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Western Australia worms, flies and lice update - June 2021

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

With pasture conditions well ahead of the situation in recent years, the annual worm cycle has jumped ahead by a few weeks. Sheep farmers need to be on the look-out for problems usually not seen until later in the year.

In particular, it’s time to start monitoring WECs on a schedule that pre-empts possible outbreaks of worm disease and makes sure that moderate — but unsuspected — worm burdens don’t cause significant production loss.

Hoggets and worm immunity

Lambs need at least three to four months of continual intake of worm larvae above a certain level to trigger their natural immunity to worms. In highly seasonal climates such as in WA, worm larvae are abundant only from autumn to early summer, generally for six to eight months, depending on the year and location.

This means that lambs born earlier in the year, say May or before, usually have plenty of contact with worm larvae and develop a reasonable worm immunity before pasture senescence in summer. This will set them up for handling worms in the following autumn and winter. In contrast, later-dropped lambs often don’t have enough worm exposure and are still relatively worm-susceptible in the next year.

But there are no hard-and-fast rules. The exposure to worms depends on how well worms were managed in the ewes prior to lambing, and on the varying worminess of different paddocks. The effectiveness of drenches in summer and autumn also has a major effect on worm levels, but can’t be predicted without checking for drench resistance. We have to assume there’s the potential for worm problems in all sheep in their first year of life.

Monitoring worms in hogget-age sheep

When hoggets show the typical signs of worms while on green pasture — especially an increasing percentage of scouring — an immediate drench is almost always advised. As there are few other causes of these signs (in sheep on green pasture), there is usually little point in doing WECs. If there is no response to the drench, veterinary advice is needed.

However, the aim is to pre-empt worm disease, and avoid lost sheep production and all the problems related to dags. An effective WEC monitoring schedule starts a month or so after pasture growth begins and continues at five- to six-weekly intervals until early summer.

Ideally, average mob WECs should stay below about 250 EPG, as this is entry-level for worms to reduce growth in some of the mob, at least. If the mob is in good condition and there are no signs of scouring, counts at this level trigger a watching brief, with another count in a couple of weeks. Often, as immunity develops the worm numbers will drop and stay at a low level.

Worms will have a bigger effect in sheep not doing well, and mobs with an average body condition score below about 2.5 will significantly suffer more from worms. A drench is likely to be worthwhile once average counts exceed about 250 EPG.

Lambing ewes

There’s no change to the advice over the last couple of months: WECs need to be very low ahead of the lambing date, and a check about three weeks out gives time for a drench if needed. Anything more than 100 EPG should trigger treatment — and I’d expect that it will be justified in more ewe mobs than was the case last year.

Barbers Pole worm

Barber’s Pole worm needs warm and moist conditions, and the danger of outbreaks is increased when there’s an early start to the season. It’s especially serious in lambing ewes, as often there is no choice but to yard them for treatment, despite the risk of lamb losses.

The risk reduces greatly with cold winter temperatures, which prevent the development of Barber’s Pole worm eggs, but the larval stages can persist on the pasture for many weeks. This explains occasional outbreaks in ewes in July or even August, long after it would be expected to be too cold.

In years such as this, it is important to monitor Barber's Pole in ewes prior to lambing, especially in the areas where it is only occasionally a problem. If worm egg counts indicate it may be present, an effective pre-lamb treatment — possibly with a long-acting drench — may be a wise precaution.

Esperance: Nicole Swan, Swan’s Veterinary Services (

Recent WECs have been low, with all having averages between zero and 50 EPG. This is a good situation to be in as it will result in minimal contamination of winter pastures. This in turn will result in lower worm burdens and better growth rates in lambs.

Lamb marking is an important time to monitor worm burdens in ewes. At lambing an ewe’s natural immunity to worms drops and worm numbers can increase. This is especially the case with haemonchus (Barber’s Pole). If ewes are carrying high worm burdens, lambs will subsequently be exposed as they graze prior to weaning. In most cases lambs will not require drenching at lamb-marking.

Lamb marking is the time to start considering conducting a Drench Resistance Test (DRT). A DRT is conducted using undrenched weaners. A mob of an appropriate size (determined by the number of drenches you wish to test) can be identified and kept on the lambing paddock to ensure adequate worm burdens for the test. An average count of 300EPG is required for the test. It is important that these are scour worm eggs and not haemonchus eggs that you are counting. As both eggs look the same it is recommended that on properties in Barber's Pole (Haemonchus) areas, the weaners are given a closantel drench four to 10 days prior to the pre-test to ensure that you are only counting scour worm eggs.

If you are unsure about conducting a DRT or which groups would be of most value to test, it is recommended that you contact your local veterinarian for specific advice.

More information on DRTs can be found on the WormBoss website.

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