As spring and warm weather approaches, horse owners typically consider deworming. Recent discussion has centered around how horse owners should switch from rotational deworming--or time-based deworming with the same drug--to strategic deworming also known as targeted or selective deworming. Following that will be consideration of ways to prevent reinfestation of horses that were dewormed.  

Rotational deworming is the use of a different class of dewormer each time deworming a horse is undertaken. One form of this is slow rotation, when one dewormer is used for one year, then a different dewormer is used the next year. Fast rotation includes the use or rotation between different classes of dewormers at periods of three to six times per year. Time-based deworming is when horses are dewormed at eight-week intervals, or another set time period.   

Parasite guidelines written by the American Association of Equine Practitioners are designed to prevent parasite resistance and increase dewormer effectiveness by reducing deworming treatment frequency using strategic or targeted deworming, and using environmental management practices.  

What is strategic deworming?  

Strategic deworming involves collecting the feces of the horse and examining it, using a fecal flotation test, to determine the number of parasite eggs present. Then, with strategic deworming, only horses with a specific number of parasite eggs or higher (called “shedders”) would be dewormed.  Shedding refers to the number of strongyle type eggs present.  Generally, deworming a horse is done if its fecal egg count was 500 eggs per gram or higher. A previous article in the Connecticut Horse Council newsletter (Spring 2011) focused on strategic deworming, its advantages and disadvantages, so that will not be covered here.  

Luckily weather conditions do provide relief with deworming. Strongyle larvae cannot survive on pasture when temperatures rise above 85 degrees Fahrenheit, and eggs cannot develop into larvae when temperatures are below 45 degrees F. Although strongyle larvae may only survive a few weeks in hot weather, in cold weather they may persist in the environment for as long as six to nine months depending on local climatic conditions. Therefore, considering ways to prevent reinfestation is important. 

What are ways to prevent reinfestation after a horse is dewormed?  

• Remove manure from stalls and small paddocks daily, and from larger paddocks on a weekly basis.  

• If at all possible, rotate dirt paddocks in the summer months so that the eggs/ larvae die off before finding a host.  

•  Composting manure should be considered since it will help kill eggs and larvae.  

•  Compost manure before spreading onto fields (heat of composting will kill the eggs/larvae).  

•  Keep weeds and grasses in paddocks short.  

•   Avoid feeding grain/hay out of wheelbarrows or manure buckets used for cleaning stalls.  

• Perform fecal egg counts on new horses and deworm appropriately before turning out in lots and/or pastures.   

•  Avoid having a lot of horses in a small area (high stocking density).   

•  Limited access to pasture or absence of grass may help prevent reinfestation.  

Focus special attention on fecal egg count monitoring in young horses, which are more likely to be high shedders than mature horses.  

• Pasture rotation may help prevent reinfestation especially if manure is picked up, and mowing is done after taking horses off the pasture.  

• Cross grazing with other species can help, they won’t be affected by the eggs or larvae and give them time to die off.   

Strategic deworming is a new way of managing the horse to reduce parasite loads.  Studies have shown that it is quite effective. Work with a veterinarian to determine the deworming method that is best.  It is also important to prevent overgrazing and manage manure in the pastures to keep from adding to the horse’s parasite load. Deworming works best when it goes along with best management practices that include good farm sanitation and manure removal.    

Resources

American Association of Equine Practitioners. (n.d.) Internal Parasites: Strategies for Effective Parasite Control.  

https://aaep.org/sites/default/files/Guidelines/AAEPParasiteControlGuidelines_0.pdf   

Brady H and Nichols W. (2009). Drug resistance in equine parasites: an emerging global problem. Journal of Equine Veterinary Science 29 (5): 285-29.  

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