Wormer resistance in sheep explained

Recent years have seen growing resistance to multiple classes of wormers across sheep and cattle. Here we look at what causes wormer and antibiotic resistance in sheep and how resistance develops. 

Click on the arrow underneath the picture to go through the six steps and you can go back or replay at any time.

See also: Strategies for combating wormer resistance in sheep and cattle

1. Worm life cycle

  • Sheep eats grass contaminated with worm larvae
  • Larvae develop into worms and mature in the gut of the sheep
  • Female adult worms lay eggs, which are passed on to the pasture in the form of faeces

2. Worming treatment

  • The sheep is treated with the correct dose of wormer
  • Susceptible (blue) and part-resistant/part-susceptible (blue/red) worms are killed
  • Resistant (red) worms survive

3. Resistant worms

  • The resistant worms remain in the stomach producing eggs
  • Their eggs are passed on to the pasture where they go through stages of development before becoming infective larvae, which are eaten by the sheep

4. Exposure

  • Worms already on the pasture are in refugia (not exposed to the treatment)
  • The sheep picks up more susceptible worms from the pasture (those in refugia) and the proportion of resistant and susceptible worms becoe balance until it is treated again

5. Resistance strengthens

  • If the same group of wormer is used to treat sheep over a number of years, selection pressure continues in favour of resistant worms
  • Over a long period, the proportion of resistance within a population of worms increases further

6. Wormer’s effectiveness

  • Treatment is ineffective
  • Sheep will start to succumb to disease