Quest may not be the best choice for this horse as an initial dewormer more because you say she is underweight rather than due to risk of impaction. In an adult horse, the risk of impaction colic from deworming isn't nearly as high as most people think. Impaction colic from deworming is more of a risk in horses under a year of age because they are likely to have a heavier parasite load due to being susceptible to ascarids which are LARGE worms.
As for a double dose of pyrantel being less likely to cause an impaction colic than Quest, that's not really true either. You are going to be killing more than 90% of the adult parasites in the GI tract with either pyrantel or moxidectin so the risk of impaction is the same.
I would recommend a fecal egg count if you know that this horse hasn't been dewormed in 3-4 months. Here in Texas we are in our high reinfection season so this is a good time to determine what kind of parasite load this horse normally carries so that you can plan a deworming program that is appropriate. And please don't go by all the old deworming plans that are posted online. Most of them are following the same old out-dated plan of rotating dewormers and deworming every 6-8 weeks---NOT a good plan at all.
Find out when this horse was last dewormed and with what. If it's been 3 months since she was dosed with ivermectin, do an FEC now if the last deworming was with Quest then you need to do the FEC 4 months later. If it was anything other than those and it's been more than 2 months then go ahead with an FEC. Then if your results are under 200 eggs per gram, you should only need to deworm this horse twice a year---spring and fall. If it's over 200 but under 500 eggs per gram, then you are going to want to deworm 3 times a year--spring, fall and once in the winter. If it's over 500 then you want to deworm 4 times a year--spring, fall and twice during the winter. In Texas, the summer weather serves to control parasite reinfection rates for adult horses so you don't need to deworm then.
For weight gain, rather than supplements just go with a good quality forage fed free choice. And definitely get her teeth checked.
(And Luvs2Ride is exactly right IndyHorse, if you have been using fenbendazole in an adult horse you have likely NOT been effectively treating parasites. Ivermectin and moxidectin can both be used in pregnant mares and at least one of them should be used to control parasites.)
Licensed Veterinary Technician
Last edited by Ryle; 02-02-2010 at 10:48 AM.