There has been growing concern about parasite resistance to available dewormers. While it is tempting to deworm frequently and aggressively, there are no new dewormers currently in development. After studying how to increase susceptibly of parasites to our dewormers, researchers have developed the idea of refugia in livestock animals.

Refugia is the natural population of parasites within an animal/environment that are unexposed to anti-parasitic medications. It’s important to remember that while an excessive parasite burden can cause severe disease, especially in young or immune-compromised animals, a few parasites will not harm a healthy adult animal.  Unfortunately, parasites are like any other organism in that they are genetically diverse. Therefore, our dewormers do not have equal efficacy across the parasite population.

Consider the following graphic: Though it is hypothetical, the graphic shows what occurs to the worm population given the efficacy of a hypothetical dewormer. Only the parasites with the greatest ability to withstand the dewormer survive and reproduce. In turn, they pass the dewormer resistance to their offspring. With each generation, more and more of the total population of worms is resistant to the dewormer.

100 Adult Worms

            “Dewormer A”

This is the importance of refugia. If the parasites are only breeding with other parasites that survived the deworming treatment, that resistance is higher in the next generation. However, if the resistant parasites reproduce with a population that has not been exposed to the dewormer, the more susceptible parasites of the refugi population will be more likely to pass on susceptibility to the dewormer, and will effectively dilute the resistance gene in the later generations of worms.

How does this theory play into your deworming strategy? A new approach has been developed called strategic deworming. Research has shown that in a herd 20% of the animals shed 80% of the parasite eggs found in the pasture. So when deworming a herd every 6 weeks, we are over-treating the majority of animals while undertreating some. Factor in the growing resistance, it is a recipe for parasite control failure. Strategic deworming involves:

  1. Identifying “high shedders” in your herd- these are the animals most likely to be passing the highest number of parasite eggs
  2. Identifying the efficacy of each class of dewormer against each type of parasite on a particular farm
  3. Determining the best time to deworm your animal against a particular parasite given the lifecycle of that parasite

Veterinarians complete fecal examinations and evaluate the number of parasite eggs in a particular fecal sample. From this, an animal is classified as a low, moderate or high parasite shedder. It is recommended to have your veterinarian evaluate a fecal sample at least twice a year. If there is a high shedder on your property, it is recommended to complete more fecal examinations. If you are having a parasite problem on your property, a Fecal Egg Count Reduction Test (FECRT) can be used to monitor the efficacy of dewormers used. A fresh fecal sample is taken before deworming, then again 10 days after deworming. The number of parasites is counted in each sample and then compared to evaluate if a specific dewormer is effective. 

Using strategic deworming with refugia allows owners to correctly identify which animals have the heaviest worm burden on a farm and take steps to decrease the shedding of parasites on the property. Strategic deworming allows owners to spend less money and energy deworming animals, all while being able to treat the herd of animals effectively.

Call Bayside Veterinary Services for more information!