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Western Australia worm update - July 2013

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

This month we have seen a number of cases of scouring sheep. FWEC averages have been between 30 and 1110 epg. Most of the high counts have been in hoggets. In 1 case the sheep had been drenched with an ML drench 5 weeks previously and then put onto a lambing paddock.
It has been periodically raining since March and until the last few weeks the temperature has been quite mild. A lot of properties especially along the coast are very wet.
 

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

As we are now mid-way through winter, sheep worms will remain a risk until late spring or early summer, especially in lambs.  Worm development won’t be limited by environmental conditions, as cool temperatures and moisture favour worm egg development, and even overnight frosts have only a minor inhibiting effect. Conditions only become unfavourable for worm development once hot and dry weather sets in, and pastures become dry.

The extent of the potential worm problem is governed by the level of pasture contamination with worm eggs, and with the early season rains in most districts, the worm risk started earlier than in most years. Whether the potential risk translates into a problem on a particular farm is determined largely by the effectiveness of previous preventative measures, especially the summer-autumn drenching program. Even if given at the most effective time, drench resistance may reduce the impact on worm numbers later in the year.

Unless there has been a recent drench test, the effectiveness of drenches is rarely known for certain. Resistance is well-established to all drenches other than the newly-released monepantel (“Zolvix”). We can no longer assume that moxidectin (“Cydectin”, Moximax”, Moxitak”) will be fully-effective, and some cases of resistance have emerged to the “triple combination” drenches (a white, clear and ML drench).

Recommendations:

  • Do worm egg counts on hoggets not drenched within the last 6 weeks. 
  • Visually check ewes for signs of worms, and if significant scouring is present, do a worm egg count. It may be feasible to drench at lamb marking – or if the count is low, the problem won’t be drench-responsive.
  • Plan a drench resistance test on weaners – ideally, at weaning time.