What is strategic deworming?

Strategic deworming is a new approach that helps to slow down the development of parasite resistance while providing horses with the protection they need to maintain optimal health and performance.

This is achieved by reducing the frequency of deworming for a significant percentage of horses on a given given farm (based on fecal egg counts).

Sound animal and pasture management practices aimed at reducing the potential for contamination and reinfection also play a crucial role in the fight against internal parasites.

When it comes to parasite management, a goal of “zero tolerance” (i.e., completely eliminating all parasites in an individual horse) is no longer realistic, especially in light of parasites' increasing resistance to common dewormers.

With new knowledge comes a new way of looking at deworming – one that rests on scientific evidence and on a few key principles.

Deworm based on evidence from fecal egg count
  • Why deworm your horse when you don’t even know if he’s infected?
  • Don't base deworming decisions on something as arbitrary as the calendar. Assess your horse's worm burden first, by asking your veterinarian to perform a fecal egg count.  
  • Your veterinarian can then recommend appropriate dewormers, based on their effectiveness against the specific parasites your horse is harbouring.
Don't encourage resistance
  • Frequent, scheduled deworming often kills all the parasites that are susceptible to the drug, leaving only the ones that are resistant to it.
  • With each deworming, there is a potential of increasing the proportion of resistant parasites, threatening the efficacy of the three classes of dewormer currently available.
  • Reducing the amount of drugs we use helps preserve what is known as "refugia" (the proportion of the parasite population that has not been exposed to the dewormer) and reduces the selection pressure that promotes resistance.
Identify resistance
  • Repeat fecal testing two weeks after deworming, to determine the efficacy of the treatment and identify any resistance problems.
  • The expected reduction in the number of eggs per gram of feces depends on the class of dewormer used.
  • Your veterinarian can determine the effectiveness of the treatment and identify potential resistance to a specific dewormer based on the fecal egg count reduction.
Don't treat all horses the same
  • Horses vary greatly in their ability to fend off parasites.
  • The immune system of some animals seems better able to control parasitic infections than others. Because of their light worm burden, these horses are known as “low shedders.”
  • Other horses are genetically more susceptible to parasites. As a result, they shed more eggs in their feces – which is why they are referred to as “high shedders.”
  • More than 80% of all parasites in a herd might be harboured by 20% of the horses.3
  • A small proportion of horses (20%) may also be responsible for the majority of the contamination of pastures with strongyle eggs.3
  • Controlling parasites in these animals can have a major impact on the risk of infection for the entire herd.
Preserve refugia
  • Learn to live with parasites to some degree.
  • Horses do not have to be completely parasite-free to be healthy – their immune system can handle a light parasite load.
  • Preserving refugia helps prevent resistance by diluting resistant parasites in the population.
Consult your veterinarian
  • Your veterinarian can offer advice regarding fecal testing and drug efficacy, and help you develop a strategic deworming program tailored to your horse's specific needs.

Putting it all together: Strategic deworming made simple

3. Reinemeyer CR. Controlling Strongyle Parasites of Horses: A Mandate for Change. AAEP Proceedings, 2009; 55:352-360.

The 20/80 rule
of parasite management

Did you know that as few as 20 % of horses in a herd...

...may harbour more than 80 % of all parasites?3