Equine Internal Parasite Control

Below is a video that shows a clip from a gastroscopy and some bot larvae they found attached to the stomach. Watch for it to start moving at about the 0:10 mark. The video is not mine and was taken by a veterinarian in Norfolk, UK.

​I remember several years ago sitting through a lecture on equine parasitology and realizing that I had really missed the boat on just how complex this area of equine medicine can be. Its easy enough to push a tube of dewormer in the horses mouth and forget about it but if you really start to dive into it, it can be a little overwhelming to digest the number of parasites that our equines are exposed to and must be protected from, each with their own lifecycle, anatomical preference, environmental considerations and drug susceptibility.

In my view, the most critical issue currently facing the equine industry in terms of parasites is the realization over the past several years that parasite resistance is real and quite widespread. This means that the dewormers we have counted on for years are not working the way they once did in many parts of the country. Also alarming is the fact that new dewormers are not hitting the market like they were 15-20 years ago leaving us with few options on farms where resistance has taken hold.

An important paradigm that I did not come up with, but have latched onto, is that due to differences in individual horses, about 20% of a herd will be responsible for 80% of the parasitism and parasite egg shedding. This means that when we treat based solely on a calendar that says its time to do it, we are likely overtreating many of the horses on the farm and possibly undertreating the horses that most need our attention. At this time, a fecal egg count test is the best tool available to find the horses that are most contributing to the overall parasite burden of the herd. The protocol is simple and requires a small fecal sample which is placed in a solution to separate parasite eggs from the organic material of the feces so they can be visualized under a microscope. The test is not perfect but is very helpful in directing treatment plans for the ranch and, through a modification of the test (fecal egg count reduction test), identifying which drugs are effective for that particular farm. I think it is important to note that the goal of deworming is not to eliminate all the parasites. The concept of parasite refugia has to do with maintaining a drug susceptible population of worms. In short, we want to keep the worm burden low enough that it has no impact on the horses health but not push so hard for a complete worm kill to select for resistance in that way.

When I set up a parasite control plan, several factors are important.

  • Population demographics—juvenile horses (<3 yrs) are, in general, going to be more susceptible to all forms of parasitism than adults. They will also have some special considerations regarding the types worms and therefore dewormers that are best to use. The needs of places with 3-4 adult horses will be very different from those with a mixed population of broodmares, weanlings, yearlings, etc.
  • Grazing management—Obviously the more densely populated a pasture is, the more opportunity there will be for parasite contamination. Additionally, rotating pastures, if possible, so that the most susceptible horses are always on the cleanest possible pasture can be very effective.
  • Manure management—Cleaning heavily stocked pastures or high traffic areas by spreading and breaking up the manure so it can dry out and be exposed to UV light from the sun can be quite effective at decreasing a pastures worm burden.
  • Some ranches/farms have steady traffic of horses both coming and going. This does not necessarily change the principles of controlling parasites in these populations but it may change the risk profiles for any permanent residents.

To varying extents, all of these factors have an effect on what parasites we need to be most concerned about. The worms that I feel have the strongest impact on the health of the horse in our area of Texas are small strongyles (cyathosomes), ascarids (round worms), possibly tapeworms and Draschia/Habronema (the cause of “summer sores”). The first three listed are probably the most important. Pinworms can be a great aggravation to the horse and deserve attention but are not a cause of illness in the same degree as those listed above. Bot larvae are undesirable and can look dramatic on an endoscopic exam of the stomach, but the adult flies seem to be a greater nuisance than the larvae themselves.

As a baseline, I am of the opinion that every adult horse that is in good body condition is going to likely benefit from being dewormed with a product labeled for encysted small strongyles and tapeworms once a year (Quest Plus) with minimal risk of resistance developing. The question that can be answered by fecal egg counts, fecal egg count reduction tests and an honest assessment of the properties grazing and manure management is which parasites seem to be causing a problem, which horse(s) need to be dewormed more than that and is the drug we have selected even effective. There is mixed opinion about the value of rotating dewormers. I am of the opinion that the most logical strategy is to maintain routine surveillance of the herd through fecal egg count tests and fecal egg count reduction tests and deworm with the most appropriate dewormer based on those findings.

I’ll close with a response to the most common question that comes up after I have a discussion about parasite control: How much is this going to cost? The answer to that question, quite frankly, is a lot less than if you develop a resistance problem in your herd. In reality, it does seem to be a little more expensive in the first year that the strategic program is implemented than the traditional deworming based on the calendar. In subsequent years, the cost is probably very comparable or less than traditional deworming protocols because many ranches will spend far less on drugs even with the additional costs of testing. But I must emphasize, resistance is a growing problem that will be expensive and very frustrating. I believe that it is in the best interest of the horse, firstly, and secondly, my clients finances to take this issue seriously and consider setting up a parasite surveillance and strategic deworming program.

If you made it this far, thanks for reading. If you have questions or want to develop a specific plan for your herd, feel free to call or shoot me an e-mail.


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