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  • Writer's pictureSarah L Rhoades, DVM

The Science Behind Parasite Control


Parasite resistance directly affects your farm and your management strategies can impact that greatly.

Where are you at on your de-worming schedule this year? Is it ivermectin every other month? Are you rotating de-wormers quarterly? Are you confused to begin with which de-wormer you should be giving when? Does your boarding stable just do it for you?


Maybe you’ve heard about fecal egg counts – or catch phrases like parasite resistance as though it was some new fad or hot topic.


Maybe you’ve reasoned the costs don’t pan out when you can get a tube of paste de-wormer from the store for only a few bucks.


But let’s face it, the real reason: the thought of de-worming your horse less, probably gives you the heebie-jeebies – especially when what you’re currently doing seems to be working just fine. Why risk it?


It did for me at one time, too, I get it.


I recall childhood stories, from men and women old enough they remember plowing fields with teams of drafts and mules as kids because their parents could not afford a tractor. Horror stories about animals they lost to massive colic disturbances, from the tragedy of horrific parasite burdens. I’ll bet you can recall at least one similar story from your elders that left an impression on you too, and I bet whispers of those words cross your mind when you hear this topic brought up.


And it was no joke, that was life before anthelminthics (the drugs we use to treat internal parasites) became more readily available. Horses met some horrific ends due to disease caused by parasites. And part of why I find this subject so vitally important – parasite control is important, and those commercial anthelminthics, those paste de-wormers, may be the single greatest invention that ever hit the market (maybe second to the vaccine) as far as our horses’ health is concerned.


If you will bear with me, I would like to take the opportunity to explain why parasite resistance is a problem, why rotational de-worming is harmful (and directly to your herd no less), why the literal diseases we’re trying to address now differ from those your grandparents faced, and how we can attack parasite control in a way that is even more beneficial to your horse and herd than what you are doing now.


Parasite Resistance – here’s how it works:

You’re probably familiar with the idea of natural selection – right now genetic mutations are plastered across the news as the general public is getting a crash course of how a virus mutates to form new strains and continue to spread and propagate.


It banks on the idea of survival of the fittest. Organisms that develop beneficial mutations are more likely to survive and reproduce, so beneficial mutations spread amongst the population until that trait becomes the norm. Organisms that don’t possess these advantages are less likely to survive and reproduce, and eventually their line dies out.


Viruses and bacteria give us an amazing insight to this process because they have short lives and time to reproduce and they have abundant numbers – so, in a day, you can see generation after generation, so fast, it amplifies the effect of adaptation to something we can see and measure with relative ease – it’s like watching an entire civilization rise and fall from the pages of history, right on your TV screen.


Parasites share two of these qualities: they have abundant numbers and very short lifespans. So, we can also visualize and measure dramatic changes within our lifetimes.


When you expose a population to a killing drug (whether we are talking about bacteria or parasites), it kills a proportion of the population, may even be ‘most’ of the population, may even be 99%. But it will never kill all of the population – 99% of an organism that has millions or billions of members in its population means that thousands or millions survive.


A sizable number, and individuals that have a beneficial trait they can now survive, to reproduce, and pass on that trait to the next generation. So the following generation then has an increased resistance to that drug, so the next time it’s given, it’s less effective, maybe 90%. But now the survivors are even tougher and more resilient, and now there are even more survivors to reproduce those traits. And so, it builds, until the drug loses all reasonable efficacy and becomes useless.


In bacterial therapy, when we use antibiotics, the goal is to kill enough (or slow the growth/replication) of the disease causing bacteria so that the host’s immune system can finish the job.


But with internal parasites in herd, grazing animals, like horses, cattle, goats, and sheep, that last part doesn’t happen – the immune system in these animals allow a certain threshold of parasites to live and propagate within the gastrointestinal tract, without causing disease. A sort of symbiosis if you will that is part of the animal’s natural flora.


The take home point: when you give a de-wormer, a proportion of those internal parasites will survive and propagate, and pass those resistant traits on to the next generation of parasites making the drug less effective with each use and a horse’s immune system naturally allows a certain proportion of parasites to live and propagate within it without causing disease.


Rotational De-worming – it is harmful to your herd, and here’s why:

Most of my life I have been told ‘make sure you use a different de-wormer every time so you don’t get parasite resistance.’


Those same sources also use to tell me not to let your hot horse drink water after a ride or to hose him off to cool him down – multiple, reproducible studies have shown this to be false in endurance horses and most famously the horses in the 1996 Summer Olympic Games in Atlanta, Georgia – here’s a bit about those myths, if you’re interested https://equusmagazine.com/horse-care/preventing-heat-stress-in-horses-8351.


These are what we call ‘old wives tails’ – real and observable associations where the conclusions about those relationships were not necessarily reliable.


To restate: if you expose a parasite population to a drug, a proportion of that population lives, the proportion that possesses traits resistance to that drug that then reproduce.


What happens when, a couple months later, the newly, slightly more resistant population of parasites gets exposed to another class of drug?


The drug works, exactly as intended: it kills off a high proportion of the parasite population, it looks like you took a resistant population of parasites and made them less resistant.


But a proportion of those parasites still survived, the portion of parasites with resistant traits. And what's worse, what wasn't intended, those parasites didn’t lose their resistance to the first drug - those parasites that survived, have traits that make them resistant to two classes of drugs now. It’s actually a more drug resistant population than what you had before, even though it looked like it was less. With each class of drug you throw in the mix, the more resistance you get.


Your elders weren't 'wrong' in their recommendations from 50 years ago - they just didn't have access to the whole picture.


Do you remember what I said about de-wormers/anthelminthics being one of the most important inventions of equine health care? I truly believe that – so, I hope you understand my grave concern when I add that we have no new anthelminthic/de-worming drug classes in development. Nothing. What we have is what we have.


So why is this relevant to you and your herd? Lets break this down:


Every time you de-worm your horse, you kill off a certain population of parasites, and a small proportion survives to reproduce and spread resistance. But the entire population of parasites on your pasture and in your horse have not been exposed to that drug at all, we call that a refugia.


The refugia is the parasites on your pasture, in the manure pile, parasites living in a dormant state, depending on the drug it can be encysted small strongyles in your horse’s large intestine. These are parasites that had no selection pressure for resistant traits at all!


If you de-worm your horse once, the resistant, surviving parasites within your horse are diluted out by a vast refugia of parasites that have not been exposed to the drug. They compete with the resistant parasites for resources and reproduction, and ultimately, dilute their effect out.


The more frequently you de-worm your horse, the more proportion of that refugia population gets exposed to anthelminthic drugs – and the dilution effect it originally had, gets reduced.


While drug resistance is a world wide problem that effects everyone, this is the mechanics of why we talk about parasite control on a whole farm basis. Individual Farms are infected with resistant populations of parasites. This is why farms that used intensive de-worming strategies for decades now host some really problematic parasite populations and now actively see disease caused by these parasites – we’re talking thoroughbred breeding operations in Kentucky that have highly resistant round worm populations ravaging their valuable young horses.


The take home point: is your management really does make a big difference, and you can make a dramatic difference in your horses’ herd health care.


What about those scary stories my grandfather told me? Here’s what’s changed:

Parasites can cause deadly disease – the recommendation to de-worm every other month and switch de-wormers was made to prevent the deadly diseases your grandfather talked about. But, again, they didn’t have all the facts when that recommendation was made 50 plus years ago and the landscape has actually changed in that time.


The idea to deworm every other month came from testing that showed it would take two months from administering an anthelminthicide to when that horse started shedding parasite eggs again.


Let me offer a different perspective here: our goal is to prevent disease caused by parasites – we can all agree on that. However, let me add a new perspective: that goal is not synonymous with killing all parasites.


The first goal is achievable. The second one is entirely not - and like we discussed, attempts to do so are actually quite harmful to your horse and herd. We can, however, address concerns for parasite resistance and offer your horse and herd superior parasite control measures – those two goals are not mutually exclusive!


Like bacteria, parasites are not evil. They are actually a very normal part of a grazing, herd animal’s gastrointestinal tract. It’s when those numbers get out of balance, or certain processes are allowed to continue, where disease is caused. We understand this relationship a lot better now than the experts of your grandfather’s time did.


The landscape, too, has also changed. Diseases, for example large strongyles, or bloodworm, infestation, where larvae enter the blood stream of a horse’s gastrointestinal tract, causing clots and occlusions to occur resulting in depriving that organ of blood and nutrient supply, often times death – those parasites have very small numbers, they are very susceptible to anthelminthics, and the disease they cause takes a very long time to develop, and consequently, are also very rare, now.


We understand now how easy it is to interrupt that disease process so that it never has the chance to occur at all. Preventative medicine.


And as a personal opinion (because you will find a billion out there): these diseases are the reason why I do recommend all horses in this region, receive at least twice, yearly de-worming with one of those incorporating a praziquantel drug to address tapeworms. That is frequent enough to interrupt that process before it becomes a clinical disease problem.


The major disease problem we are faced with in this day and age is not the large strongyle (bloodworms), that plagued your elders, but rather small strongyles. These guys live in vast numbers in your horse’s gastrointestinal tract. Because of that, they behave a little more like the bacterial-antibiotic relationship with respect to anthelminthic administration – whereas large strongyles are much fewer in numbers, and a single dose will entirely interrupt that disease process they cause, small strongyles will always have a significant number of resistant survivors that are left to propagate.


Small strongyles cause disease when their numbers get overabundant, by a couple of mechanisms - but it takes a lot, and also takes time to develop. For the most part, they live within the gastrointestinal tract and don’t cause a lot of problems.


At this point, I think it’s important to mention, control strategies are slightly different for breeding operations and young horses under the age of 3 years – and part of that is the typical parasites you’re treating – those too are different than what plagued your grandfather. It’s just, I could type all day about this, and for the moment, we’re focused on mature horses and a typical herd situation.


So how do we effectively prevent disease in horses while also reducing parasite resistance? Lets face, not one of us ever want our horses to undergo a horrific event like what our elders told stories about - this entirely about doing what's best for your horses.


I’m going to say this again for emphasis: We’re not trying to kill all parasites – we are trying to prevent disease caused by parasites as effectively as possible. Rotational de-worming only appears effective for a time, and, further, it contributes to the problems we are facing and can cause great harm to your horse and herd health.


So, the first thing – you can remove some parasites from the environment without using anthelminthic drugs at all. These are your strategies like pasture rotation, and manure handling or removal. They are important and deserve to be first mentioned, but there are also an abundance of resources available that you can utilize, like your university extension office – every farm is set up a little different, so every strategy must be tailored to that. Space is limited, so I’m going to focus on the things that I do, personally as a veterinarian to address these problems.


At my level, we reduce the amount of de-wormer we are using by targeting the horses spreading the most parasites.

I think it’s important to clarify here: I’m not talking about treating clinical disease, I’m talking about reducing the parasite load on the pasture.


See, most horses don’t shed a lot of parasite eggs. About 80% of the parasite load on your pasture, originates with 20% of the horses in the herd. That means 4/5 horses, will only rarely come up positive on their fecal egg counts, even if only being treated with de-wormer once or twice a year (depending on the environment and husbandry practices mentioned above, of course!). (Interestingly, this statistic is true across pasture grazing, herd species like cattle, goats, and sheep.)


These 1/5 horses, the 20%, it’s important to recognize they don’t suffer from clinical disease, they are perfectly healthy horses - their body simply allows those parasites to live in their gastrointestinal tract in higher numbers. The problem is, it is exposing the rest of the herd to greater numbers of parasites – we’re using a targeted approach to treat the herd’s exposure to parasites.


To be clear (if I wasn’t already) - we’re not talking about the skinny, poor doers, and starvation cases here – those horses are sick and need treatment for an active problem(s), we’re talking about disease prevention in healthy appearing horses – because that’s our ultimate goal: to prevent disease caused by parasites in your horse – and these permissive shedders are not identifiable by body type.


And the good news, is we have a relatively inexpensive test we call a Fecal Egg Count – runs about as much as a Coggin’s Test – to not only identify those horses, so that we can target and treat them individually, but also to monitor your farm’s overall parasite control strategy and figure out if you’re running into problems, before you run into problems.


That is what targeted and effective parasite control means. That's what preventative medicine is all about.


And, I feel this is so important, I tried to incorporate it into my wellness plans (check them out here) in line with current AAEP guidelines (check those out here) and Platinum Performance’s Colic Surgery Reimbursement Program (check that out here).


I hope you learned something, I know I actually learned and re-learned quite a bit on this subject developing my wellness plans and writing this post - I try my best to only pass on up to date information - and boy does it get out of date fast! For that I must give you all a huge Thank You, because you’ve helped make me a better practitioner, and helped me help you better.


Sarah L Rhoades, DVM

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