Causes of drench resistance

Resistance genes are naturally present in every worm population, although at very low levels. When the proportion of worms with resistant genes in a worm population increase to such a level that a significant percentage is no longer removed by the particular drench, then ‘resistance’ is said to occur. For most anthelmintics, a reduction of less than 95% of a worm egg count after treatment is considered to indicate resistance to that drench.

Many factors can influence the development of resistance, including the worms themselves and their environment, however, the way that cattle are managed also plays an important role.

A parasite population can develop resistance to a chemical through:

  • Frequent use of the same chemical or chemicals within the same chemical group. Avoiding unnecessary treatments will help avoid the development of resistance.
  • Under-dosing with drench, which allows the more tolerant pests in a population to survive. Common causes of under-dosing include under-estimating the weight of animals being treated, poor application technique, and mis-calibrated application equipment. Untreated cattle licking animals recently treated with a pour-on product can also result in under-dosing.
  • Unintentionally exposing non-target parasites to chemicals (e.g. pour-on products to treat worms can also affect tick and lice populations).
  • Treatment of cattle when few worms are in refugia (the number of drench susceptible worm eggs and larvae on the pasture is low).