<< My vet recommends rotating Ivermectin, Strongid, Benzelmin, and
Anthelcide,
with the Ivermectin after a frost . . . >>
I'm interested in any opinions about rotating anthelmintics. I've always
known it was an important practice to prevent resistance, but that's all I
knew.
Then some years ago I did contract PR work for Merck, and attended their
salesrep training sessions about Ivomec (ivermectin) for sheep.
Rotation was a very big part of the discussion, and I learned a couple of
interesting principles that I think must apply to all species. The ones that
stick in my mind are:
1) Be sure you know the chemical class of the dewormers. Rotating between
two different products of the same chemical class (e.g., oxfendazole and
fenbendazole are both benzimidazoles) leads to side-resistance.
2) Some chemical classes have the same mode of action of other chemical
classes; therefore rotating between these classes will cause
cross-resistance. Ivermectin is the only product in its chemical class,
therefore you can rotate between ivermectin and any other anthelmintic.
3) If you use a product only once before rotating (fast rotation), you can
develop multiple resistance. This is because the worms are exposed to each
chemical class with too great a frequency.
4) Parasitologists generally recommend a "slow rotation" program. Merck
defines "slow" as using the same product for a year before switching.
5) Ivermectin is used on camels. I would love to see a photo of someone
drenching a camel.
With this in mind (all except the bit about camels), I have been doing a
"slow rotation" on my horses, using ivermectin (whichever brand is cheapest)
every six weeks for a year, then switching to a benzimidazole (whichever
product is cheapest) for a year, then back to ivermectin for a year. I do,
however, as Suzanne points out, use ivermectin at least once in the fall
after the bot flies are dead, even if it's a non-ivermectin year. I worm at
the same time the horse is shod, which is my way of not forgetting when six
weeks or so are up.
I once asked my vet if she thought I should have a fecal egg count done. She
said she has done fecal counts but never finds anything because most horse
owners are pretty conscientious about deworming, and also because ivermectin
is widely used and is extremely effective against the internal parasite
spectrum we have in Northern California.
So have I been brainwashed??
Linda (San Francisco) and parasite-free Chaco and Solace (West Marin County)
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Forwarded message:
From: mountss@usfca.edu (Suzanne Mounts)
To: ridecamp@endurance.net
Date: 97-11-06 11:08:47 EST
Lysane Cree asked about Benzelmin and Anthelcide--
My vet recommends rotating Ivermectin, Strongid, Benzelmin, and Anthelcide,
with the Ivermectin after a frost (which is sometimes tricky, since we don't
have all that many frosts in northern California). I must admit that I have
not examined him closely regarding the basis for his recommendation.
Suzanne Mounts
Oakland, CA