DRENCH RESISTANCE

Resistance is a significant problem in maintaining effective parasite control in livestock. Reports demonstrate that at least 60% of Australian sheep farms have resistance present in one form or another.

The problem of resistance is that you can’t see it. You may use your drench ‘faithfully’ with effective results, and believe that you don’t have to worry about resistance.  Yet resistance is a problem that affects every farmer and it is a problem that will not disappear.

Drenches play a vital role in farm production and in animal welfare, and that’s why farmers have to take measures to ensure the long-term viability of current available drenches. A short definition of drench resistance is “the emergence of a parasite strain that is no longer killed by a particular class of chemical at the recommended dosage”.  

Significance of resistance

The level of resistance means different things to different people. A farm is said to have drench resistant parasites when the normal dose of a drench fails to reduce the FEC of a group of sheep by more than 95%.  This criterion is generally recognized by research workers and veterinary diagnosticians for declaring the presence of resistance. There are some limitations to this guideline.  For example, the presence of a worm species that the drench would not normally be expected to adequately control.

The farmer wants to know what drench resistance means to him. The answer to this question was specifically investigated in Australia, where productivity was compared in young sheep exposed to moderate parasite infections for which drenches with varying levels of efficacy (100%, 85% and 65%) were used. The results are sobering. The greatest difference, was seen between the highly effective treatment and the least effective, which translate into a reduction of approximately Aus$4.50/head in wool production.  There was also a substantial difference (Aus$1.25/head) between the middle and least effective treatment.

In Australia at least, the experts in the field of resistance detection and management know that producers generally do not recognize the significance of resistance in their flock. Despite the evidence that the overwhelming losses due to parasites are sub-clinical, which have been recently estimated to be in the order of Aus$150 million annually to sheep farmers in Australia. They simply consider that worms are a problem only when signs of clinical parasitism are apparent.

Benefits of drench use

It is easy to lose sight of the benefits that a well planned parasite control program provides, because there is a lot of publicity surrounding drench resistance. Perhaps the most vivid illustration of the effects on sheep production in the absence of such a program was from a series of New Zealand trials - production potential was affected through:

• Increased deaths – 23% average over first three years
• Lower fertility – 20% reduction
• Reduced wool production – 35% reduction
• Poorer liveweight gains – 43% reduction over first year of life.

Such figures are not only an indication as to how drench use can benefit, but also serves as a timely reminder of the need to avoid resistance through the wise use of drenches.

Management of resistance

To gain these benefits on a long-term basis a sound policy of resistance management is necessary.  Resistance cannot be eliminated. Therefore, the goal must be to manage existing resistance in such a way as to ensure that no rapid escalation in the extent or spectrum of resistance occurs. The following measures are considered to be the ways to achieve this:

• Strategic drenching

Strategic drenching are treatments given at critical times of the year in relation to the epidemiology of parasite infection, often in association with some form of animal management. The aim is to maximize the effect, and thus reduce in number, of treatments required to achieve effective control.

• Pasture management

It is beyond question that greater efficiency of parasite control can be achieved if drenching is combined with some form of grazing management, whereby treated animals are moved to pastures of lower infectivity.

• Monitoring

Important benefits can be gained by monitoring changes in parasite burdens by means of feacal egg counts. Effective monitoring is a key requirement in managing anthelmintic resistance.  Veterinarians are able to provide a number of diagnostic tools to monitor the effectiveness of drenches.

• Preserving Efficacy

Drench efficacy can be preserved by:

• Making use of combination drenches - Formulated combinations of benzimidazoles and levamisole have been available for several years and are useful when the individual components are no longer effective. Computer predictions have shown that the use of cocktails of anthelmintics, in which all drugs have high levels of efficacy, are the most powerful means of maintaining long-term drug efficacy.
• Ensuring sheep always receive a full dose of drench. Ensure the drench gun is working accurately and test it often during the drenching session.  Weigh sheep to ensure the appropriate dose is being used
• Using correct drenching technique to ensure sheep do not spit out the drench.
• Quarantine drenching bought in sheep with a combination drench and/or an endectocide drench to ensure resistant parasites are not introduced to the property.
• Check treatment efficacy before the introduction of new treated stock
• Do not use expired products and respect storage recommendations

Types of resistance

The parasite population on a farm is made up of many different types of worms. When resistance occurs it is highly likely that each of these types will be affected to a varying degree. This can lead to quite complex resistance patterns within the overall parasite population. Knowing the drench resistance status (if any) is present on a farm is therefore essential to planning an effective drenching program. Types of resistance are:

• Single resistance

A situation where the farm has worms that are resistant to a single drench group only. If more than one worm type is involved in the resistance the term ‘multi-generic’ may be used.

• Dual resistance

This occurs when the farm has some types of worm that are resistant to one drench group and other (different) worm types resistant to another group. For example, in sheep the benzimidazole and levamisole drench groups are implicated in such resistances on 60% of Australian sheep farms. 

• Multiple resistance

This describes a situation where a farm has one or more types of worm that are simultaneously resistant to two or more drench groups. On farms with more multiple drench resistant worms, the specific nature of the resistance will determine whether the combination drenches are effective or not. Only drench testing can determine this.

• Side resistance

Side resistance occurs when any member of a particular drench group does not kill resistant worms. For example, when a strain of worm is resistant to albendazole it will also be resistant to all other benzimidazole white drenches.   This is the rule rather than the exception.

The task of achieving sustainable nematode parasite control in grazing livestock not only rests with the farmers but also with the veterinary chemical industry, research and extension workers. Consumer education is also important, because these changes will inevitably be associated with an increase in commodity price. However, on the basis of precedent with the success of various integrated pest management schemes in insect pests, there are grounds for optimism that effective parasite control programs which slow the development of resistance will be achieved in the future.