The recommendation for de-worming protocols has changed quite drastically in the past 5 years. While new research continues to point out the risks associated with over doing it with your de-worming the word has been hard to get out to horse owners, which is hampered largely by the anthelmintic companies continuing to tout the rotational every other month de-worming schedule.
Typical parasite programs have been based on schedules that have nothing to do with parasite transmission. For years the recommendation was to de-worm every time your farrier came to reset your horse's shoes. The goal of this recommendation was to make it easy for horse owners to remember. You are standing there holding your horse, great time to get this little chore done. Or if you are lucky your farrier might even give you hand.
The problem is that this method of medicine delivery did not take into account what was actually going on in your horse's body. Repeated use of common anthelmintics given at inappropriate doses has led to vast resistance in our parasite populations. This not only complicates our ability to deal with significant parasite loads but also leads owners to believe that they are de-worming their horse when they may be doing nothing more than giving ineffective drugs. While the thought of any parasite lurking undetected in the bowels of your favorite mount can give any horse owner the heebie jeebies, recent evidence suggests that horses may actually be healthier with a small well managed parasite load.
What?! How can this be possible?
Make no mistake, after all the progress we have made in equine health in the field of parasite control since the development of ivermectin I am not advocating that parasites are good and should be encouraged. But, there is evidence that maintaining a completely parasite free digestive tract may predispose a horse to digestive insufficiency as well as allergies.
Horses evolved right along with the parasite and their digestive tract has carried and supplied the parasite with a viable home for eons. In the wild as horses are constantly on the move, overt overburdening with parasites is rare, though parasitism is not. It is our artificial containment of horses restricting their natural grazing patterns and concentrating the parasite load they are exposed to that has caused problems in our domestic horses with parasite loads.
There are very good reasons to control parasites in our horses. Overloading a horse with parasites can have disastrous effects. Arterial aneurysms, larval migration damage, worm impactions, ileo-cecal colic, poor health, poor weight gain, and overall damage to the intestinal tract all are consequences of an overabundance of parasites. However, a few helpful parasites in the intestinal tract of a healthy adult horse help may (there is conflicting research in this area) help to stimulate the horse's immune response in a positive way, as well as increase overall gut motility and in some cases aid in digestion in the large colon.
In our area we are blessed with incredibly cold parasite unfriendly winter temperatures as well as a period of very dry hot weather in the summer. During both of those times there is very little transmission of parasites happening because the larva do not survive to be transmitted. The exception to that would be horses that are kept indoors in the winter or on irrigated pastures on the summer. High stocking densities and poor manure management also increase the ease of parasite transmission and may lead to a larger worm burden in your particular herd.
In most cases we recommend twice annually de-worming. I like to use a product in the fall that will take care of both tape worms and bots (both of which are common in our area in the summer months). These products include Equimax, Quest Plus and Ivermectin Gold. Equimax is my preferred choice. I have had a few horses react badly to Ivermectin Gold I do not recommend that one in pregnant mares. In the spring I recommend one of the drugs that targets roundworms and strongyles such as Strongid, Panacur, or Safeguard. Typically we are de-worming towards the end of October to coincide with freezing temperatures and therefore less chance of larval transmission and then again in the spring when there is warmer temps and thawing generally March or April.
In the between months I recommend fecal exams on your horses to be sure that this program is working for your particular herd. Sometimes a horse may be extra likely to be carrying or shedding high numbers of parasites despite de-worming. Those horses will often need more frequent de-worming in order to prevent shedding or a build up of parasites. Foals need to be de-wormed more frequently during their first year of life as well as they are extra sensitive to an over abundance of parasites. I recommend that foals be de-wormed at one month of age, again at weaning or 3-4 months of age, and then again at 6 months of age.
There are several options out there for folks wishing to go more natural and not use the commercially available products. In some herds with very strict manure management these can be effective but I do recommend periodic fecal exams to be sure that worm loads aren't building up creating trouble. Diatomaceous earth, garlic, and tobacco have all been used by horse owners attempting to manage their horses more organically. Of these options diatomaceous earth would be my first choice. Garlic can cause anemia when administered at levels high enough to discourage parasites. Tobacco also comes with other side effects and is not generally recomended for horse or human!
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