How to tell if dewormers are effective, and if drug resistance is developing:
Usually, when an animal (our ourselves) is sick, we know when a medicine/drug has worked because the problem has gone away. That is because the illness is considered ‘clinical’ – it has visible symptoms. Worm infections, on the other hand, are usually subclinical or asymptomatic, where we cannot even tell if the animal has worms or not. (Hands up those of you who have had an egg count done and the results didn’t match what you thought?)
To check if an animal needs deworming (especially horses), you may instead get a FEC done first. If the egg count comes back high, even though your pony is fat and shiny, you go out, buy the dewormer and treat.
But then how do you then know if it has worked? If the pony was fat and shiny to start with, then you cannot use an improvement in condition as.
What you should do, is get a follow up FEC done, 2 weeks after treatment. When a dewormer is effective (and there is no drug resistance), you should expect an egg count of 0EPG. Dewormers are wonderful drugs, and they will clean out all of the adult worms, therefore no eggs will be shed in the manure.
If there are eggs present in the manure 2 weeks after treatment, this is a very strong indication of drug resistance. What it means is that the dewormer did not remove all the adult worms, leaving some behind to continue laying eggs. If this is the case, it usually spells trouble, and that you have a proportion of worms that cannot be killed with the dewormer.
When we get to this scenario, the only thing to do is choose a different drug and treat again, then check again to see if that one worked. But once we get to this point, it is difficult to fix. However, you can monitor the development of drug resistance before it gets to that point.
After you deworm a horse (from now on I’ll be using horses as the example; the concept holds true for other livestock, the drugs/timeframes differ slightly), you have a period of time before eggs reappear in the manure. This is called the Egg Reappearance Period (ERP). On the box of the moxidectin drench that is ever popular with horse owners, it states “Protects for 14-16 weeks”, referring to the ERP. Moxidectin did have an ERP of 16 weeks when it was first introduced, ivermectin was at 9-12 weeks, and oxfendazole was 6-8 weeks.
But guess what?
We’ve got drug resistance developing.
We’re not at full on “there are still eggs present 2 weeks after treatment” resistance, but more like “you only wormed this horse 6 weeks ago with ivermectin and there’s already eggs again” resistance. This year I did a FEC on a pony with a EPG of over 1000, and said pony was treated with moxidectin SEVEN WEEKS PRIOR. I had a heart attack.
If you want a more complete picture of how the dewormers are working on your property, it may be worth thinking about testing their ERPs. It is more FECs and more work, but it can give you an insight into if resistance is developing and allow you to make changes to your deworming practices to avoid resistance getting worse.
Example of how to use ERPs: Get a FEC done before treatment and then two weeks after treatment (to ensure that all the worms are in fact removed). Then, depending on what drug you used, at set weekly or fortnightly intervals afterwards get FECs done to see how long it takes for eggs to reappear. For oxfendazole, check at 4, 5 and 6 weeks post treatment. For ivermectin/abamectin, 5, 7 and 9 weeks. And for moxidectin, 7, 9 and 11 weeks.
If the ERP is within a reasonable time frame, that means drug resistance has not started to develop yet, and while you should still continue to use strategic deworming, there is no need for immediate concern.
If the ERP is shorter than ideal, this suggests that you may want to start thinking about how you can reduce deworming treatments even more, and which drugs should be avoided.
As with all things, prevention is better than a cure. And this is especially true when there is no actual cure for drug resistance worms.