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Western Australia worms, flies and lice update - May 2019

Albany: Brown Besier, Brown Besier Parasitology (brown@bbpara.com.au)

WA outlook

Prolonged dry conditions across much of the agricultural areas will result in limited worm pick-up by sheep, but worms will be ready to go once effective pasture growth commences. It is important to check that sheep aren’t adding to the risk by putting significant numbers of worm eggs onto the pasture.

On dry pastures, worm eggs dropped onto the pasture in sheep dung can develop to the larval stage, but they need moisture for maximum development rates. Moisture is also critical for movement from the dung pellet onto the herbage, and the amount of green pasture is, therefore, a good guide to whether sheep are actively picking up a worm burden. 

In general, late-break seasons mean a shorter “worm year”, and depending on the weather in the next few weeks, we may have a relatively light worm year. Where pasture growth has started, however, the worm year will have started—this will be the case along parts of the south coast.

While sheep grazing dry pasture may not have significant worm burdens at present, there are a couple of caveats. If sheep have dropped to a lower than ideal body condition score, they will be more susceptible to the effects of worms when they eventually develop. Keeping ewes at the condition levels recommended by LifeTime Ewe guidelines (average close to Score 3 at lambing) will help reduce the size and impact of worm burdens.

Further, short pastures mean that sheep are grazing close to the ground, where most worm larvae are found. If there is even a small amount of green growth, the worm cycle will keep ticking along, and sheep may be carrying more worms than expected.

For these reasons, it is important to monitor worm burdens as we move into winter and the worm potential increases. In areas with perennial pastures or unusually heavy rain events, the risk will be higher, and on some properties, significant worm egg counts may have already developed.

Ewes

If drenched in autumn, counts will be low and no pre-lambing drench should be needed for mobs due to lamb in May and early June. A worm egg count check is wise either if lambing later than early June, or there is some doubt over the effectiveness of the most recent drench. 

If ewes were given summer drenches, ideally with a small proportion left undrenched (for drench resistance management), it is close to impossible to predict pre-lambing worm burdens. Chances are that these will be low, especially this year, but for late-lambers at least, maybe not.

Pre-lamb drenches are recommended if mob averages are above about 100 eggs per gram—this is a very low figure, but the aim is to reduce paddock worm contamination, so the lambs are not exposed to heavy intake. Many ewe mobs will have worm egg counts at this level or more, but it won’t be visible to the eye—hence the advice to check egg counts.

Weaners/hoggets

By early winter, worm egg counts should be done every 4–5 weeks, at least on some representative mobs of this age group, and especially where body condition scores are lighter than ideal.

If scouring occurs in more than a few individuals of this age group, it will almost always be due to worms—this can be confirmed by a worm egg count, but in most cases a drench is necessary. If there is little response to a drench, a worm egg count should be taken, and further advice sought.

Barber’s pole worm

In susceptible areas, the barber’s pole worm risk could last for another month. Although cold conditions prevent the progression of the eggs to the larval stage, larval development generally occurs at least until mid-June, and the larvae can remain on pasture for some weeks despite low overnight temperatures.  This explains outbreaks of barber’s pole worm disease in ewes in winter, when it usually seems too cold for this worm.

Adding to the risk, the temporary loss of worm immunity in ewes during lactation allows burdens of all worm types to increase, and for barber’s pole worm burdens the consequences can be fatal. In known barber’s pole worm areas, a worm egg count check on a few mobs is a wise precaution.

Hopefully, by the time of the next ParaBoss newsletter, there will have been decent rains across the southern part of the state, even if it does mean that worms will be on the move.