Types-HorseWormers

There are many different brands of wormers and hundreds of different wormers. However, all of them are based on one or more of the following four types of chemicals. Consequently, an understanding of the following four medications will give one a good understand of all the different wormers. For related topics (such as how to give wormer to your horse, see our guide to worms and worming).

  • Macrocyclic Lactones. This group includes Ivermectin and Moxidectin.
  • Benzimidazoles. This group includes Fenbendazole,  Mebendazole and Oxibendazole.
  • Pyrantel.
  • Praziquantel.

Within these groups, there are six common types of chemicals for treating horse worms but each type will treat only certain types of worms, as the following table shows.

  Ivermectin Fenbendazole Pyrantel Oxibendazole Praziquantel Moxidectin
Bot Larvae X         X1
Large Roundworms       X    
Large Strongyles X X4 X4 X4   X3
Small Strongyles5 X X X X   X
Large-mouthed Stomach Worms X          
Lungworms X          
Pinworms X X X X    
Redworms X X X      
Roundworms X X X   X  
Stomach Hair Worms X X        
Tapeworms   X2 X2   X  
Threadworms X   X X    

The above table has superscript numbers to indicate notes. Following are the corresponding notes:

  1. Although both Ivermectin and Moxidectin can be used to treat parasitic bot fly larvae, Moxidectin is effective against only 2nd stage larvae but Invermectin is effective against all three stages. See Horse Bots for further discussion.
  2. Although Praziquantel is effective against all three species of tapeworm, Fenbendazole and Pyrantel are not. See Horse Tapeworms for more details.
  3. Ivermectin is effective against all forms of large strongyles and moxidectin is effective against all forms except the adult form of strongylus equinus.
  4. Fenbendazole, Pyrantel, and Oxibendazole are only effective against the adult version of large strongyles, not the larvae stages.
  5. Moxidectin is the only medication effective against all stages of small strongyles. Ivermectin is effective against adult and larvae stages, with the exception of encysted larvae. Fenbendazole, Pyrantel and Oxibendazole can be used to treat the adult stages. 

As can be seen from the above table, no medication is effective against all worm types. Consequently, to cover all types one needs to either:

  • Use Combination Medications. A number of horse wormers will combine two or more of the above chemicals (medications), to give a broader spectrum coverage. Note that combination medications (e.g. one tube of wormer paste which includes multiple medications) have been approved by the applicable national animal medication regulatory bodies, but one should not give multiple medications (e.g. a dose of one wormer type and a dose of another wormer type) at the same time as this risks overdosing the horse. 
  • Rotate Wormers. In this approach, one gives a different type of wormer every couple of months, so that over the year your horse will receive medications to treat all types of worms.

For more details on these approaches, see worming schedule.

Advantages and Disadvantages

The four types of wormers vary not only in terms of which worms they treat (see above table); some other notable differences are:

  • Benzimidazole group (Fenbendazole, Mebendazole, Oxibendazole). In some areas, worms have developed partial resistance to these wormers. This can be partly offset by using higher dosages or by using repeat treatments (consult the product literature or your veterinarian before doing either). However, due to increased resistance to this medication, its use is declining in favor of the other medication types.
  • Ivermectin. Treats more types of parasites than any other, so it is the first choice for a general purpose wormer.
  • Moxidectin. The only medication effective against encysted small strongyle. One needs to be especially careful with very young horses or weakened horses, as these can be relatively easily overdosed with Moxidectin.
  • Praziquantel. One of the two medications effective against tapeworms.
  • Pyrantel Pomate. Although effective against tapeworms, one may need to use a double dosage to have a high kill percentage.

The above table has superscript numbers to indicate notes. Following are the corresponding notes:

  1. Although both Ivermectin and Moxidectin can be used to treat parasitic bot fly larvae, Moxidectin is effective against only 2nd stage larvae but Invermectin is effective against all three stages. See Horse Bots for further discussion.
  2. Although Praziquantel is effective against all three species of tapeworm, Fenbendazole and Pyrantel are not. See Horse Tapeworms for more details.
  3. Ivermectin is effective against all forms of large strongyles and moxidectin is effective against all forms except the adult form of strongylus equinus.
  4. Fenbendazole, Pyrantel, and Oxibendazole are only effective against the adult version of large strongyles, not the larvae stages.
  5. M