So you got your results back + it showed your horse has a worm burden!
And you have been told not to worm your horse!
Why don’t you worm your horse?
Worms are becoming resistant to the active ingredients in dewormers.
And as a result, the wormers are becoming less + less effective!
There is always part of the parasite population when worming that doesn’t come into contact with the drug during treatment.
This can be because:
- the eggs + larvae are on pasture
- the parasites are in untreated horses
- the parasites, in horses, are in stages where they are not affected by treatment. For example, in the encysted stage when treated with a non-larvicidal drug
This stage is known as Refugia.
REFUGIA HELPS OUR HORSES?
We know that resistance to current worming drugs is extremely high.
If we can INCREASE the number of worms we have in refugia we can DECREASE the rate of resistance.
Some people want to worm their horses even when they have a low burden (100/150/200 eggs per gram).
But this is the whole point of refugia – we don’t want to worm them at a low level.
We need those worms to be in the population to use them to help break future life cycles.
In all worm populations, some worms are resistant to a drug at the time it is given + some are not.
Those that are resistant will survive coming into contact with the drug.
They will produce more worms that are in turn resistant to that drug.
So, the more horses that receive the drug, the lower the number of worms killed by that drug + resistance grows.
Worms in horses that are not treated because they have a low worm burden, + worms on pastures, are ‘in refugia’.
As they are not given a drug, all of them survive.
If they were to come into contact with a drug, some would be resistant to it + survive + some would not.
But their resistance hasn’t been ‘decided’ through exposure to a drug.
The unresistant population will grow to outnumber the resistant population.
Which is what we want to happen!
WHAT CAN WE DO TO INCREASE REFUGIA?
Pasture
We can treat against worms less often when the number of parasites on pasture is lower. This being when the weather is either extremely hot or cold.
We no longer recommend moving a horse onto new pasture immediately after worming but to worm them BEFORE moving to leave the resistant worms on the old grazing.
When we worm a horse, only those worms resistant to the drug survive. If the horse is then moved to a new pasture it increases the risk of the new pasture being exposed to the resistant worms.
Selective Therapy
We can practice ‘selective therapy’.
This is where we worm count + leave horses with a low burden untreated.
It’s actually healthy for them to have a low burden + can be a really useful tool for the horse population.
Only treat when necessary
It’s estimated that in any population of horses, 20% of them carry 80% of the worm burden.
We can worm count + only treat those with a medium-high burden.
This way, many horses will be left untreated which will result in a significant level of refugia.
So, to sum up:
Horses are never free of worms – that’s why the lowest result we can give when testing is a burden of less than 50 eggs per gram.
And we don’t want them to be worm-free!
It’s actually good for them to have a low worm burden.
That’s why we don’t worm at the slightest indication of a burden.
We need those parasites there to help increase refugia and lower the population of worms resistant to drugs.
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EFECS provide independent faecal egg counts for horse owners – buy worm counts here
