Three steps to slowing down the development of Anthelmintic Resistance on Sheep farms

Anthelmintic resistance is a major and growing problem on Irish sheep farms. Head of Sheep Michael Gottstein outlines the actions that sheep farmers should take to slow down the development of Anthelmintic Resistance.

Figure out what wormers are still working on your farm

Irish research has shown that anthelmintic resistance is a major and growing problem on Irish sheep farms. That means that the wormers are no longer killing all of the worms inside in the sheep’s digestive tract. It is not the case that the wormers have changed, it is the worms that have changed. They have developed the ability to survive coming in contact with a wormer that would normally kill them. This is what we call anthelmintic resistance. It is the ability of the stomach worms in the sheep’s digestive tract to survive a dose that would normally kill them.

Faecal egg counts

Working out what products still work on your farm is pretty simple. It involves doing two faecal egg counts either one week or two weeks apart. The first faecal egg count is to identify the number of worm eggs that are present at the time of dosing. This will be the reference number. Once the animals have been treated for a period of seven days in the case of Levamisole (yellow) based products or 14 days in the case of either Benzimidazole (white) or Macrocyclic Lactone (clear) based products a second faecal sample should be taken to see if the egg count has been eliminated or sufficiently reduced. We are looking for a reduction of at least 95%.

It is important that we take faecal samples from at least 10 lambs and it needs to be the same 10 lambs that are sampled on both occasions. Therefore remember to identify the lambs with spray marker when they are first sampled.

Don’t dose your ewes for stomach worms unless there is a demonstrated need.

Dosing ewes for stomach worms significantly increases the selection pressure for resistant worms. In the absence of a demonstrated need - hogget ewes rearing lambs, Haemonchus Contorus infection identified etc. there is no benefit in terms of improved performance from treating adult ewes for stomach worms. The big down side of treating ewes for stomach worms is that from that point on and for a period of time - depending on the product used, the only worm eggs being passed out by the ewes are from worms that have survived the treatment - resistant worm eggs. These eggs will hatch into resistant worm larvae and may then infect lambs where treatment will be necessary to maintain performance.

Avoid inadvertent treatment of ewes for stomach worms

Also make sure you avoid inadvertent treatment of ewes for stomach worms. This is the most common way in which ewes are treated for worms. For example ewes are treated for liver fluke infection which is warranted but the product used is a combination product - fluke and wormer. So now all the ewes have inadvertently been treated for worms as well even though this was not intended. Another example is where ewes are treated for external parasites - scab or lice, using and injectable macrocyclic lactone and again here the product being used will also have anthelmintic activity. It will select for resistant stomach worms despite this not being intended. Alternative treatments such as plunge dipping may be more appropriate.

So to avoid inadvertent treatment of ewes for stomach worms we need to ensure that we use products that are targeted only at the conditions that we are trying to treat. So for example if we are targeting liver fluke then use only a product that will kill liver fluke

Take care when you are purchasing sheep.

Stomach worms can only move in the digestive tracts of sheep. When buying sheep or moving sheep onto your farm from another holding always be conscious that they may be carrying worms that are resistant to anthelmintics. Preventing the importation of resistant stomach worms onto your farm can be done by following a simple three step procedure.

1. Quarantine drench purchased sheep using either of the following suggestions given sequentially: • Zolvix and either Levamisole based product or a Macrocyclic Lactone based product. Using Startect would also be an option but it is currently not available in Ireland.
2. House any sheep for a period of at least 48 hours after treatment and arrival on your farm
3. Turn the sheep out onto dirty pasture that has been recently grazed by sheep to help to dilute the effect of any resistant parasites that may have survived the quarantine treatment.

The development of anthelmintic resistant parasites is a given on all sheep farms. It is a natural phenomenon that occurs. What is however under the control of each sheep farmer is the speed at which resistance develops on their own farm. This can be as short as four years or as long as 50 years. Therefore taking a few simple steps can help sheep farmers to identify where they stand in terms of resistance to the three commonly available active ingredients and also steps that they can take to prolong the efficacy of usefulness of the commonly available wormers on their farm.

More information

Watch the videos below and visit the Anthelmintic Resistance section of our website.