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Resistance to drenches in sheep internal parasites

This material addresses aspects of the following syllabus outcomes:

A student:

H2.1 describes the inputs, processes and interactions of animal production systems.

The work presented in the following section contributes towards understanding the following syllabus content areas:

the problems of pesticides and chemical resistance in target organisms
the importance of agricultural chemical labels as they relate to safe practice and correct usage.

Extract form Stage 6 Agriculture Syllabus NSW Board of Studies 1999

Resistance in sheep internal parasites

Lets go back to the February 1998 Farming Ahead article, 'Effective drench use reduces resistance' (this article was also used in the section Internal parasites in sheep.)

"Evolving resistance"

There are several reasons identified which contribute to the development of resistance to drenches by worms. Overuse of drenches is one of the most obvious causes.

For many years treatment for worms was fairly ad hoc with producers drenching up to 12 times a year, often treating the symptom of scouring sheep not necessarily attributed to worms.

This problem has largely been addressed by producers after extencive publicity by State Departments of Agriculture over the past 10 years that frequent drenching helps the development of resistance.

Underdosing has also been identified as a major contributor to the selection for drench resistance.

Again the need to weigh sheep, calibrate the drench gun and dose to the heaviest in the mob has been adopted by most producers.

Underdosing is one of the ways worms can be exposed to sublethal doses encouraging the selection for resistance.

Poor drenching techniques is another way to encourage resistance which has received less attention and adoption.

If the drench gun is placed over the tongue and the drench is swallowed, the majority of the drench will reach the rumen and allow maximum utilisation of all the active ingredient.

Repeated use of the same drench group increases the rate at which resistance develops. This problem is lessened by rotation between all of the effective drench groups on an annual basis.

Understanding drenches

Understanding how drenches come into contact with worms will help to improve the efficacy of the drench.

After a single dose of an oral drench the drench is absorbed from various parts of the gastrointestinal tract and excreted from the sheep's body.

It is critical that a lethal concentration of the drench remains in the sheep's body long enough to kill all the worms.

If the drench is excreted too quickly worms can be exposed to a sublethal concentration of the drench with a similar effect to underdosing.

Slowing the movement of the gastrointestinal contents increases the exposure of the worms to the drench and therefore increases the efficacy.

Keeping sheep off feed for 24 hours before drenching will slow the movement of feed and improve the effect of white and ivermectin drenches.

This effect is particularly evident when sheep are on green, highly digestible feed. Sheep need to be provided with water while restricting feed intake.

Worm control options

Breeding sheep with resistance to worms is seen as one long-term solution to worm control

Programmes have now been developed to a stage at which stud producers can select for ram lambs with a reduced faecal worm egg output.

It is unlikely that a new drench group will be developed in the short term so best use of the current drenches is required.

Integrate their use with whole farm planning to minimise worm population build up and reduce the reliance on the drenches.

Activities

The first paragraph in the Farming Ahead article above, claims that the overuse of drenches is one of the most obvious causes of the development of resistance to drenches by worms. What do you think is meant by 'overuse of drenches'?
Find out what is meant by ad hoc, so that you can explain what is meant by 'For many years treatment for worms was fairly ad hoc'.
Contrast ad hoc drenching with the Wormkill program that is described in the Controlling sheep internal parasites section. Describe how these two methods differ.
What do you think would occur if a farmer who was experiencing problems of worm resistance to a drench, failed to change the type of drench he or she was using?
What do you think 'dose to the heaviest in the mob' means?
What is a 'sublethal' dose of drench?
This Farming Ahead article suggests another method of controlling worms, a long-term method of worm control. What is this method of worm control?

Answers

Now try Interpreting a drench label.

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