Horses are exposed to parasites that are found in their environment. The amount of exposure depends on where your horse lives, how many other horses share the same environment, and how susceptible the individual horse is to parasites. The most common dewormers today are fenbendazole (Panacur or Safeguard), pyrantel pamoate (Strongid), ivermectin (Zimecterin, Equimax), moxidectin (Quest), and praziquantel.
We no longer recommend following the rotational deworming programs of the past. The problem that comes with indiscriminate deworming is the parasites build up a resistance to the deworming products, and these products are may no longer effective. We recommend deworming horses based on fecal samples collected and tested to determine parasite load and therefore verify the need for deworming. All horses need to be dewormed twice a year, even with negative fecal egg tests, to
take care of tapeworms and bots whose eggs do not appear in feces.
Common Equine Parasites
Roundworms (Ascaris) – large round worm most commonly seen in young horses or immune compromised horses. Large numbers living together in the intestine can cause an intestinal blockage.
Small Strongyles – small reddish worms that can encyst in the lining of the intestine. Few dewormers can touch encysted larva. Mass emergence of encysted larva can cause severe diarrhea and colic in horses. This parasite can be resistant to several classes of dewormer.
Tapeworms – small, segmented worms that attach to the intestinal lining typically around the junction of the small and large intestine. Large numbers of tapeworms can cause colic. Typically a double dose of Strongid (Pyrantel) or a single dose of praziquantel will take care of the problem. Horses should be dewormed once a year with one of these products. The eggs will not always show up in a fecal egg test.
Bot Fly Larva – round, red, bristly larva that are found attached to the lining of the stomach.
Horses should be dewormed one time a year after a hard frost with Ivermectin or Moxidectin.