The process of deworming your horse involves more than pulling any old dewormer off the feed store shelf. You need to consider your horse's age, the drug being used, the health of your horse, and risk of exposure to parasites.
Most at risk. The very young (under 1 year), the very old, and debilitated horses have lowered resistance to parasites. Because of this, even the most effective dewormers often will not reduce egg counts to zero as they will in healthy adults. These horses are constantly reinfesting themselves and can build up large parasite burdens very easily. In these high-risk groups, if you're not going to do fecal exams to see whether the horse needs deworming, you would be wise to stick very strictly to the deworming intervals in the Dewormer Drugs Chart on the next page.
On the flip side, healthy horses who are not very young or old often have a very vigorous immunity to parasites. The immune system cells lining their intestinal tract can manage to keep parasite infestations very low or even to none at all, especially if they live in uncrowded pasture conditions where they never have to eat close to fecal droppings.
Choosing dewormers. When buying a dewormer drug, train yourself to look at the active ingredients information on the label, rather than the brand name. There are several different brand names for many drugs. If you are trying to rotate dewormers, you need to make sure a different brand name isn't actually the same drug you used last time. Knowing the actual active drug in the dewormer is also essential for knowing what the correct deworming interval is.
Parasite resistance to dewormer drugs is widespread. For small stronglyes, resistance has been found to all drugs except ivermectin and moxidectin. If you choose to use one of the drugs with known resistance problems in your program, you should plan to have your veterinarian perform a fecal exam after using that drug at least once a year. If you don't do that, you may be inadvertently increasing the parasite burden in your horses and putting them at risk of building up high parasite levels during some stages of your deworming schedule.
Exposure. Exposure is a big part of risk. Low levels of parasite infestation do not necessarily have any obvious health consequences, but they do keep parasites alive and well in the environment. Because no dewormer gets each and every life stage of parasites, when you deworm your horse you will always be missing some immature worms. These are the ones that eventually mature and begin laying eggs during the deworming interval. The bottom line here is that no horse will ever be 100% parasite free, even if it's the only horse on the property, because he will be a source of parasites himself.
Exposure can come from other horses too, of course. The more horses your horse is turned out with, and the more crowded the living conditions are, the higher the risk is of being exposed to parasites from another horse. Rotating horses through small paddocks or round pens on crowded facilities is also a high-risk situation. If you take your horse away from home for shows or trail rides, always be picky about how you let him graze (if at all) in high traffic areas, and never feed directly on the ground. A few mouthfuls in the wrong place can result in a heavy parasite infestation.
If you are like most people and have a horse who is heavily infested with parasites, or always has a negative reaction to dewormers, the thought of deworming him probably fills you with fear. This is truly a situation in which you are "between a rock and a hard place." However, the answer is definitely not to avoid deworming. Letting the parasites continue to drain the horse's energy, nutrients, and health and damage his intestinal tract only further weakens him-including his immune system.
Deworming the heavily infested horse. First, involve your veterinarian. There are a variety of strategies for making the process easier on the horse. Anti-inflammatory drugs-such as flunixin meglumine (Banamine®)-can help control systemic reactions like fever or laminitis. Some veterinarians also use antihistamines. For the actual deworming, using a schedule that first deworms with a product that primarily targets only adults, like pyrantel or regular-dose fenbendazole, is a common strategy. Other veterinarians prefer to use ivermectin or moxidectin but at a lower-than-usual dose. After these less-aggressive approaches have reduced the parasite burden, your veterinarian will recommend a regular deworming appropriate to your situation.