WA WormBoss Worm Control Programs
WA WormBoss Drench Decision Guides
It seems that an old-style typical winter worm pattern has set in, with a series of cold fronts following the early summer and autumn rains. As noted in earlier comments on parasites, what’s good for pasture and crops is good for worms and blowflies, and this is a year to be especially on guard for worm problems.
Worm egg counts through the laboratory have been both high and low recently. We always expected an early worm season, due to the early pasture growth, but suspect that many cases of high counts occurred where drenches given in summer or autumn were not fully effective.
Scouring: Reports of scouring are not unexpected, but it’s not always something a drench will fix. Scouring in a percentage of sheep is relatively normal when the pastures changes from dry to green, but it’s never clear how much is due to the sudden intake of lush herbage, and how much is due to worms.
What we do know is that sudden outbreaks of scouring may be a reaction to the intake of large numbers of worm larvae from pasture. This can occur as the natural immunity that sheep develop is re-set, after waning when sheep on dry pastures are not exposed to worm larvae. Normally, it seems that sheep face a trickle of worm intake from pasture after autumn rains, and their immunity re-develops with little drama. However, if they are exposed to a large larval intake—whether due to seasonal effects or a shift into an unsuspectedly wormy paddock—a proportion can experience an over-the-top immune reaction, leading to gut inflammation and scouring.
“Worm larval scours” is very similar to an allergic response after sudden exposure to an inciting substance, but this hypersensitivity settles down after a couple of weeks, and scouring usually lasts only a short time in affected sheep. The good news is that the immune reaction generally expels worm larvae, so they never become adult worms—but this means they have very low worm egg counts, and drenching won’t help.
How do we sort this out? A worm egg count will show whether sheep really do have a significant worm burden, justifying a drench, or if it’s a temporary response to worm larvae that will eventually cease, leaving the sheep with their full natural immunity to worms.
Where you see a few animals scouring badly, but most are clean, this indicates an individual genetically-based disposition. These sheep should be culled, as they will be repeat-offenders, as will many of their offspring.
Pre-lamb drenching: It’s important to keep worm burdens in ewes low prior to lambing, as the number of worm eggs they deposit onto pasture largely governs the likelihood of worm problems in their lambs.
We use the very low threshold of about 100 eggs per gram to indicate the need for a drench, best given a couple of weeks out from lambing. Ewes that were given an autumn drench often don’t reach this level, but the later that lambing starts, the more likely that a specific pre-lamb drench will be needed.
Barber’s pole worm: We’ve seen several cases of sheep deaths due to barber’s pole, often not immediately suspected. Typically, farmers report finding a few dead sheep over a week or so, with no indications of obvious disease. However, it’s easily confirmed by a worm egg count, or a post-mortem examination by a veterinarian.
Blowfly strike: There’s been numerous reports across the state, ranging from a few struck sheep to full-scale outbreaks. It will die down with the cold weather, but where there have been autumn strikes, it’s likely the flystrike risk will be particularly significant during spring.
After 47 years with DAFWA, Brown Besier is retiring in mid-July. During his many years with DAFWA, Brown has made major contributions to the improvement of worm control practices, not just in WA, but both nationally and internationally. In the last few years, we have seen his work produce practical and effective guidelines for targeted treatment to slow development of drench resistance and he has worked with Moredun Research Institute to produce the Barbervax vaccine, a hugely significant achievement with major benefit for producers where barber's pole is endemic. Fortunately, Brown will continue with parasite work as a consultant (Brown Besier Parasitology) and will still be a regular contributor to ParaBoss through monthly reports and his ongoing membership on the ParaBoss Technical and Steering Committees. ParaBoss wishes Brown all the best in this new phase of his career.
Recent worm egg counts have, in most cases, been fairly low with drenching not required. Counts on pre-lambing ewes identified the need for a pre-lambing drench to ensure against clinical outbreaks of worms post lambing. Most of these ewe mobs had not been drenched since last year.
In the majority of cases we recommended that ewes not drenched at the beginning of summer, receive a drench before the autumn break. The break came early this year due to significant rains early March. In most cases, it was recommended that ewes receive a Zolvix or Startect drench, as these are still fully effective. Now that everything is green, there will be a significant percentage of the worms on the pasture, and ewes that required drenching pre-lambing may still need to be drenched with a tactical drench. This tactical drench will lower the burden significantly, but will not necessarily be fully effective.
In areas susceptible to barber’s pole infestation, it is important to monitor regularly. Protection against barber’s pole pre-lambing with closantal is recommended. This drench has a protection period of 4 weeks (plus a further 3 weeks or so before eggs will again start to contaminate the paddock), so if given in the 2 weeks prior to lambing, ewes will then be protected for a major part of the lambing period. This will also reduce contamination of the lambing paddock with barber’s pole larvae.
For some farmers lamb marking is not far away. A worm egg count of the ewes prior to yarding will determine if they require a drench. The lambs should not require drenching as they should be too young to have been grazing long enough to have acquired a significant burden. If you are concerned that they have worms then a WormTest is recommended.