Before Jaz's bout with strongyles (strongylus vulgaris), my worming routine was pretty simple: CW Daily Continuous Wormer and ivermectin paste in April and October. It's what Dr. G recommended and that's what we did. I never had fecal egg counts done because I assumed my regimen made it unnecessary.
Pyrantel tartrate is the active ingredient in CW. It was discovered in 1966, and was found to be effective in the prevention and eradication of large and small strongyles, pinworms, and ascarids. Ivermectin (avermectin), discovered in 1975, was introduced to the veterinary market in 1981. It was found to be effective against some ectoparasites (lice, fleas and ticks) and nematodes (roundworms). These drugs became widely used because while lethal to many parasites, they have a wide margin of safety for the host mammal.
The widespread use of this limited array of drugs has created the same dilemma as the overuse of antibiotics and antibacterials in humans — resistance. That's how Jaz, despite being dewormed religiously since he was a foal, ended up with a belly full of parasites. Large strongyles (strongylus vulgaris — read about it here and here) was thought to have been eradicated, which contributed to the delay in Jaz's diagnosis.
David Pugh, DVM, MS, DACT, DACVN, of Fort Dodge Animal Health, has studied anthelmintic resistance. He says that the problem is, no dewormer kills all targeted parasites (emphasis added):
"Obviously, parasites should be controlled when they negatively affect the horses’ health. But the formulas of the past (monthly or bimonthly administration of anthelmintics), although historically effective, have contributed to the level of anthelmintic resistance that now threatens modern horse production ...Simply put, Dr. Pugh's solution (read the full article here) to anthelmintic resistance is "less frequent deworming or (by) using an anthelmintic less frequently and only giving it to a portion of the horse population on any given farm or facility". The first part of his solution flies in the face of conventional veterinary wisdom that says to deworm more frequently, not less. But Dr. Pugh makes sense.
The more frequently the animal is dewormed with a particular class of anthelmintic, the greater potential for survival of an increased number of parasites that are not killed by that particular agent. Thus, a genetic shift or selection for parasites not killed by the particular class of dewormer occurs. On a farm, if all animals are dewormed frequently, then parasite resistance in all or most horses on a particular farm may occur."
Dr. E, the vet who diagnosed and treated Jaz's strongyles, subscribes to the rotational school of thought, though he does recommend bi-monthly deworming. Here is Dr. E's rotational schedule for Texas horses 1 year and older. I have not included the schedule for foals, weanlings, and broodmares.
January — ivermectin (1ml/100 lbs body weight)
March — oxibendazole paste (e.g., Anthelcide)
May — oxibendazole paste
July — ivermectin
September — oxibendazole paste
November — oxibendazole paste
January — moxidectin paste (e.g., Quest)
March — double dose pyrantel pamoate— 12 ml/100 lbs body weight (e.g., Strongid)
May — standard dose pyrantel pamoate
July — moxidectin paste
September — double dose pyrantel pamoate
November — moxidectin paste
The keys to this rotation are the January and July treatments. Dr. E says that most parasites on the ground will die during those periods in Texas because of the extreme temperatures. That means all the parasites are camped out in the horse's gut. I will follow this schedule, but rather than using a straight ivermectin paste, I will use one that also contains praziquantel (e.g., Zimectrin Gold or Equimax). Interestingly, Dr. E also said he "doesn't have a problem with" the continued use of a daily wormer. I will also have fecal egg counts done at least once a year, as part of their annual wellness exam.
My take-away from Dr. Pugh's article is that anthelmintic resistance is more or less inevitable, and the best we can hope for is to slow its onset until the invention of another array of drugs.