If you are interested in reading other parts of this series:
Barber Pole Worm Series Part 1-The #1 Killer Of Small Ruminants
Barber Pole Worm Series Part 2-Controlling Worms Without Dewormer
Barber Pole Worm Series Part 4-Breeding Parasite Resistant Sheep
Barber Pole Worm Series Part 5-How To Do a Fecal Egg Count
In the next part of our series on barber pole worms, we are going to discuss proper deworming. It is extremely important to follow the guidelines to deworm properly. This is because there are no new dewormers being developed, and there is a lot of dewormer resistance building in this country.
A lot of this resistance can be attributed to our lack of understanding and inappropriate deworming practices. Unfortunately, many of these same practices are still widely used and are being recommended by people who do not understand how much harm they are causing. Thus, as a small ruminant owner, it is important to educate yourself on what the current recommendations are.
Dewormer resistance is due to improper use and over use of dewormers.
So first lets take a look at historical deworming practices.
Believe it or not, dewormers are actually pretty new to the scene. Modern anthelmintics, the official name for dewormers, didn’t come onto the scene until the 1960’s and 70’s. Prior to that options for controlling parasites were pretty limited.
As we settled into using dewormers, we settled into a lot of bad habits that have gotten us to the resistance problem we have here today. We had this new miracle cure and decided that it was a good idea to deworm all of our animals very frequently. Probably on the faulty assumption that we could eradicate the worms. When parasite resistance started to rear its head, we started making the recommendation to rotate dewormers every time we dewormed. Somewhere in here we also decided it was a good idea to move the animals to a new pasture immediately after deworming.
Unfortunately, while well intentioned, we now know that these practices are some of the best ways to create dewormer resistance.
We now know we cannot eradicate worms. We know a lot more about the lifecycle of parasites and barber pole worms are alarmingly persistent on pastures. We know that if you deworm and rotate your animals back and forth between two pastures every 30 days, you are NOT putting them in a clean worm free pasture. You are actually putting them back on the pasture at peak infectivity.
Not only were we not reducing parasite populations, because we were deworming every single animal frequently, we were creating dewormer resistance simply by the high quantity of exposure. Rather than helping, rotating dewormers just increases the parasites’ exposure. Resulting in worms resistant to not only 1 but 2 or even all 3 dewormer classes.
This deworming schedule also makes animals with good natural parasite resistance look exactly the same as those who are extremely susceptible. Ceasing to select strong healthy animals makes the problem a viscous cycle of weaker animals needing more deworming, so we deworm more making more weaker animals and more parasite resistance.
Every single dose of dewormer comes with risk
The risk of creating a resistant worm is there every single time we administer a dose of dewormer. However, your risk of creating a resistant worm goes up exponentially with increased dewormer useage. For example. If I administer 1 dose of dewormer to 1 animal in any given year, I have 1 chance of creating a resistant worm.
What if I give 12 doses to 100 animals in a year?
Well, then I suddenly have 1200 chances of doing it instead of 1.
If I rotate between the 3 dewormer classes, then I have 400 chances of a worm developing resistance to any one of the 3 dewormers. What happens if one of those 400 chances creates a resistant worm that just happens to win the lottery? That worm also is lucky enough to develop resistance to the next dose of dewormer you give. Suddenly you have a parasite resistant to 2 kinds of dewormers.
This wouldn’t have been possible if I had not been frequently deworming and rotating dewormers. If I had only dewormed a couple animals instead of every single animal on a schedule, I would have reduced the risk even more.
We now realize that we should not deworm on a schedule, and that we need to care for what we call ‘refugia’
Every farm has a population of worms. This includes both the worms inside your animals and on your pastures. Your refugia is the portion of that worm population that HAS NOT been exposed to your dewormer. These non-exposed worms are important because they dilute the resistant worms.
To maintain refugia you CANNOT deworm all your animals at the same time. You should also not put dewormed animals out onto a clean pasture. They should go back into the same wormy pasture that they were in prior to deworming. Putting the animals back into a wormy pasture is doubly important if you have to deworm a large portion of your group (such as with lambs).
Why is this so important?
We know that if we deworm an animal that has a resistant worm, that the animal is only shedding resistant parasite eggs onto your pasture. What if all of your animals are only shedding resistant worms on your pasture? This is what happens if you deworm all of your animals or put dewormed animals onto a clean pasture. Before you know it, there are only resistant worms on your farm because you selected for them. As you can imagine, this could pretty quickly turn into a huge problem. And it has. This is the problem we see on farms across the country and world.
So how do we deworm properly to protect our refugia and slow the progression of resistance?
Firstly, lets start with some DON’TS. For the reasons laid out above:
- Do not deworm any animal unless they actually need it (as determined by things like FAMACHA)
- Never deworm all your animals on a schedule.
- Never deworm all your animals at the same time.
- Do not deworm your animals and move them to a new pasture.
Additionally,
- Do not under dose dewormers
- Do not use injectable or pour on dewormers.
Under dosing, injecting or pouring on dewormers can contribute to resistance by exposing worms to sub-lethal doses of dewormer giving them more opportunity to adapt to it.
What we should be doing?
- Use FAMACHA and or fecal egg counts to determine what animals to deworm.
- Only deworm the animals that need to be dewormed.
- Return dewormed animals to a dirty pasture with other animals that have not been dewormed.
- Make sure that you weigh your animals accurately prior to deworming so that you can give them a proper dose. As mentioned above under dosing encourages resistance. 2 of the 3 classes of dewormers are extremely safe (a 10x dose will do no harm), so if in doubt, err on the side of over rather than under dosing. The 3rd class needs an accurate weight to be dosed correctly.
- Deworm with all 3 classes of dewormer at the same time, eg. Cydectin®, Valbazen®, and Prohibit®. Don’t put them in the same syringe, but give one right after the other.
- Get fecal egg count reduction tests done periodically to determine the efficacy of your dewormers
- Remember it is normal (and even healthy and beneficial) for your animals to carry some level of parasites.
What are the 3 classes of dewormers are, and why are they important?
Despite different branding, each of the dozen different dewormers on the store shelf is not a unique entity. Any time we are discussing dewormers, it is important to keep in mind that there are only 3 types or classes of dewormers. Within each class there are several different dewormers sold under various brand names. For example macrolytic lactones include ivermectin (Ivomec®) and moxidectin (brand names Quest® or Cydectin®). Benzimidazoles include fenbendazole (Safeguard®) and albendazole (Valbazen®). Levamisole (Prohibit®) and morantel tartrate (Rumatel®) are nicotinic agonists.
Why does this matter? Because each family has a unique mode of action. Resistance is to the mode of action not the brand. If your parasite is resistant to ivermectin, it is also resistant to moxidectin. This means that in practical application we have 3 dewormers total. Giving 2 from the same class doesn’t benefit your animals or your wallet.
We also mentioned above that you should give a dewormer from all 3 classes at the same time every time you deworm.
Right about now, you might be confused by the recommendation to give all 3 dewormers at the same time. After all, we just talked about how scheduled rotational deworming between different classes of dewormers increased resistance. Now we are telling you the current recommendation is to deworm with all 3 classes of dewormer at once.
Seems backwards doesn’t it? It isn’t though. Let’s look at why you give all 3 dewormers together.
At this point in time, every single farm in this country has some level of dewormer resistance. We are beyond the point of preventing resistance by not exposing worms. We are to the point of reducing resistance. This is where deworming with all 3 dewormers comes in. It is actually much more effective than rotational deworming because you are not allowing the resistant worms to reproduce.
How this concept works is easy to show with some simple math.
In the table below, we have a list of 3 hypothetical dewormers. The percentage listed in each column are how effective each dewormer is by itself. The total column shows how effective the dewormers are together.
| Dewormer 1 | Dewormer 2 | Dewormer 3 | Total |
| 90% | 90% | Not used | 99% |
| 90% | 90% | 90% | 99.9% |
| 50% | 70% | Not used | 85% |
| 50% | 70% | 90% | 98.5% |
| 50% | 50% | Not used | 75% |
| 50% | 50% | 50% | 87.5% |
| 30% | 50% | Not used | 65% |
| 30% | 50% | 80% | 93% |
As you can see from this table if we use dewormer 1 by itself, 10% of the worms live to reproduce and shed eggs on your field. Adding dewormer 2 means only 1% of the resistant worms are left. If we add dewormer #3 to the mix, there are only 0.1% of parasite resistant worms left.
This is very significant.
Rather than letting the population resistant to dewormer 1 reproduce you are immediately killing them with dewormers 2 and 3. A single barber pole worm can produce 10,000 eggs per day, so that 10% left alive can do some serious damage in the 60 days before you killed them with dewormer 2 on a rotational deworming program (or 120 days for dewormer 3).
Looking at a different example for less effective dewormers, we see that even if all 3 dewormers are only 50% effective by themselves you can still kill almost 90% of the worms if they are used together. This is also very significant. This leaves you with only 10% resistant worms (instead of 50%), out there reproducing in your fields.
More importantly, this increased kill rate from combination deworming makes them effective enough to save an animal.
A dewormer that provides 50% kill is unlikely to save a severely anemic animal. 90% kill on the other hand might not be ideal, but it will save that animal. And that is information worth taking home with you!
So as we can see, using all three dewormers together, and only on animals that absolutely need them is the best way to prolong their usefulness for our animals. However, keep in mind. This is just buying us time.
The end of dewormer effectiveness is inevitable, and has already come in some areas. Thus it is important to also practice good management to minimize your animals exposure to parasites. If you are ever unsure of what is best check the American Consortium For Small Ruminant Parasite Control for the most up to date information. Most importantly, it is important to work towards selecting parasite resistant animals. In the next portion of this series, we are going to discuss how you go about selecting a parasite resistant animal.
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