Wormers are not always effective.
There is a complicated interaction between worms and wormers that not everyone knows about. First of all, worms can become resistant to wormers, just like bacteria can become resistant against antibiotics. This can be caused by underdosing, frequent exposure and using the wrong product.
- Underdosing happens more often than we’d like to admit. No one has a horse weigh scale, so everyone’s estimating their horse’s weight. Research has shown that owners often underestimate their horse’s body condition score. This means that weight can easily be underestimated as well. Using a weight tape can be helpful, but even those are not 100% accurate. Wormers are made to be safe even when overdosed a bit. To prevent resistance to wormers, it’s best to use a weight tape and dose your horse for the measured weight + 100kg (50 for minis). Make sure your horse doesn’t drop the wormer out of its mouth.
- Every time worms come into contact with a wormer there is a chance of some worms being strong enough to survive it and acquiring resistance. Frequent exposure means that the more often the worms come into contact with a wormer, the higher the risk of creating resistance. Ways to prevent this is by only worming based on confirmation of the worm burden by doing a faecal egg count. A faecal egg count tells us whether worming is necessary. The other way of preventing frequent exposure and resistance, is by proper paddock management. When worm eggs are passed with the faeces and end up on in the paddock, at takes a few days for them to hatch and migrate from the faeces to the grass where they can be taken up by the horse again. By removing the faeces from the paddock before they are able to hatch and migrate, the worm burden on the pasture stays low. Adult horses generate a certain amount of immunity against worms. However, this only works on low worm burdens. Thus, when the worm burden on the pasture is kept low by removing the faeces twice a week, the horse can stay healthy. Results show that horses kept on well managed pastures often have a low faecal egg counts and don’t need to be wormed as often as otherwise. Exposure of both the horse and the worms to wormers therefore stays low.
- The horse can get infected with different worm species. Wormers have different mechanisms with which they kill a worm. Not all wormers work on all worms. Additionally, a product might state that it’s effective against a long list of worms, but it doesn’t state the effectiveness in percentages and the information is not re-evaluated for present status. Using a wormer that only kills the bulk of the worms means that the healthiest and strongest worms survive. These will then reproduce, and the new population of worms will be even healthier and stronger and harder to get rid of than the worms that were there before. This creates a problem for the horse and the wormers that we use. It is important to know which worms are present in your horse so that we can target them with a wormer will be most effective against that worm. By doing a faecal egg count we can find out which worms are present and what product to use. Another misconception is that using a combination product with three different active constituents in it will kill anything. The same issue is present with these products. In addition, when only the bulk of the worms are killed, the ones that survive have now not only been exposed to one, but three active constituents, that they can then develop resistance against. By removing the faeces from the paddock twice a week and rotating, horses don’t have to be wormed as often so that we don’t constantly select for the healthiest and strongest worms.
- A faecal egg count can not only be used to see if worming is necessary, it can also be used to check the effectiveness of your worm product. By checking the faeces 2 weeks after worming we can see if the product has had effect. We advise you to do this once or twice a year.
How you manage the pasture is more important
than worming when trying to keep parasites down to reasonable levels. With proper paddock management adult horses often only need worming once a year. Best practice is to remove faeces twice a week and rotate. If possible, let sheep graze the paddock down after horses have been on it and keep stocking density low.
- Many people prefer harrowing a paddock over picking poo. In theory, harrowing exposes worm eggs and larvae to sun and wind, which could kill them. However, this is only effective in hot dry climates like in Australia or southern Europe. However, the climate in New Zealand is not suitable for this method. When applied in climates that don’t kill the worms, it actually leads to increased pasture contamination. This technique is therefore is not recommended in New Zealand! Research shows that removal of faeces reduces the numbers or worm larvae on pasture to only 10-20% of the levels on pastures grazed by horses being regularly treated with wormers. If your paddock is too big, dividing the paddock up helps not only with removing faeces, but this also makes it possible to rotate and rest paddocks more often.
- By rotating paddocks every 2-3 months the used paddocks can be rested. When a paddock is rested, the horses don’t ingest the worms and enable the lifecycle, thus replication stops. This is most effective when horses are removed from a paddock before they graze it close to the ground, where the parasites mainly sit. Under winter conditions, when the ground freezes and thaws repeatedly, a proportion of the worms that are on the pastures dies. By growing grass on used paddocks for hay and cutting it, a portion of parasites will also be removed.
- Cattle and sheep can also be used to reduce the worm burden. Sheep graze closer to the ground and don’t trample the ground as much, so they are preferred. The only horse parasite that could infect ruminants is Trichostrongylus, which lives in the stomachs of cattle, sheep and goats. However, if infection occurs it is usually only minor. We advise paddocks are left for 2-3 weeks before sheep or other ruminants are introduced, to ensure that worm eggs have hatched.
- Parasite numbers tend to be greater when more horses are grazing on the same pasture and horses are overgrazing the land. The lifecycle of a worm needs the horse to replicate in. A higher stocking density means more horses, more worm replication, higher worm numbers on pasture and more horses on the pasture to ingest the worms again, etc. When too many horses are kept on a pasture and the grass is grazed down too fast, horses will eat grass next to manure piles and pick up a heavy worm load. Alternatively, when stocking density is kept low and pastures are rotated in time, horses only get exposed to a few worms. Adult horses have a certain degree of immunity. If the worm load is low, they can handle it.
If you are managing a lot of horses on a small pasture you must be especially careful about managing stocking density and grazing rotations. You can determine whether your management practices are paying off by monitoring horses’ faecal egg counts to determine their worm burdens.
In 2017 the European Medicines Agency (London, UK) released a reflection paper on anthelmintic resistance. They concluded the following:
There is a common understanding among experts that measures to reduce the need of anthelmintics (wormers) and promoting an appropriate use of these drugs are important to delay resistance development. Examples of prudent use advice are: to base treatment on confirmation of worm burden or solid epidemiological information, to employ targeted selective treatment approaches and to avoid routine and frequent use, to dose correctly and particularly avoid under-dosing, to use combination products only when all substances are necessary for effective treatment, to manage pastures properly and to maintain an appropriate level of refugia, in particular by keeping horses that don’t need treatment untreated.
How does a faecal exam work?
For the faecal exam to be reliable, we need to have a reliable faecal sample. As we’ve mentioned before, when the faeces are expelled, the worm eggs hatch. Once they’ve done that, we can’t find them anymore. Faeces therefore need to be freshly picked, and if necessary, stored in a fridge.
How to take a sample:
- Make sure the faeces are fresh (preferably not older than an hour)
- Take about a hand full from the top of the pile, which has not been in contact with the ground
- Put it in a ziplock bag or container and write your name and the horse’s name on it
- If you’re not able to bring the sample straight to the clinic, store it in the fridge (preferably no more than one day) until you can bring it in
- Samples can be taken to our Feilding or Awapuni clinic during opening hours
Once we receive the sample we mix a certain weighed amount with a special solution to make the eggs come out and show themselves. We then count the amount of eggs that we’ve found. Because we know exactly how much faeces and solution we used, we can say how many eggs there are per gram of faeces, which gives us an “EPG” (eggs per gram) value.
By doing a faecal egg count we can help you with the right worming strategy. We can also give you information about the worm burden on the pasture. By working together on a strategic parasite control program, adult horses may only need to be wormed once a year.
Worming can only be done right when there’s knowledge of what’s going on inside your horse and on your pasture. Therefore, our worming recommendations are not the same for every horse. Our recommendations are based on information about your pasture management, whether your horse grazes together with other horses, your horse’s age, any health issues at the time and a faecal exam.