Wednesday, March 2, 2011

Parasite Challenges - Deworming Strategies for Today

The seminar conducted by our vet last evening was excellent and very well-attended.  We turned in our (equine) fecal balls for testing beforehand and got our tickets for the door prize drawing - there were close to 100 people there (and no, I didn't win any of the door prizes).  The title of the talk was "Parasite Challenges in the 21st Century".  The big takeaway was that deworming strategies need to change, but also that there's no one-size-fits-all solution and that each barn and/or owner has to develop their own deworming strategy in consultation with their vet, and that the right strategy will vary depending on the circumstances.  So neither deworming every two months using a rotation of dewormers, nor just doing two fecals a year and only treating what shows up (or not treating or continuing to test at all if fecals are negative), are necessarily the answer.  Hang in there - this is a long post with lots of information - so here's the poop (couldn't resist!).

The biggest challenge for the 21st century in equine deworming will be to prevent parasite resistance.  Small ruminants - sheep and goats - worldwide already have a huge problem with parasite resistance - most of the dewormers are ineffective.  All equine dewormers are started to produce some resistance as well.  One striking example is in Germany, where resistance of strongyles to dewormers is high - lytha, this is the reason you can't get dewormers there except by veterinary prescription.  The last new dewormer for equines was developed almost 25 years ago, and there's nothing new in the pipeline, so we have to preserve the efficacy of the dewormers we have for as long as possible by delaying the development of parasite resistance.  The old days, before effective dewormers, weren't the good old days - horses died and were very sick - worms were a major cause of colic and failure of horses to thrive and roundworms caused pneumonia in foals.  We don't want to go back there, but if our current dewormers become ineffective due to the development of parasite resistance, that's where we and our horses will be.

The answer is to be smarter about how we de-worm - 80% of the parasites are transmitted by 20% of the horses, and stable management practices can also make a big difference.  Also, our vet pointed out that it isn't necessarily a bad thing for horses to have a few parasites - the objective doesn't have to be zero parasites - carrying a few parasites actually strengthens the horse's immune system.

The strategy involves using effective products properly - the right dosage administered properly at the right time, monitoring fecal egg counts if possible and deworming only when appropriate.  One big issue our vet has found is that people administering dewormers often don't get it all down the horse due to poor technique and also often underdose.  He said 80% of the horses he sees are overweight and that owners often seem to dose at the ideal rather than the actual weight - some larger horses might need more than just one tube of de-wormer and weight-taping isn't all that accurate. What happens with underdosing is not that you only kill x% of the worms, but that you selectively kill the most susceptible of the worms, leaving the stronger ones to reproduce - this is one of the primary ways parasite resistance develops.  Our vet also said that one reason some people underdose is from a fear of toxicity - he said almost all dewormers in use today are very safe for horses, although he said care must be used when deworming with Quest in skinny, debilitated/immune-compromised or very young (foals) horses as toxicity can result - this has to do with the fat levels in the horse's body and how Quest is metabolized.

But he did make the very important point that even if your fecal egg counts are negative, you likely will have to still deworm - he didn't want the take-away to be that you can stop using dewormers completely.  The two culprits are bots and the three species of tapeworms that affect horses.  In both cases, only a couple of the dewormers are effective against these, and they aren't easily detectable using fecal egg counts.  Encysted strongyles that don't show up in fecal egg counts also are an issue. Interestingly enough, exposure to tapeworms varies greatly in different parts of the United States - in some parts of the desert Southwest the exposure rate is as low as 19% but in our part of the world - upper Midwest - the exposure rate is 96%.  (No one's entirely sure why this is, but it's likely due to moisture/temperature/humidity conditions.  Unrelated factoid - over 70% of horses in our part of the world have also been exposed to EPM, which occurs only through exposure to possum manure.)

To develop an effective deworming strategy, a number of factors should be taken into account.  Particular products administered with particular timing should be recommended depending on a number of factors: time of year, humidity, temperature, how are horses housed/turned out (stalls, paddocks, dry lots, pastures - strongyles is primarily a worm of pastures - degree of mixing of the horses), age (foals and immune compromised horses such as Cushings horses may need more treatment as their immune systems don't cope as well with worms), health status of the horse and the prepotency period of the parasites.

A brief digression about prepotency period.  The prepotency period of a particular species of worm is the time from the hatching of the egg until the adult is sexually mature and produces eggs itself - the full life cycle.  This period can be 3 to 4 months or in some species up to a year.  So effectively your horse has "rolling exposure" to worm species in its environment, meaning that any fecal egg count you do is just a snapshot from a moment in time.  This is another reason why not deworming at all after one or a few negative or low fecal egg counts may not be wise.

Three very important factors in establishing a worming program for your own situation are sanitation, identification of high shedders (more on this below) and the concentration of horses at your facility.  Although the vet said (and I agree) that horses on pasture are the happiest, horses on pasture are at highest risk for worm exposure.  Removing manure from paddocks and dry lots, pasture rotation and dragging pastures at least several times a year can significantly reduce worm transmission - most are sensitive to heat, drying and UV radiation so the more air and sunlight the manure particles can be exposed to, the better. Heat and moisture promote worms.  Horses that are in stalls or paddocks or dry lots (provided manure is picked up) are at lower risk.  Are horses in separate groups or are they mixed together? The medical history, body condition, age and occupation/travel of each horse is also important.  Horses with Cushings present special issues as they are often high shedders due to having compromised immune systems.  Foals and broodmares also require special regimens.

What you decide to do may be very different if you have a few horses at home or if you manage a large boarding stable where horses frequently move in and out.  Some barns may decide to continue with every two months deworming, and some horse owners may be able to monitor, deworm and manage in such a way that only deworming for tapes and bots is required once a year - our vet believes that not deworming at all isn't a good option.

Here's our vet's recommendation for year one - develop a specific program based on discussions with your own vet, that takes account of your circumstances and needs and the specifics of your own horses.  Do a minimum of two fecal egg counts for each horse in year one - spring and fall - to establish a baseline.  (Our vet says it is possible to learn to do this yourself using an ordinary microscope, and several people in the audience were doing it themselves.) Determine which horses have some degree of resistance to parasites - which horses are high, medium or low shedders of parasite eggs - this cannot be done just by looking at a horse's health and condition as some apparently very healthy horses are high shedders and a source of worms for other horses. Also, horses can change status - to a higher load in case of persistent exposure to high shedders, or lower due to treatment. In some cases, doing a mixed sample for a pen or paddock may be OK, although if the results show there is a high shedder present, it will probably be necessary to determine which horse it is in order to treat appropriately.   Depending on how each horse is classified, treat appropriately - when to do fecal egg counts for each horse, which products to use and when, the correct dosage for each horse (this is really critical to prevent parasite resistance) - the best thing is to end up with a written chart summarizing the recommended protocol for each horse in the barn, specifying in which months testing and/or treatment should be done.  In year two and thereafter, continue to sample and modify the program as needed.  Our vet emphasized that it isn't just a case of worming less, or not at all - it is necessary to monitor over time as well and take account of parasites that don't show up in fecal egg counts.

Our vet defines a low shedder as a horse with an egg count from zero (a negative horse) of up to 200 eggs/gram of stool.  This may seem like a lot of eggs, but it really isn't.  A low shedder should have a fecal egg count twice a year or perhaps once if the horse is in a low-risk environment.  These horses should be dewormed three times a year - spring, summer and late fall (some vets will recommend twice a year).  Specific products should be recommended by the vet for each time of year.

A medium shedder has eggs counts of 201-400 eggs/gram of stool.  These horses should have fecal egg counts two to three times a year, and should be dewormed four times annually - spring, summer, fall and late fall.

High shedders have egg counts over 400 eggs/gram of stool.  These horses should be a focus of testing and treatment as they are the vector for transmission to other horses - the objective is to break the cycle of transmission.  They should have a fecal egg count prior to and two to three weeks after each worming - to determine their resistance levels - and should be dewormed five to six times a year until their status changes.

None of this is costless, of course.  Our vet currently charges $18 for a fecal egg count - the one last night was free (Dawn was negative and also was negative on her last count in the fall).  Our vet will not be charging for the consultation necessary to set up the program for each horse.  Each barn and owner will, in consultation with their vet, have to come up with a program that works for them and their horses and practical considerations will enter in.  If you have less control over your barn's management practices or how other horses in your barn are dewormed, your choices may be different.  If there are multiple vets involved, some coordination will be required by the barn management and the barn management may have to decide what practices are required rather than leaving it up to individual owners.

I found this information very valuable.  Our barn has three different vets, but I expect we can work out a program that meets the needs of all horses and owners.


  1. I'm very excited to be armed with this information for my visit with the vet today. Thanks for the great information!!!

  2. Thanks for a great report. I shared it on FB on a boarding barn business page that I am the admin for.

  3. Very good information. Thanks for posting. I've been wondering about this.

    Did he recommend any specific product over another?

  4. SprinklerBandit - no, he said he didn't want to get into that level of detail last night or we'd be there all night - he said it would depend on the specific needs of each horse and also I expect would make use of the few dewormers that get bots and tapes.

  5. Our vets held a seminar last year that my daughter and I attended. Basically, they had the same protocol. Good information on the subject.

  6. GREAT stuff here! Thank you so very much for writing it all out. It makes perfect sense that the solution is somewhere in between the two extremes....

  7. Wow--this is wonderful information! Thanks for sharing it.

  8. What an excellent write up and how wonderful that you were able to go to a local seminar about deworming. I am always amazed at the varying views people have on deworming.

    I bought a microscope last year and started doing my own fecals in the fall. It is really fun actually! I will pair my own observations with actual fecals done by CSU for the first year to be sure I am doing it correctly. I hope I can because it would be a substantial savings with 8 horses!

    Thanks for the excellent write up!
    Sue and the DVR Crew

  9. Very informative Kate, thank you.

    Where my girls are currently at, rotational worming is supposed to be done by barn owner, and is covered by our board payment. I can tell you that 1) she does not do it regularly, 2) she can not tell you the last time she did worm 3) she does not use proper dosage.

    There are three of us owners who now worm our own horses on an 8 week schedule. How does the other, 20 horses not being treated for parasites (including one Cushings), effect our efforts? Is what we are doing with our own horses even effective?

  10. Two years ago I got all the equipment to do home fecal tests. I've identified our high shedders (donkeys, Jackson) and monitor them closely. We give an all worm type wormer twice a year to get the stuff (like tapeworms) that don't show up on fecals. We muck twice a day, the horses are on dry lot, and we use fly predators. It works really well and I like that the worming is targeted, not given just because its been six weeks.

  11. Jeni - Based on what I understand, what you're doing should be effective - it's the protocol that our vet said many larger barns with high turnover might use. That said, your horses, depending on sanitation and how/with whom they're being turned out, might be getting constantly reinfected by high shedders in the general population. But with your schedule and a proper rotation of wormers, as I understand it, you should be OK. You could always have fecals done from time to time to see how things are for your horses - ask your vet and see what they say.

  12. Thanks for taking the time to share this info with us Kate. I wouldn't mind learning to do the fecal count myself, as our vets here are really expensive.

  13. Great informative post. My vet is holding a "clinic" about General Wellcare, Spring Grass Grazing, and Parasites March 26. I was wanting to attend, but after reading this, I really plan to now!

  14. Excellent post. Sounds like a super seminar. I "attended" a webcast seminar on the topic last year and discovered much of the same info.

    As usual, your write up was very accessible. Nice job.

  15. I attended a seminar similar to this a few years ago, and it was very good. Our dry large pasture area, they say has very little parasites. But seing as last year was exceptionally wet, I think that might have messed it up. Thanks for sharing, i had forgoton about most of it though.

  16. Great post! I love that the info my own vet gave ME is the same info your vet gave YOU. LOL! Sometimes it makes me feel better to know that. ;-)

    For some reason, my vet is not doing the regular (and cheaper) fecals. Instead he was doing something more "precise"...for $150 for each horse! :-O I opted to NOT follow his guidelines last spring as I could not swing an extra $450 on top of the already high $800 spring vet visit for my three mares (plus all the extra $$ I had to spend to vaccinate my horse to attend WEG). He is coming in a few weeks and I am sure will bring it up geez, it is hard to follow such expensive protocol!

  17. Thanks for the informative (as usual) post Kate :)

    So nice you have a resource like that seminar - lots of info - and really good for it to be passed around as ultimately the resistance issue affects us all.

    A (quasi) local vet made up a wormer schedule tailored to our climate / locale. Using that combined with fecals several times per year makes me think Val and I are on the right track.

    Thinking seriously about getting a microscope and learning how to test myself :)

  18. Thanks so much for your post on Worming horses you have some great informative blogs, much appreciated...good reminder for me to go get that faecal count done.

  19. It amazes me how often people underdose. Whenever I ask what a new resident was last dewormed with I also always ask the amount given. The answer is usually "the whole tube." Most of the horses here get a tube and a half to make sure they are full dosed. The ponies obviously get less.

    We try to identify the high shedders and de-worm them aggressively and then worm the rest less often. We also make sure to drag the pastures in the summer regularly as that is the best time to kill the little suckers.

    There is a reason the daily de-wormers are banned in Europe, they've had devestating effects on resistance problems. Now with ivermectin some research is finding the ERC (egg reoccurence period) is dropping from 8 weeks to 4 weeks. Still getting the same "kill level" but the time of efficacy is shortening. Not good.

  20. Very good information. Just wanted to add that there is a school of thought that perhaps some of our chronic human ailments (allergies, etc.) may be occurring because WE are so parasite free.
    Yeah not a chance in hell I would infect myself with parasites to cure my allergies.

  21. My vet and I went over this information last year. It is good to read through a reminder. Thanks for posting.

    Managing a large herd on limited acreage has its own challenges in this regard even with the horses being stalled.

  22. Great post!!
    We have a lot of horses and do our own fecal egg counts ourselves with a lab we have set up right at the barn! It makes it easy to keep track of parasite levels, is much more cost-efficient, and allows us to determine who our low and high egg shedders are.
    It's a great system... and though I realize doing the counts ourselves might not be as accurate as sending them to the vet who puts the sample in a's efficient enough for its purpose.
    What I really like is knowing we are worming horses "as-needed" and not administering chemicals blindly to horses that have natural resistance.

  23. Great post!

    Thanks for sharing this. It's terrifying that these types things evolve and resist the medicine, but that's evolution for you. I'm going to have to look up the exposure rates for the Southeast now.

  24. I'm glad your post has been read by so many people. This is the same information I've been getting lately -- maybe you should put it in your sidebar for people to easily come back to.

    "The whole tube" is what my 900 pound horse gets because I always thought about that saying, "What doesn't kill you makes you stronger." (speaking to the worms)

    I just learned that cats get worms by eating mice, and since mine eats mice every day, our vet said I must worm her every season. This means my cat will be getting wormed more often than most German horses.

    Wormers aren't the only thing that is controlled here--everything is controlled here. Not only do I need a prescription for a vaccine, I am not allwowed to administer it. Also, if I have a stomach ache, headache, or a cold, I have to go to a pharmacy and ask for something. If I have a headache after 5 pm, I have to wait 'til morning when they open for Aspirin. These are freedoms I took for granted back home.

  25. Great information! Thanks for sharing. I've considered getting the equipment to do fecals on my own. I did a few when I worked at the vet clinic so I'm familiar with the process. However I just don't know if it's cost effective for one horse, one donkey, two dogs, one cat and a bunch of chickens . . . it probably is because we could do them as frequently as we wanted. Anyway thanks for sharing.

  26. Very helpful information! As a horse owner deworming is part of your responsibility. You need to ensure that your horse does not suffer from the burden of having too many parasites.


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