You might think that the principle of rotating horse drenches has been with us for as long as we've had drenches. Not so.
For a start, we were heading into the 1960s before we had much in the way of drenches that could be considered suitable for rotation.
Rotating drenches as a sound management practice became established during this period, but not for the reasons you might think.
Drenches were not effective against all parasites so the sensible course was to rotate them. Drench resistance was not then the problem it is today, but finding drenches capable of doing the whole job was.
Scientists began learning more and more about the resistance problem, but strategies for the most effective use of wormers were not really taking shape until the mid-60s.
Until then, most horse owners would drench their horses when they felt they needed it. Perhaps it was the pot-belly appearance or a dull coat or tail-rubbing that persuaded them it was time to invest in a tube of drench.
Of course, by the time such signs were showing in the horse the worm burden would already have been significant.
The recommendation that we drench horses every six to eight weeks came to us in 1966 when parasitologists Gene Lyons and Harold Drudge published a paper on the subject.
They looked long and hard at the life cycles of parasites and saw that, with drenches, horse owners could not only kill the parasites inside their horses but, with correct timing, could reduce the number of eggs being deposited in pastures when a horse passes dung.
Pastures with fewer eggs meant a slower rate of re-infection among the horses grazing it.
They recommended drenching horses with a particular drench combination at six to eight-week intervals. Their principles were adopted widely. Tubes of drench began carrying their dosing recommendations and horse owners began marking their calendars and dosing their horses regularly.
It made a huge difference. Horses that once carried major worm burdens had their parasite problem dealt with and it was kept under control long-term.
The principle of drench rotation remains important but times have, to a considerable extent, moved on. Drench resistance is now a much bigger problem than it was 40 years ago.
Rotation is still important and some horse owners rotate through a different family of drench every six or eight weeks. This is unlikely to be a sound strategy.
Firstly, researchers have yet to be convinced that rotating drenches for every drench cycle delivers any particular benefit and, secondly, it can make it much harder to establish the effectiveness of any particular drench.
Rotation is important, but not every cycle. Horse owners are best to use drenches they can show to be effective on their property, that target the right parasites, with very occasional rotation to slow the advance of worm resistance.
It is quite possible that an entire round or two of drenching is effectively a waste of time and money, due to resistance.
That is where faecal egg counts can prove so valuable. By monitoring the faecal egg count of horses, owners can over a period of weeks build a picture of the worm problem on their particular property.
Worm resistance is on the one hand a universal problem, but its prevalence across different countries and regions and worm species is variable.
Many factors will feed into the parasite picture on your property, including the pattern of drench use in preceding years and the comings and goings of horses and the parasites that they carry. Past management practices of the pasture will also have a big bearing.
Faecal eggs counts will also prove invaluable for those who choose to use natural or herbal worming agents. Egg counts will allow you to determine once and for all whether a particular worming strategy is successful - whether you're using a recognised family of drenches or a herbal concoction.
No matter what you use, if the egg counts are high or fail to drop adequately after drenching, it's time to change your tactics.
The following strategies will help in the fight against worm resistance:
* Never under-dose a horse, except on veterinary advice. A vet may recommend such a course to ride a young horse of a heavy ascarid burden. For accurate dosing, you will need an accurate estimate of your horses weight. The following calculation method may prove useful.
* Use your faecal egg counts to monitor the effectiveness of your drenching programme.
* Don't drench unnecessarily.
* Use pasture management techniques discussed here to minimise the re-infection rate.
* Quarantine new horses on your property and dose them with a good, broad-spectrum drench. The last thing you want to do is introduce drench-resistant worms to your property.
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We've discussed the issue of resistance, which is a parasite's ability to tolerate the chemical agent employed to kill it. If the resistant parasite gets to reproduce, it is likely that trait will be passed on to future generations.
Can horses themselves deliver a killer blow to parasites? A healthy immune system can overcome a raft of viral and bacterial infections. Why doesn't the immune system do the same with parasites?
Partial immunity does appear to develop in horses against some parasites.
In fact, in the case of two parasites, horses seem to be particularly effective in developing a high level of immunity.
Strongyloides westeri (threadworms) can be a major problem in horses up to six months of age but then are rarely present. Parascaris equorum (ascarids or roundworms) pose their biggest threat in animals up to two years of age, then largely disappear from a horse's gut.
Unfortunately, immunity is no get-out-of-jail-free card.
No horse owner should assume that a horse's immune system will deal with its worm burden. Nor should they think that supplements to boost a horse's immune system will in turn deal with the animal's parasite problem.
That said, individual horses will vary in their ability to handle worm infestation, and the effectiveness of their immune system will play a part in this.
We already know that some horses develop high levels of parasite infestation quite quickly while others tend to consistently return low fecal egg counts.
Clearly, some horses are more susceptible than others to worm infestation.
Evidence also suggests some horses are better at coping with a worm burden than others. In other words, two horses with similar levels of infestation may present in quite different ways. One may have a pot belly and dull coat, the other still appear quite healthy.
Using faecal egg counts, we can identify the more susceptible animals and tailor their worm-management programme accordingly.
So immunity is important in the worm-fighting equation, but it doesn't mean you can leave your drench in its packet.
Many drenching experts appear to agree that ensuring the drench you're using is proving effective is just as important - or possibly more so - than routine rotation of drenches. It is a sensible argument.
The introduction of broad-spectrum drenches containing a number of active anthelmintics has not only improved the efficiency of drenches, but arguably reduced the need for regular rotation.
The key is ensuring that the drench you're employing is doing the job. If you cannot establish that, you may not only be failing to deal with worm burdens adequately but you're potentially helping to grow worm resistance.
» Next: Meet the nematodes
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