I have seen many changes in worm control since I graduated from veterinary college in 1970. At that time most horses were dewormed with a combination of Piperazine, Phenothiazine and Carbon Disulfate for bots; the Bendizoles were just coming on the market. They mostly were administered by a veterinarian with a stomach tube because they tasted terrible. Then Ivermectin came along, it was an injectable dewormer that claimed to rid horses of worms forever. Shortly thereafter Ivermectin started selling it as a paste dewormer. Originally deworming was recommended in the spring and fall, 45 days after the first hard frost or freeze. Fall worming was intended to get the bot larvae before they left the stomach to disrupt their life cycle. Then it was thought that deworming 4 times a year was even better. Eventually it was widely believed that every 6 to 8 weeks might even be better. Here in North America it was thought that a daily dewormer might be best of all. Unfortunately, as with most good things there is generally a down side to them and the development of resistance to the dewormers soon became evident.
So what might be causing this resistance to develop? In studies it has become evident; just as in other areas of medicine such as antibiotics and bacterial resistance, it was largely due to the over use of the dewormers. It appears that there was a small portion of the worm population, especially strongyles that were resistant to the deworming medication. The theory is that each exposure of the worm population to a dewormer killed off the sensitive worms, but allowed the resistant ones to reproduce and flourish allowing them eventually to become the majority of the worm population on some farms and in some pastures according to research data collected by Dr. Kaplan at the University of Georgia.
So what can we do to slow the development of resistance to deworming medications? It appears that only deworming when needed will reduce the number of times the worm population is exposed to deworming medication. This will stop killing off all the sensitive worms and allow them to mate with the resistant ones so that a high percentage of their off spring will be sensitive. The only way of knowing when to deworm and which horses to deworm is by doing fecal egg counts. The results of fecal egg counts can tell you which horses are passing the highest number of eggs in a pasture and allows you to target those horses. We may also use a before and after deworming comparison of fecal egg count results to determine the effectiveness of the deworming medication and the possibility resistance has developed to the dewormer used.
Fecal egg counts are not intended to replace deworming, but should be a guide to indicated when to deworm. Fecal egg counts will also indicate which horses are contaminating pastures and their environment and should be dewormed. This will also give horse owners the information needed to classify each horse as a low, medium, or high shedder of strongyle eggs and then they can target the horses’ deworming program accordingly.