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Western Australia worms, flies and lice update - December 2018

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

WA State outlook

With the header back in the shed on most sheep properties, except perhaps on a few far-south properties, it’s time to think of holidays… but not before you’ve taken care of sheep worms!

As noted in the recommendations for sheep (and goat) worm control for Western Australia last month, effective worm control in summer is the key to sustainable and effective control for the coming year. The same recommendations apply this month—on most farms, summer drenches will have been given once the crop came off, but if not, it is important to ensure worm burdens are low in young sheep now. There is more leeway with adult animals, but whatever program is used, their worm counts should be low by early autumn.

Young sheep: summer drenches

Most young sheep mobs (this year, orange and white tags) need a drench in late spring or early summer and then go onto either a crop stubble or a dry pasture paddock. (Green pastures in summer are a particular worm risk—more worm egg count checks are needed.)

Weaners typically have significant worm burdens by the end of spring, as most will not have developed their full worm immunity, and worms usually accumulate over the first few months of grazing. Unless the lambs have gone onto a prepared low-worm paddock in spring (e.g. weaned onto a cattle pasture, hay paddock or forage crop), chances are that worms will be having some effect—whether or not it’s visible to the eye.

Last year’s lambs should have a reasonable worm immunity unless they did not have sufficient worm exposure (hard to predict), and in any case, there may still be some individuals not fully immune-competent (a genetic effect).

For mobs thought likely to have a very low worm burden and a drench not necessary, a check of worm egg count is recommended. Even a small leakage of worms onto pastures in autumn may lead to winter worm problems, so counts should be well less than an average of 100 eggs per gram.

In lower-rainfall wheatbelt areas, a drench to lambs at weaning may be sufficient to keep worm burdens low, especially if given late in the year. A routine summer drench will not always be needed for lambs in this zone, and often not at all for older sheep—this is easily shown by a worm egg count.

Ewes: what program?

Summer worm control in adult sheep is a key part of drench resistance management. The aim is to allow the survival of a small number of non-resistant worms that dilute the resistant worms likely to be present in summer-drenched younger sheep. At the same time, we don’t want excessive worm burdens in any mobs, or too many worm eggs to be dropped onto autumn pastures.

Adult sheep in good body condition have a strong resistance to worms and tolerance of their potential ill effects. This gives us options to ensure that resistance management doesn’t conflict with good worm control.

The two alternative approaches to sustainable control in mature sheep:

Option 1: Autumn drenching: instead of a summer drench back in late November or early December, delay drenching until mid-March or early April, which allows a small percentage of the worm eggs dropped onto pasture before this to develop to the larval stage. These can survive beyond that time, ensuring a small population of less-resistant worms in winter. This strategy has been used with success on many farms for some time, provided that the ewes were in good condition over summer and that this drench is given by mid-April.

Option 2: “Targeted treatment” at summer drenching: leave a small percentage in each ewe mob undrenched if they are drenched in summer. The worms surviving in these sheep provide a low-resistance population to dilute resistant worms in summer-drenched mobs (such as younger sheep). Trial work confirms that due to their high level of worm-tolerance, individual sheep in good body condition (3.5 or more) can stay undrenched for many months without problems. In areas where barber's pole worm is a low risk, 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.

Drenches must be effective

No long-term worm preventative strategy will work if the drench doesn’t—and if worms survive treatments in summer or autumn, they will have a head-start for the usual build-up over winter. What’s more, because worms that survive treatment will be drench-resistant, the level of resistance on the property will increase.

Unless using one of the newest drench types, a check of drench effectiveness is recommended after summer treatments. Worm egg counts can be taken from the mob when in the yards for drenching, and then again 10–14 days later, as a paddock collection. Anything less than a 95% drop (at a minimum) means the drench was not up to scratch.

So, Merry Christmas from the ParaBoss team, and we look forward to good parasite control in the New Year!


Esperance: Nicole Swan, Swan’s Veterinary Services (

Now that stubbles are available, it is a good time to check sheep before giving a summer drench. Weaners and hoggets are the most susceptible age groups to worms and should receive an effective broad spectrum drench. Worm egg counts will confirm the need for this, but regardless of how low (unless zero eggs per gram (epg)) are the egg counts, drenching in these classes of sheep is recommended to reduce the likelihood of a worm problem over the summer. Older sheep should be drenched if the average worm egg count for the mob is greater than 200 epg. If older sheep need drenching, a small percentage (5%) of them should be left undrenched (this number can be higher in areas that do not have a high barber's pole worm risk). Leaving some older sheep undrenched will ensure that there is a small number of a mixed population of worms left in the sheep. Then, if the summer drench is not 100% effective, the property is not populated with only resistant ‘super’ worms in the following winter. This phenomenon is called “refugia” (worms in refuge from drenches).

Recent counts from lamb/weaner mobs have confirmed that drenching is required, as worm egg counts have averaged between 300 and 525 epg. The majority of these mobs have been from coastal properties, and it is highly likely that there is a significant percentage of barber’s pole in them, as many properties have perennial pastures. Intermittent rain events have ensured continued pasture growth, and climatic conditions have been conducive to barber’s pole survivability.

Over the summer months, if sheep have been drenched onto stubbles or pastures are dry, worm survival should be low. In hot, dry conditions, eggs should not hatch and for those that do, their larvae should not survive. However, if there is a significant rain event over the summer months, and especially if there is some green pasture growth, sheep should be monitored 3-4 weeks after the event. For sheep grazing perennial pastures, monitoring is recommended if there are any signs of scouring, or a ‘tail’ in the mob when moved, or sheep are pale, or lethargic, or not doing as well as expected.

Wishing everyone a safe, Merry Christmas and a Happy New Year.

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