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Western Australia worms, flies and lice update - November 2018

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

With summer conditions upon us or coming soon, the main worm control issue across the state is to plan strategies to keep the worm risk low for the following months.

For a long time, “summer drenching” has been used to minimise worm burdens so that few worm eggs are shed onto pastures in autumn, with the aim of reducing worm numbers in winter and spring. However, we also know that drenching all sheep in summer can significantly increase drench resistance. Worm management strategies to get the best of both worlds are necessary, and possible.

Current-year lambs (orange tags)

These sheep are at the most worm-susceptible time of their lives, and need the highest level of worm protection. All should be drenched in early summer, either onto a crop stubble or onto a dry pasture (for pasture paddocks, allow a couple of weeks after it completely dries off to kill any worm larvae).

A word of warning: if the lambs have not been drenched for a few weeks and a summer treatment is not due by early December, worm egg counts should be checked. The lambs may already have significant burdens, leading to worm problems before a stubble or dry pasture paddock is ready.

This can be a problem when late rains extend harvest times, and especially where there is some perennial pasture, or if green growth takes time to die off, such as in higher rainfall areas. In these situations there is also the risk of barber’s pole worm, and with the potential for sheep deaths, worm egg counts are recommended at more frequent intervals than in lower-rainfall zones.

The cautions above apply to all breeds, including prime lambs—don’t risk a loss of slaughter weight due to an invisible, but significant worm burden!

Hoggets (white tags)

These also need a summer drench, as there is a variation in the rate of development of worm immunity, and some may still have significant burdens. However, a worm egg count may show that in fact their counts are low and no drench can be justified, giving an opportunity to reduce the selection pressure for drench resistance.

Adult sheep

As these typically have far lower worm burdens than younger sheep, and are also more resilient to the effect of worms, there is some latitude to use them to manage the development of drench resistance. The aim is to ensure the survival of some worms that are not resistant to drenches, so they can dilute the number of resistant worms that survive summer drenches.

(Editor’s note: In any sheep, a proportion of the worms (hopefully only a small percentage) will be resistant to a specific drench and the remainder will be susceptible to that drench. By not drenching some adult sheep in a mob, the susceptible worms live on and contaminate the pasture with more susceptible worms. If all sheep are drenched then only the resistant worms survive to contaminate the pasture. When pastures are dry and only resistant worms live in the sheep over summer and become the only source of new worms to contaminate the pasture at the break of the season, then the overall population of resistant worms on the property can rise rapidly.)

There are two main options:

  1. Autumn drenching: delay drenching until between mid-March and mid-April, as by then a small percentage of any worm eggs dropped onto pasture will develop to the larval stage. This strategy has been used with success on many farms for some years, provided that the ewes are in good condition over summer and the drench is not left too late. For early-lambing ewes, this drench also doubles as a pre-lamb treatment.
  2. “Targeted treatment” for summer-drenching: leave a small percentage undrenched—specifically adult sheep. Trial work and field reports confirm that individual sheep in good body condition (3.5 or more) have a high level of worm-tolerance and can stay undrenched without problems. A minimum of 10% of a mob should be left undrenched, and ideally, 20% or more—those in especially good condition are easy to spot as you move along a race.

If in doubt, a worm egg count will indicate the most effective and safest strategy, and confirm that it is on track over the coming months.

Drench resistance testing

Many drenches previously expected to provide good control are no longer fully effective. Worms resistant to the widely-used abamectin are now known to be present on a good two-thirds of Western Australian sheep properties, and to its bigger brother, moxidectin, on a quarter or more of farms. Resistance appears to be still uncommon to the “triple combination” (ML—Macrocyclic lactone and BZ—benzimidazole ('white') and LEV—levamisole ('clear')) drenches, but some cases have been seen, and they should also be checked.

A full drench resistance test involves drenching small groups of sheep with different drench types, and comparing their worm egg counts after treatment with counts in an undrenched control group. Testing is ideally done in lambs that have not been drenched for at least a couple of months, and depending on when drenches were last given, some may be suitable for an early-summer test. Next best are last-year’s lambs and a worm egg count will show whether counts are high enough for a test. It is best to talk to a consultant or veterinarian to plan a drench test.

An alternative is to simply check worm egg counts before and after a drench by taking samples when sheep are in the yards for drenching, and then from the paddock 14 days later (no need for a second yarding). Over time, several drenches can be checked for effectiveness, however, this does not indicate how well the individual actives within a combination drench works, which is valuable information if you are using combinations to help manage development of resistance.

There can be a significant penalty for using a partially ineffective drench in summer or autumn: not only are control programs less effective due to surviving worms, but as these will be resistant to the drench used, the level of resistance on the property will increase.


Goats kept in paddock situations are especially susceptible to worms, and they can pick up all the worm species carried by both sheep and cattle. The general pre-emptive strategies recommended for sheep apply to all breeds of goat, as it is especially important to keep worm populations low so that the usual increase in winter comes from a lower initial base. As in sheep, worm egg counts are a major tool for monitoring worm burdens and the success of control tactics.

However, there are only a few registered drenches available for goats, and while other types are commonly used, this “off label” use should be on veterinary advice. In addition, goats require higher dose rates of drenches than for sheep or cattle, and again, advice should be sought from your veterinarian regarding both the use of drenches, appropriate dose rates and their place in a preventative program.


With some recent cooler weather and late rains, the flystrike risk will remain present for a bit longer, especially along the south coast. It may be cold comfort, but where harvesting is on hold due to mild weather, there may be time to check the sheep !


Esperance: Nicole Swan, Swan’s Veterinary Services (

Recent rain and sunny weather means that the pastures are cranking. Harvest has started in parts of the district, but rain has slowed things down.

Worm egg counts have been variable over the last six weeks. Mob averages have ranged from 5 eggs per gram (epg) of faeces for ewes drenched late in March with Startect® (abamectin and derquantel), to 1555 epg for undrenched lambs on a coastal property.

Weaners from a well-managed property and tested prior to a drench resistance test (DRT), had an average worm egg count of only 35 epg. To conduct a valid DRT, an average count of 300 epg is required after eliminating the barber’s pole count.

Barber’s pole worm (Haemonchus) should be eliminated prior to testing as in WA, all drenches will kill it, and if the sheep have mainly barber’s pole, the majority of worms in all sheep will be killed and the drench will appear better than it is.  At the start of the test, the sheep should be drenched with closantel, a narrow spectrum drench that kills only Haemonchus. This drench will not interfere with the test.

If you are considering a drench resistance test on your weaners, it is recommended to contact your veterinarian prior to starting the test for specific farm advice.

Summer drench time is fast approaching. Ideally sheep requiring drenching will be drenched with an effective drench onto stubbles. All weaners and hoggets should receive a summer drench. For older sheep a worm egg count is recommended. Mobs with counts greater than 200 epg should be drenched.

Drenching at this time of year has a strong selection pressure and using an ineffective drench will contribute to drench resistance, as worms that are not killed by the drench will become the main worm population on the property. For summer drenches to adult sheep,  it is recommended that 2–5% of each mob are left undrenched to ensure that a mixed population of worms survive. This is called refugia and is an important aspect of overall farm worm management.

Editor’s note: In areas where barber’s pole worm is not a risk, 10–20% can be left undrenched, but where barber’s pole worm is a known problem, fewer sheep should be left undrenched.

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