Slowing the development of anthelmintic resistance

The continuous use of wormers in sheep leads to drug resistant worms, known as anthelmintic resistance. Resistant worms are those that can survive a dose of the wormer that would normally kill them. Teagasc researcher Orla Keane Ciaran Lynch, Damian Costello and Michael Gottstein have advice

Main messages

• Infections with gastrointestinal nematodes (stomach and gut worms) reduce thrive in grazing lambs.
• Good intestinal worm control is highly dependent on effective anthelmintics (wormers).
• Anthelmintic resistance is defined as the ability of worms to survive a dose of a wormer that would normally kill them; resistance is now common on Irish sheep farms.
• In order to slow the further development of anthelmintic resistance 4 key actions are recommended:
• Do not treat adult ewes for intestinal worms unless there is a demonstrated need.
• Use only benzimidazoles (white wormers) to control Nematodirus.
• Quarantine treat bought in sheep with a wormer containing a new active.
• Use faecal egg counts to determine when to treat and what products are effective on the farm.

Introduction

Grazing sheep are naturally exposed to parasitic roundworms that live in the intestine and infection can result in chronic disease, ill-thrift and occasionally even death. Good worm control is highly dependent on effective worming products. However, a direct and unavoidable consequence of continuous use of wormers is the development of drug resistant worms, known as anthelmintic resistance. Resistant worms are those that can survive a dose of the wormer that would normally kill them.

Drug resistant worms

Despite the large number of wormers on the market, there are currently only 5 classes of wormer licenced in Ireland for the control of intestinal worms in sheep and all products fall into one of these classes. These classes are 1) benzimidazoles (white wormers), 2) levamisole (yellow wormers), 3) macrocyclic lactones (clear wormers), 4) amino-acetonitrile derivative (orange wormer) and 5) spiroindole (purple wormer). The orange and purple wormers are available on veterinary prescription only and are not commonly used. Wormers from different classes have different modes of action. However, within the same class all products share the same mode of action. Therefore, when resistance develops to one product within a class other products in the same class are generally affected. A complete list of all the products licenced for the control of parasites in sheep in Ireland and the class they belong to is available from your Teagasc adviser or here on the Teagasc website

A recent study carried out by Teagasc tested the efficacy of the 3 commonly used wormer classes on Irish sheep farms. Resistance to white wormers was found on 100% of farms tested, resistance to yellow wormers was found on 17% of farms tested and resistance to clear wormers was found on 61% of farms tested.

Strategies to slow the further development of anthelmintic resistance

Given the evidence for widespread anthelmintic resistance on sheep farms in Ireland it is important that sustainable strategies to manage gut worms and to delay the further development of anthelmintic resistance are now implemented. There are many steps that can be taken to slow the further development of anthelmintic resistance.

Four key steps, that are applicable on most Irish sheep farms, have been identified.

1. Older stock have generally developed good immunity to intestinal worms and so mature ewes should not require routine treatment for these worms. Lactating yearling ewes or thin or immunocompromised ewes may require treatment but this should be targeted to individual animals on the basis of demonstrated need. If mature ewes do require routine treatment for intestinal worms then contact your veterinary surgeon or Teagasc adviser as there may be an underlying health or nutrition issue that is making the flock susceptible to worms. Mature ewes may require treatment for fluke.
2. The first treatment administered to lambs in spring is generally to control for Nematodirus. This worm has a different lifecycle to the mid-season worms and anthelmintic resistance in Nematodirus is rare. In contrast, there is widespread resistance, particularly to the white wormers, among mid-season worms. Therefore, only use white wormers to treat Nematodirus in lambs and preserve the other classes for later in the season.
3. Resistance to all the commonly available wormers has now been detected in Ireland as have multi-drug resistant worms. In order to prevent buying in resistant worms in purchased sheep, it is important that all sheep coming onto the farm receive a quarantine treatment for intestinal worms.  Incoming sheep should be treated with one of the new anthelmintics (orange or purple) and housed for 48 hours. They should then be turned out to contaminated pasture recently grazed by sheep.
4. Use faecal egg counts to determine when to treat and what classes of wormer are effective on the farm. In lambs a group faecal egg count of greater than approximately 500-600 eggs per gram may have an impact on performance and may indicate a need to treat for gut worms. Ensure that the correct dosing technique is used and animals are treated according to the manufacturer’s instructions and dose rates. It is important to use an effective product and determining which anthelmintic classes are working on the farm is the first step in ensuring the right product is used. Discuss how to test which anthelmintic classes are working on your farm with your veterinarian or adviser.

Contact your vet or Teagasc adviser for further farm-specific advice on additional steps that can slow the development of anthelmintic resistance.