Re:Quest

[Trish Dowling <dowling@skyway.usask.ca>]

Quest is moxidectin, and a cousin of ivermectin. There is no difference in the mechanism of toxicity between 
the two drugs. The reason for more problems with Quest, is that it is given at twice the dose of ivermectin. It 
has to be given at 400 micrograms/kg of body weight, versus 200 micrograms/kg for ivermectin, in order to 
kill bots. Quest is also more fat soluble than ivermectin. So you start with twice as much drug as ivermectin, 
and as the drug is absorbed, most of it goes to the horse's body fat stores and is slowly released back into 
the bloodstream. Unfortunately, if the horse is overdosed (common in young horses and miniature horses) or 
given to horses with little body fat, there will be an increased amount of drug in the bloodstream. If there is 
enough, the drug will cross into the horse's brain and cause signs of toxicity - weakness, staggering, falling 
down. (I can think of no reasonable way to blame Quest for a weight loss problem.). Because young foals do 
not have much body fat, they should not be given Quest (despite the very misleading picture of a foal on the 
box!). I have no problem with using Quest in my horses, but I'm also able to actually weigh them. I'll bet 
most ridecampers just guess, or give the whole tube to each horse - which was safe with ivermectin but not 
so safe with Quest. I do have a lot of problems with their dosing syringe. It's flimsy, and very difficult to use 
partial tubes, and the little lock mechanisms fall off and I had to do Heimlich manuevers on my shelties to 
get them to spit them out! I also know of no documented reports of resistance in horse worms to ivermectin 
or moxidectin. Sheep and goats, yes, especially in Australia. So I will continue to use Quest on my own 
horses, and I teach my students to tell clients to at least use a weight tape when dosing their horses.
Cheers,
Trisha

Trisha Dowling, DVM, MS, Diplomate ACVIM & ACVCP
Associate Professor, Veterinary Clinical Pharmacology
Western College of Veterinary Medicine
Saskatoon, SK S7N 5B4
306-966-7359/FAX 306-966-7376