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The Worms' Turn

Dan Salmon, Regional Veterinarian, NV1942

Sheep worms are a significant problem. Not just the scouring, weight loss and dying that they cause but the work and expense of drenching to control or cure them. Our Regional Veterinarian, Dan Salmon, gives some timely advice on "to drench, or not to drench...

West of the Newell Highway worms tend to be less of a problem than in the higher rainfall areas to the east. That does not mean that they are not a problem, just that they are not a problem every year.

The variation from year to year means that the people who do not drench have worm problems every few years, and the people who drench routinely waste a fair bit of time and money.

As well as the trouble and expense, drenching more than needed increases the chance of the worms that are there developing resistance to the anthelmintic being used.

Even if there are not many worms in a mob there will be some and it is probable that some of them will survive any drench. There will not be many, but they will be the parents of the next generation of worms and they will be a bit more resistant to the drench than the worms that the drench killed. Eventually the only worms left in a mob of sheep will be the descendants of worms that have survived a drench.

We can be pretty confident that no mob of sheep is ever truly free from worms because when the season is right for the worms (or wrong for the sheep) worms appear out of nowhere. Even worms that we have not seen for decades such as barber's pole worm or black scour worm in the far west will turn up in force.

Keeping sheep alive and slowing the development of anthelmintic resistance presents a balancing act. Moving too far in either direction is not good for the future. It is not much good having worms that are susceptible to all known anthelmintic if they are killing all of the sheep and it is just as bad to have sheep with hardly any worms if nothing will kill them: they will quickly become a lot.

There are a few things that will help to maintain the balance.

In the long term sheep that are resistant to worms offer the best hope. It is possible, but there is not much incentive for breeders to go down that path just yet.

In the short term the balancing act means being prepared to live with a few worms.

The first thing to do is to reduce the amount of drench worms we see. This is pretty hard in the high rainfall areas, but in the Riverina there are plenty of years when the worms do not cause much of a problem. Sometimes undrenched sheep may not do quite as well as drenched sheep, but they don't scour and they don't die and the anthelmintic will work for a few years more.

The key is to know which year to drench and which year to leave it. That is where worm egg counts come into their own. They are easy and cost about as much as drench for a hundred sheep (or two hundred or three hundred depending on the anthelmintic).

The other thing that makes a fair bit of difference to the development of anthelmintic resistance is the timing of the drench.

The original work on summer drenching had a recommendation that the first drench be given when pastures were just haying off. We gradually drifted further into the summer. We also drenched sheep onto stubbles because the stubbles had very few worm eggs in them.

A lot of this meant that the worms that survived the drench were the only worms that survived the summer and come autumn their descendants were the ones that bred up.

The answer is a little illogical, but once again it is a question of the sheep living with a few worms now or dying with a lot of worms later on.

One approach is called (for some reason) refugia. It involves not drenching a percentage of sheep during the summer. If the sheep left undrenched are those in the best condition they will generally hold that good condition but the next time worms breed up the worms in the undrenched sheep will provide some susceptible offspring and possibly mean the next crop of worms have a lot that will be killed by an anthelmintic.

The other approach is to drench late in the spring (when the pasture is just haying off!!) or during the autumn before it gets too wet or cold so that there will be a few susceptible worms surviving to contribute to the next generation.

This approach does mean that the worms will build up a bit quicker than would otherwise be the case, but they build up pretty quickly under the right conditions so t he difference is only a matter of a week or so.

Strategic worm control works well, but there is a bit of fine tuning needed to avoid problems in the future.