How often should you deworm your horse?

The recommendations for deworming horses have changed in the last few years. Due to the widespread development of parasite resistance to two of the 3 families of dewormers commonly used and the fact that the old "deworm every 6-8 weeks rotating dewormers" plan was based on targetting a specific parasite that is no longer the main threat and on the use of old dewormers and doesn't take into account the effects of the currently used broad-spectrum dewormers, the current recommendation is to deworm strategically. This means focusing on the times of year when parasites are an issue based upon geography/environmental conditions and on deworming AS NEEDED based upon fecal egg counts.

It is known that horses can develop quite strong resistance to certain parasites when mature and around 50% of adult horses maintain low parasite burdens with a minimum of deworming. Another 30% tend to carry moderate burdens and need to be dewormed slightly more frequently and 20% are high egg-shedders and need to be dewormed around 4 times a year. Foals are a different story and recommendations for them are based upon their increased susceptibility to parasites.

The choice of dewormers is also more complicated than "just rotate" because in more than 90% of areas tested world-wide, strongyles (the main parasite of concern in adult horses) are resistant to fenbendazole (panacur) and in around 50% of areas they are resistant to pyrantel (strongid). So the only drug choices still highly effective for strongyles are typically ivermectin and moxidectin which are in the same drug family. Fenbendazole and pyrantel should ONLY be used in situations where fecal egg counts have been done prior to dosing and again 10-12 days after dosing to determine that they are still effective in a given population/herd.

The basics of deworming are that all horses should be dewormed spring and fall for bots and tapeworms, therefore with a combination of either ivermectin and praziquantel or moxidectin and praziquantel. Beyond that, fecal egg counts need to be performed at specific intervals (3 months after dosing with ivermectin or 4 months after dosing with moxidectin) to determine if any further deworming is necessary during the season when parasite reinfection rates are highest. Whether or not to deworm is based upon the number of eggs per gram of feces with over 200 eggs per gram bein moderate and indicating need for further deworming and over 500 eggs per gram indicating a high burden and likely a horse with a lower resistance to parasites. Seasonal conditions in which parasite reinfection rates are going to be high are when temperatures exceed 45 degrees but are lower than 85 degrees farenheit. So for climates where winters are snowy and cold, deworming is typically not recommended during the winter months and in climates where the summer is extremely hot, deworming is not typically recommended. This is because the weather controls parasite reinfection rates at those times and the goal is to minimize the use of deworming drugs to help slow the development of resistance to the drugs that we have as there are no new deworming drugs set to hit the market anytime soon.

If we don't minimize our use of chemical dewormers by practicing strategic deworming, we will end up with parasites that are resistant to all of our deworming drugs before we have anything new to use.

These recommendations are based upon Dr. Craig Reinemeyer's presentation at the 2009 American Association of Equine Practitioners conference. These are for adult horses. Again, foals are more susceptible to parasites and besides simply concentrating on strongyle control, we must also concentrate on ascarids. Ascarids are showing resistance to ivermectin and pyrantel...add that to the resistance of strongyles to fenbendazole and pyrantel and you see that it is more difficult to plan a deworming program that is going to be effective for foals because of the chance that the drugs you are using may only target one of the two common parasites of foals.