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Western Australia worm update - March 2014

Nicole Swan, Swan’s Veterinary Services, Esperance (nicole@swansvet.com)

We have had a few mm of rain yesterday but really it has been a dry summer for us here.

Overall WEC have been low (20-30 epg) However we did counts from a mob of lambing ewes with an average of 1115 epg (range 0 - 2400). None of these ewes are scouring. The farmer is going to drench them with closantal. These sheep were from a coastal property.

Brown Besier, DAFWA, Albany (brown.besier@agric.wa.gov.au)

Ewe worm control due now

Drenches for adult sheep (especially ewes) are due at the end of March or early April – presuming that you have moved on from summer drenching sheep of this class.

The aim of autumn drenching is 2-fold:

  • To remove worm burdens that have built up since last spring
  • To prevent pasture contamination with large numbers of worm larvae, ahead of the season’s break.

The program specifically aims to minimise the development of drench resistance. This is linked to summer drenching, as any worms that survive a summer drench become the main source of the future worm population. Delaying the drench until autumn allows worms not recently exposed to a drench to put out worm eggs, and some will develop to larval worms from late summer onward. However, we don’t want excessive numbers of them, hence the recommendation to drench by early April.

The program can potentially be more efficient by taking worm egg counts from each mob, and drenching only those over 100 eggs per gram. However, in the great majority of cases, drenching will be warranted, hence the general recommendation.  (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 - more research needed to explain this !)

Recommendation:

Either:  Drench all adult sheep with a fully-effective drench between the last week of March and mid-April.  (A “fully-effective” drench is as close to “100%” as possible - a 95% reduction in worm egg counts is the cut-off to indicate drench resistance, but is the lower limit for critical drenching times.)

Or:

Take dung samples for worm egg counts from all adult flocks and drench all flocks over an average of 100 eggs per gram.

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.

Note: summer drenches are still recommended for sheep under 2 years of age – if not given so far, either drench now, or take worm egg counts.