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Fenbendazole is an extremely safe wormer, able to withstand overdoses of 100-200 times the standard dose without causing harm to your horse. For this reason Fenbendazole is often used to clear immature parasites (which are particularly resistant to wormers due to their slower metabolism) from a horse by applying a dose twice a day for five days.
The most popular Fenbendazole-based wormer on the market is Safe-Guard.
Oxibendazole-Based WormersEffective Against:
Whereas Oxibendazole may not target as many parasites as others, its success rate against the above listed parasites is very impressive: 97 – 100% effective! In addition Oxibendazole is a safe wormer, able to withstand overdoses up to 60 times the standard dose without causing harm to your horse.
The most popular Oxibendazole-based wormer on the market is Anthelcide EQ.
Ivermectin-Based WormersEffective Against:
As you can see, Ivermectin is one of the most effective and well-rounded wormers available, though as with all wormers it does have its weaknesses since it is ineffective against small encysted strongyle and tapeworms. A very safe wormer, Ivermectin can be given in doses up to 60 times the standard dose without causing harm to your horse.
The most popular Ivermectin-based wormers are Zimecterin and Equimectrin.
Praziquantel is not effective against many types of parasites, but it has been shown to target the abovementioned parasites very effectively when paired together with Ivermectin. As such, Praziquantel-Ivermectin blends are available in the forms of Zimecterin Gold and Equimax. These blends should be included in any rotation schedule as they are extremely effective.
Pyrantel Pamoate-based WormersEffective Against:
Note that although tapeworms are listed above, Pyrantel Pamoate will not prove terribly effective against them unless a double dose is applied. Pyrantel Pamoate wormers are safe for horses up to 20 times the standard dose.
The most popular Pyrantel Pamoate-based wormers are Strongid and Exodus.
Moxidectin-Based WormersEffective Against:
Moxidectin has been the subject of controversy and for good reason. It is the only horse wormer that is capable of killing the above larvae in a single dose, which is a very impressive feat. Its liability is that the overdose tolerance threshold is far less than other wormers; 5 times the standard dose can cause significant damage to your horse.
Whereas it may be difficult to give an adult horse an overdose 5 times the standard dose, it's extremely easy to make this mistake when worming a foal. As such, at no time would I recommend using this wormer on foals or weakened horses.
The most popular Moxidectin-based wormer is Quest.
Knowing which parasites each of the above wormers are effective against is a good first start, but in the world of parasite prevention the difference between success and failure ultimately lies with how you plan your horse worming schedule.