Before we jump into what to deworm with this time of year, it would be more helpful to understand the current approach to parasite control. We used to counsel each farm based on its own particular age range and population density, but now it is far more appropriate to do it on an individual horse basis. By this means we would determine each horse's own fecal egg count score and place them into a low, medium, or high category. These categories, in turn, would have their own yearly treatment frequency with the lower count horses being dosed maybe twice a year and the high ones 4 or 5 times yearly. Any less than than twice yearly treatment and we run the risk of large strongyles reappearing.
Young horses less than 4 years of age do fluctuate in their respective counts over time and then seem to level off after age 4, hence we wait until age 4 to start counts. The more mature horses seem to be consistent from year to year in their count range, and so we wind up checking them only once every 3 or 4 years and readjusting if necessary. The reasoning behind this approach is that we don't have any new dewormers coming down the pike, and secondly that resistance is becoming a big concern. If we over treat then the most susceptible of the worms are eliminated and only the strong survive; by contrast, tailoring the treatment frequency to the individual worm burden level allows some of the weaker wormy genetics to survive and water down the worm gene pool and at the same time address the horses who are contributing most to the pasture contamination rate (the most worms on any farm are in the fields). So for instance, a horse that might spit out 2000 eggs per gram (EPG) of poop is shedding 10 times more than a horse who is kicking out 200 EPG (or as much as 10 horses shedding 200 EPG each); we want to deworm that horse more frequently in order to keep the pastures cleaner.
Strategic times to deworm for all classifications of horses are spring and fall (March and September); use Quest plus or Zimectrin gold or Equimax in the spring. Use only Quest plus in September. Foals, weanlings, and yearlings have to be dewormed additionally for roundworms, which are resistent to ivermectin and quest. For the roundworms we can use Safeguard or Strongid, and it is wise to double the dose per pound for both of these dewormers. We recommend deworming foals at about 4 to 6 weeks of age for roundworms and doing so montly up to around 6 months of age. At that time we like to then run them through a cleansing 5-day, double-dose of fenbendazole or safeguard which will clean out all the immature worms.
Jon A. Koella, Jr., DVM Bridlewood Equine