Decoding Dewormers: Types, Resistance Concerns, and Use for Horses

it will have an effect on the horses parasite load. To do this we have to make sure there is a population of parasites that are not exposed to dewormer called refugia. Any parasites not exposed to the dewormer those on pasture encysted in the gut and in horses that are dewormed less frequently will not benefit from developing resistant genes and will remain susceptible which dilutes the resistant parasites within the population. This susceptible parasite population is maintained by deworming some horses less than others and allowing them to shed a small number of susceptible eggs back into the environment. Once resistance is present in a parasite population on a farm it will not go away but the rate of developing resistance can be slowed using refugia.
If parasites develop widespread resistance to all three drug classes we will be left with no effective options for controlling parasites. So what can we do to slow down resistance? The best thing you can do is make strategic deworming decisions to increase refugia by reducing the dewormer treatments for horses that have good natural immunity which is determined by fecal egg counts FEC. Consistently monitoring the efficacy of dewormers used on your farm through a fecal egg count reduction test FECRT every 3 years can also help you make decisions on which treatments should be used
Current Resistance
In many locations worldwide some level of anthelmintic resistance has been documented in small strongyles and ascarids. Small strongyles have widespread resistance to benzimidazoles moderate resistance to pyrimidines and early indications of resistance to macrocyclic lactones. Ascarids have widespread resistance to macrocyclic lactones and early indications of resistance to both benzimidazoles and pyrimidines.

Chemical Class	Small Strongyles	Ascarids	Large Strongyles
Benzimidazoles (fenbendazole/oxibendazole)	+++	+	-
Pyrimidines (pyrantel)	+++	+	-
Macrocyclic lactones (ivermectin/moxidectin)	+	+++	-

Chemical Class

Small Strongyles
Ascarids
Large Strongyles
Benzimidazoles fenbendazoleoxibendazole
Pyrimidines pyrantel
Macrocyclic lactones ivermectinmoxidectin
indicates widespread resistance. indicates moderate or common resistance. indicates early resistance. indicates no known resistance at this time.
Note. Information in table gathered from the American Association of Equine Practitioners Internal Parasite Guidelines.
Because horses develop immunity to ascarids at a young age small strongyles are the main parasite of concern in adult horses because of the growing anthelmintic resistance problem. It is important that horse owners take action to reduce incorrect use andor overuse of dewormers to ensure drug classes remain effective. The American Association of Equine Practitioners AAEP has created guidelines for deworming that focus on increasing refugia and these should be consulted when creating your deworming strategy with your veterinarian.

How frequently does my horse need to be treated with dewormer?
When creating a wholefarm deworming plan you must evaluate each individual horse and treat it specifically. Horses vary in immunity to internal parasites. Therefore two key factors should be assessed the horses shedding rate and age. These two factors can guide treatment plans and frequency based on the horses current parasite load and risk for developing health problems from the parasite load.
Shedding Rate
A horses shedding rate is the number of parasite eggs it releases into the environment through its manure. This rate provides insight on the horses current level of immunity and the amount of treatment needed. To evaluate a horses shedding rate an FEC is conducted which analyzes the horses manure to identify the type and number of parasites within it. FECs reveal primarily small strongyle and ascarid eggs it is rare to find other parasite eggs in an FEC even if the horse is infected. Horses are then placed in one of three categories low moderate or high shedders based on the number of eggs per gram EPG of feces
small strongyle and ascarid eggs are counted separately.
Most adult horses have good immunity against small strongyles with 4060 of adult horses being low shedders 2030 moderate shedders and 1030 high shedders. Therefore about 80 of eggs shed on a horse farm will come from only 20 of the horses. Horses that are high shedders should be targeted with more frequent treatments than horses that are low shedders. This allows for the overall population of small strongyles on your farm to be reduced while reducing exposure to dewormers in horses with lower shedding rates thus promoting refugia.
Age
A horses immunity to parasites