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Western Australia worms, flies and lice update - October 2020

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

Until paddocks are fully dried off and temperatures are consistently hot, the main sheep worm control priority in WA is to prevent severe worm disease in the current-year’s lamb drop. While there is still some green pasture present, worm larvae will survive and pose a potential risk to young lambs.

One of the few upsides of the risk is that worm egg counts (WEC) will be high enough for a drench resistance test (i.e. DrenchCheck), presenting an opportunity to ensure that the drenches intended for the up-coming summer treatments will be fully effective.

Lambs yet to be weaned

As noted in earlier articles, lambs should almost always be given a drench when they are weaned. At three months of age or older, this is the most worm-susceptible time of their lives, and even relatively low levels of intake of worm larvae will lead to at least moderate worm burdens.

Should WECs be checked before drenching at this time? In most cases there is little point, as even if counts are on the low side of usual drenching thresholds, the chances are that they will be carrying immature worms and WECs will increase as these develop.

An exception to this is where specific action has been taken to keep worms in the lambs as low as possible by the pre-lamb treatment of the ewes. This could be either where they received a fully-effective drench before a move to a (known) low-worm paddock, or a long-acting drench treatment. Even in this case, it’s a good idea to check worm counts to ensure that the intended worm-control effects have been realised. (Note: both of these pre-lamb tactics can also increase the level of drench resistance unless managed specifically to avoid it, and it is worth talking to an adviser to get the best of both worlds.)

Lambs already weaned

Lambs weaned earlier in the year will still be susceptible to worms. If no drench was given at weaning, do so now, or at least check WECs!

Regardless of when lambs were drenched, WECs should be checked every 4–5 weeks during the spring months. In many cases, no further treatment will be needed between weaning and the usual summer drench (on dry pasture or crop stubbles). This may apply to much of WA, due to the dry year across most of the state. But there will also be situations where lambs pick up significant worm burdens between weaning and summer, and additional treatments are warranted.

Testing for drench resistance

As noted earlier, lambs at weaning present an ideal opportunity to test for drench resistance.

The recommendation to conduct a full drench resistance test (i.e. DrenchTest), to check several drench types side-by side in a single test, will always remain. If done every 2–3 years, it ensures that changes in the list of effective options are shown in good time.

However, a good alternative is to run a series of DrenchChecks. Samples are taken firstly when sheep are in the yards for drenching, and then two weeks later (as paddock samples). Comparing the counts gives a good idea of whether the treatment was effective. If some worms are present, it is worth talking to an adviser regarding whether samples should be sent to a laboratory for worm species identification, as typically, only some will be resistant to the drench. (See WormBoss for details on resistance testing.)

Barber’s pole worm

Problems with barber’s pole worm are rarely predictable in WA, but are especially difficult at this time of year. The two key requirements for this worm are some moisture at ground level and relatively warm temperatures. In most sheep areas of WA, pastures are dry by the time conditions are sufficiently warm, and we rarely see barber’s pole worm at any time.

However, as most sheep producers in areas with a history of barber’s pole worm outbreaks will know, deaths due to barber’s pole worm can occur with little warning, especially in lambs. The risk is highest in years when temperatures are higher than normal while pastures are green, or when pasture dry-off is delayed due to late rains. Avoiding unpleasant surprises means more frequent checking of WECs in these areas compared to areas where the worm is not commonly present.

Lice treatments

Many sheep will be shorn at this time of year, and on many properties a lice treatment is given as a routine. However, it is worth asking: is this justified? If treatment in the previous year was successful, and no new introductions occurred from either purchased sheep or strays from other farms, treating for lice may be a waste of effort and funds.

If treatment is necessary, the questions are whether a particular product will be effective, and how should it be applied for best effect? A number of factors determine the infestation risk, and resistance by lice is widespread to some chemical classes. The LiceBoss website has all the information needed to help navigate the decision pathway.

And blowflies …

It goes without saying that vigilance for flystrike is high on the sheep management agenda at this time of year, until conditions dry off. Checking the FlyBoss website will bring you up to date on the most efficient prevention and treatment options.

Esperance: Nicole Swan, Swan’s Veterinary Services (

With warmer weather the pastures are starting to take off. Unfortunately worms thrive in the same conditions. Worm egg counts (WEC) to date this month have been from this years lambs. In all cases counts have been significant.

On one property lambs were drenched with Startect in August. Unfortunately they were drenched onto a worm-contaminated pasture and six weeks later counts averaged 1,440 eggs per gram (epg).

On another property counts were being conducted as a pre-drench resistance test (DrenchTest) count. These sheep had received a closantel drench six days earlier to rule out any Haemonchus (barber’s pole) from the pre-test count. These weaners had an average of 895 epg with counts as high as 7,200 epg. It is interesting to note that for the last five years this property has been doing pre-DrenchTest counts and despite multiple counts each year has not had enough eggs to conduct a test. On this property regular monitoring of sheep has resulted in good worm control.

To conduct a statistically significant drench resistance test, the test sheep require a minimum average WEC of 300 epg of scour worms.  Therefore, a pre-test drench with closantel to rule out barber’s pole will ensure that all eggs counted are scour worm eggs.

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