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Western Australia worm update - March 2013

Nicole Swan, Swan’s Veterinary Services, Esperance (

Early March saw a big rainfall event with much of the Esperance district receiving 50mm rain. This was followed by warm sunny weather. Since then, we have received a regular few mm’s. This has been great for those with perennials but also great for the worms.
Recent counts have revealed that some properties need to drench. Samples sent for a Lectin test for barber’s pole show that this is a problem on some properties. Given the recent weather conditions this was expected.

Brown Besier, DAFWA, Albany (

Autumn is a key time for sheep worm control in Western Australia. The ‘summer-autumn drench’ program depends on summer drenches for young sheep, but delaying drenching of adult sheep until autumn.

Recommendation: Drench all adult sheep with a fully-effective drench* between the last week of March and mid-April.

This minor change from summer drenching all sheep is a major step towards reducing the development of drench resistance. It allows a small number of non-resistant worms to survive so that resistant worms don’t become dominant in the total farm worm population.

To fine-tune this program, you can take worm egg counts from each mob, and drench only those over 200 eggs per gram. However, in the great majority of cases, the counts will warrant drenching. (The reason for this is that although worm burdens in adult sheep are usually extremely low in early summer, they usually rise during late summer as dormant immature worms develop and start laying eggs. We need more research to explain this!)

If you did give summer drenches to adult sheep: take worm egg counts now, and drench if counts are over 50 eggs per gram. This is a very low level for drenching, and such a small worm burden will have no sheep production effect. However, it is essential to remove these worms as almost all will be drench resistant, and we don’t want to start the worm-control season with a high level of resistance in the worm population.

A seasonal note: Rains over much of the agricultural areas in mid-March have led to widespread pasture germination. With relatively mild temperatures, worm egg development will produce worm larvae earlier than in most years. Even if dry conditions resume, the larvae will stay in sheep dung pellets for many weeks, and emerge when there is green pasture and moisture at ground level.

* What’s a “fully-effective” drench? As close to “100%” as possible—we use a 95% reduction in worm egg counts as the minimum to indicate drench resistance, but that is the lower limit for what is needed at critical drenching time.

Jim McMahon, Northam Veterinary Centre (

Recent rains within the Central Wheatbelt and the dew now being found in the mornings has set the right conditions for larval worm development. If follow up rains are received in the approaching autumn break of the season then it is time to assess the potential risk of worm burden and if drenching is required, as this can vary with age, paddock rotation, condition score and pregnancy. It is safer to test rather than run the risk of worm contamination in young pregnant ewes, with the benefit of follow up advice. Call in and pick up your faecal sample kits so that they are on hand and ready for the break of the season!

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