Dewormer Resistance

Anthelmintic (dewormer) resistance

Anthelmintic (dewormer) resistance is the heritable (genetic) ability of a parasite to survive a dose of anthelmintic, which would normally be effective

Worms that are resistant to a dewormer can survive treatment at the recommended dose and pass on this resistance trait to future generations. Of particular concern, is the fact that gastrointestinal parasites such as Haemonchus contortus and Teladorsagia circumcincta (prominent parasites in Alberta), have developed resistance to the two main dewormer classes that have been used in western Canada for many years such as ivermectin (Ivomec®) and the benzimidazoles fenbendazole (Safe-Guard®) and albendazole (Valbazen®). Anthelmintic resistance is a problem in Canada but also in the rest of the world.

Causes of resistance

Anthelmintic resistance occurs because of genetic diversity: some parasites can carry mutations in their genes that confer the ability to survive anthelmintic treatment.

At its most basic level, anthelmintic resistance occurs due to the high genetic diversity that exists within populations of gastrointestinal roundworms. Some individual parasites may carry mutations that they inherited from their parents which help them survive exposure to dewormers. The parasites that carry these genes may become more prevalent on your farm due to selection pressure associated with regular dewormer use.

What is selection in the context of anthelmintic resistance?

Selection is the process by which resistant worms survive anthelmintic treatment and increase their frequency in the population over time

Susceptible worms are the ones that will be eliminated by the dewormer and resistant worms are the ones that will survive the treatment. Selection is the process by which the treatment with a dewormer favours the survival of the resistant worms at the expense of the susceptible worms. This results in a gradual increase in their numbers in the population over time. For example (Figure 1), suppose you were treating ewes with a high Haemonchus contortus burden:

Figure 1: Illustration on development of anthelmintic resistance over time

Figure 1: Illustration of the development of anthelmintic resistance overtime

Initially, in a population of worms, there are many that are susceptible to dewormers (in blue) and very few resistant worms (in red) in the sheep and even less on pasture. Treatment is effective and clinical signs disappear. However, the few resistant worms survive deworming and are able to reproduce.

Overtime and after multiple treatments, the percentage of resistant worms increases. Unfortunately, the development of resistant worms can go unnoticed for a while.

As the percentage of resistant worms increases, they survive treatment and multiply. The treatment becomes less and less effective.

Note: this is an over-simplification of the process of selection which occurs over many years, one treatment will not kill 100% of susceptible worms as many are in refugia in the sheep and on pasture (For more details, see section 5).

Since resistance has a genetic basis, resistance to a drug may lead to resistance to drugs from same class (for example, parasites that acquire resistance to Safe-Guard become also resistant to Valbazen), however, the worms may still be susceptible to drugs of other classes.

How to identify anthelmintic resistance?

Anthelmintic resistance affects the flock’s health and productivity. The Anthelmintic resistance in a flock is suspected if:

  • Several sheep of the flock show reduced weight gain, poor body condition, anemia, diarrhea or deaths despite regular deworming.
  • Fecal egg counts of several animals remain high after treatment with the dewormer.

If anthelmintic resistance is suspected based on these signs, you should review the drug label to ensure the drug is being administered properly and at the correct dose.  Also, consider the different reasons why a dewormer may not appear to be doing its job in controlling the parasites on your flock. More information is available on page on Dewormers.

If you suspect anthelmintic resistance, consult with your veterinarian to investigate the problem, and develop a plan for the situation on you farm.

One way to diagnose resistance is to perform a fecal egg count reduction test (FECRT). The FECRT is a field test to diagnose anthelmintic resistance, based on the reduction in the number of parasites eggs after anthelmintic treatment. When a dewormer is effective, the fecal egg count should decrease by 95% approximately 2 weeks after deworming. Although it is a good tool, FECRT cannot be interpreted in isolation but in the context of the class of animals, time of year, and the flock and grazing management situation.

Important decisions should be made after consulting your veterinarian.