a problem in lambs. Control of stomach worms is generally achieved by the administration of wormers. Despite the large number of wormers on the market, there are only 5 different classes of wormer for the control of stomach worms in sheep. These are white wormers (benzimidazoles), yellow wormers (levamisole), clear wormers (macrocyclic lactones) and 2 new classes, orange wormer (an amino acetonitrile derivative) and purple wormer (spiroindole). The orange and purple wormers are currently veterinary prescription-only medicines and not very widely used in Ireland.

In recent years it has been shown that worms are developing resistance to the wormers that we use to control them; this is known as anthelmintic resistance. Resistance is a particular problem in the white, yellow and clear classes of wormers. The overuse of wormers leads to the development of anthelmintic resistance. Therefore, it is important that wormers are used only when needed and not used too often or in an incorrect manner. If there is resistance to one product in a class on a farm then generally all products in that class do not work. Therefore, farmers have a limited number of different wormer classes available to them.  Lambs should always be treated with a wormer that is effective on the farm.  Given the prevalence of anthelmintic resistance in Ireland it is important to know what classes of wormer are working on your farm. The only way to do this is by testing.

The simplest way to test wormer efficacy is with a drench test.  Briefly, this involves collecting dung samples before treatment from a group of lambs and sending it to a lab to test for the worm egg count.  The lambs are then treated with the wormer to be tested.  Dung samples are collected a set number of days after treatment to check if the wormer used was effective. If it was, then the worms have been killed and there will be no (or very few) worm eggs in the dung sample after treatment. If some worms survive treatment then there will be worm eggs in the dung sample after treatment.

Instructions for completing a drench test

1. Select a group of 15 lambs at random and hold them in a clean pen. Place a mark or record the tag numbers to identify these lambs.

2. Allow 1 to 2 hours for the lambs to dung. Then collect up to 10 separate fresh dung samples and place them in faecal sampling containers.

3. Treat the lambs with the wormer to be tested according to the manufacturer’s instructions ensuring that the equipment is calibrated and the lambs are treated to the weight of the heaviest lamb in the group.

4. Send the faecal samples to the lab as soon as possible, preferably on the day of collection. If there is a delay prior to posting place the samples in a fridge (do not freeze or leave in direct sunlight).

5. Record the pre-treatment egg count (faecal egg count; FEC) when the results are returned from the lab.

6. Gather the marked lambs in a clean pen a set number of days after treatment (see below). Allow 1 to 2 hours for the lambs to dung. Then collect up to 10 separate fresh dung samples and place in faecal sampling containers.

• If testing a yellow wormer collect the second faecal sample 7 days after treatment
• If testing a white or clear wormer collect the second faecal sample 14 days after treatment

7. Send the faecal samples to the lab as soon as possible, preferably on the day of collection. If there is a delay prior to posting place the samples in a fridge (do not freeze or leave in direct sunlight).

8. Record the post-treatment egg count of the lambs when the results are returned from the lab.

9. Calculate the reduction in egg count after treatment as described below

Reduction = Pre-treatment FEC – Post-treatment FEC  x  100

                   ______________________________________

                                      Pre-treatment FEC

If the reduction is >95% then the treatment is effective. If the reduction is <95% then the wormer is most likely not effective on the farm. In this case, contact your vet or adviser for steps to take to slow the further development of resistance.

Example

Pre-treatment FEC: 750 eggs per gram (epg)

Post-treatment FEC: 300 eggs per gram (epg)

Efficacy = 750-300  x  100   

               _____________    =   60% = drug not effective

                        750