This month I thought it would be interesting to have a look at resistance to wormers: which wormers have the most trouble with resistance and what causes it in the first place.
Let us first look at what it really means. To clarify the situation it is the worms not the horse which are resistant! Now you may think that is an obvious statement but you would be surprised at how many horse owners thought otherwise! When a worm is resistant to a wormer it simply means that it cannot be killed by the chemical in the chosen wormer. Like all things in life, there are weak and strong worms and it is the weak worms that remain susceptible but the strong ones that survive. I say this because you should not be confused into thinking that it is a species of worm that is resistant, it is not the case to say all strongyles are resistant but all threadworms aren’t. No, there are strong and weak strongyles, strong and weak threadworms, strong and weak pin worms – you get the picture. The trick with worming is to use the correct wormer, at the correct time of year, in the correct dosage. It’s also about knowing when it’s ok to leave a few worms in the horse for its own immune system to kill them rather than relying on chemicals all the time. Current thinking is let the horse have a count of between 250 and 350 epg before getting the wormers out but that really is a personal preference and should be discussed as a yard policy. Get that right and you should be well on the way to being able to only use one wormer a year and worm counts the rest of the time!
So that’s what resistance is, now how do we cause it? Well, we have many opportunities to allow worms to become resistant! Probably the easiest way is simply to under-worm the horse. Wormer companies have spent many millions of pounds and years of research getting the dosage exactly right. They mark the syringes with critical measurements, make it easy for us to use the syringe by putting a clever ring device on the plunger and write screeds of information on the package about kilogram to milligram ratios etc. What do we do? We give a pony half a tube and a horse a whole tube! How many of us actually know how heavy our horse or pony is and how much wormer is in the tube? Perhaps you use a weight tape but do you really know how to use it accurately? Are you also aware that most weight tapes are inaccurate to about 50 kg. That means that whatever it says on the tape measure, add 50kg! But do you know if your tape is one of the inaccurate ones or not? The only way to make sure is to calibrate it by measuring your horse then weighing him on a weighbridge and comparing the two. Dodson & Horrrell and Dengie and several Vet practices offer weighbridge days when they will come out to your yard and weigh your horses, usually for free. The point of knowing this measurement is that you can then give your horse the right amount of wormer. Too little and you will only kill the weak worms – the strong ones will be dancing about yelling “cheers sucker!”
Over-worming is another easy way to resistant worms taking over. If you use an interval worming programme which can involve worming every 6 weeks for a year if you really want to push the boat out, you will be taking away the horse’s own immune system’s ability to fight the infestation. It will make your horse reliant on chemicals to fight disease and like our own difficulty with antibiotics and certain diseases becoming immune to the cure, worms are fast going the same way. Combine this over worming with chemicals to the above underworming for the size of the horse and you may as well throw the money you spend straight onto the muck heap!
We know that there is widespread resistance to a chemical called Fenbendazole. This is the basis of Panacur. The only way to tell if Panacur still works for your horse is to worm with it then, three weeks later have a worm count done. There should be a very low count of worms. If there are still worms present, and you are sure you have got the dose right, then Panacur is not working for your horse. Better still, have the worm count done first then if there are few or no worms present in the count then why bother worming at all??? We have found also that there is very localized resistance problems with Ivermectin – the basis of wormers like Eqvalan, Eraquel, Equimax. This is mainly on a farm by farm basis and is thought to be a historical problem due to over use when the land was used for cattle. Again, the only way to tell if you can use these wormers is to have a worm count done.
A further way to improve the chances of your horse’s worms becoming resistant to your expensive wormers is to use the wrong wormer for the time of year. Don’t use a tape wormer in the summer, use it either in autumn and spring or once in winter. (The only way to tell if you have a tape worm problem is to have a blood test done by our Vet. Ask about the ELISA test). Worming at the wrong time of year will give free rein to the worms that you have missed.
Worming is a very difficult subject to get right. The British Veterinary Association have recognized this and have issued guidelines for this very reason. Put simply they are encouraging horse owners to limit the amount of times they worm their horses, use worm counts instead to ensure that your horse has an acceptable number of worms and target only those whose counts are moderate or high. Worming only once per year with a dual wormer (Equest Pramox or Panacur Guard) will kill any tapeworms, encysted redworms and in the case of Equest Pramox, bots too. This will reduce the amount of wormer you use which will reduce your opportunities for mucking it up, be a lot cheaper for you and will ultimately be better for your horse. The environment will win too as getting rid of the used wormer syringes (especially those containing Ivermectin) by chucking them in the bin (or even on the muck heap as some people do) can cause death to other animals including birds, fish and even your dog or cat!
Hopefully I haven’t frightened you too much here but for more information on worming and betters still, on worm counting then visit our website www.wormcount.com to find out much more information and how to order your really good value £8.50 worm count kit – cheaper than worming and better for the horse and the environment!