Thursday, February 4, 2010

Worms Crawl In and Out

Disclaimer: The information contained in this post is not complete, and should not be construed as veterinary advice or an endorsement of any treatment or product.

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 ...

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."
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.

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.

YEAR 1
January — ivermectin (1ml/100 lbs body weight)
March — oxibendazole paste (e.g., Anthelcide)
May — oxibendazole paste
July — ivermectin
September — oxibendazole paste
November — oxibendazole paste

YEAR 2
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.

11 comments:

Stephanie said...

Very interesting made me think about my worming schedule - which is also rotational - similar to your new one, but more for the parasites that are prevalent in Eastern Washington.

We get to feeling safe and secure in our schedules and don't stop to submit a sample every now and then - I think I might do that this summer just to be sure. It's a small fee - for a little peace of mind.

Great post.

AareneX said...

You know you're a Horse Person when...

reading about internal parasites and treatments over breakfast is no big deal!

Breathe said...

Is it Lytha who says they only worm once a year in Europe? Or only after a fecal sample?

Less is more, then. But what if yourhorse is already on a monthybregime for years. Is it too late to switch? We do quarterly here. I used to do monthly

on the non scientific side of things all my horses have been cooperative about being wormed. How are your boyz?

Leah Fry said...

Prior to administering the first dose of Quest, they were fine about being wormed. But that stuff must taste especially bad. They even spit out the cookies I tried to give them to take the taste away. I won't be surprised at all if they give me trouble from here on out just because of how bad Quest tastes.

Desert Rose said...

There is so much to concider when treating your horses. Mine get strongid daily and the vet sends me a wormer that they get 2x a year. So glad Jazz is doing great now!

Kate said...

I've done rotational, and also daily plus 2x per year paste. You're right - the worms will always keep ahead of us!

Cactus Jack Splash said...

Interesting information. Because of health issues with several of my horses they can't have chemical wormers, I have to use an herbal tonic, diatomatious earth, and excell to treat worms...seems to work effectively according to the fecal samples.
Worms are difficult to manage and keep all horse owners on their toes

Hipstercrite said...

Love your banner and blog name! Thanks for posting on my Pee-Wee story!

City girl turned Country Girl said...

Absolutely a great post!! Awareness is key!! It all makes total sense, thanks so much for sharing!! Hopefully we all learn from it! I too keep on a rotational deworming with mine but I think even that should be changed up a bit!!

Michelle said...

I've been hearing a lot about this lately. I'm a long term subscriber to the rotational plan, but I'm going to have a fecal test done and re-evaluate my options. It makes perfect sense that resistant parasites would evolve - now we need to figure out what to do about them. I wonder if there are any holistic wormers that might help.

Laughing Orca Ranch said...

I suppose in other areas of the country deworming schedules will be different. Up here, with our cooler, dryer temps worms aren't as much of a problem.
I used fenbendazol once a year right before winter and then I used Invermectin once in late Spring and at the end of summer. I had fecal counts done on Baby Doll and they were always negative with my worming schedule.
I probably could have dewormed even less, but that schedule and dosing worked well for us.

~Lisa

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