Risk Factors For Parasite Resistance

The rate of development of resistance in a worm population to a particular anthelmintic drug varies depending on a number of factors including the characteristics of the worm population, the nature of the drug and the genetic mechanism by which resistance is developed. More importantly, there are a number of important ways in which we can influence the rate of development of resistance. The following practices are likely to increase the rate of resistance development and should be avoided:

• Overuse of anthelmintic drugs, including frequent (and often unnecessary) use on a property, continuous use treatment regimes and using the same product (or the same drug) year after year.
• Under-dosing with anthelmintic drugs, generally due to inadequate estimate of animal weight or to poor worming / drenching technique.
• Worming horses and/or farm animals when pasture refugia levels are low (e.g. in the peak of summer in Australia). Parasites in refugia are those that are not exposed to the anthelmintic drug at the time of treatment. The size of the parasite population in refugia is believed to have a major effect on the rate of development of anthelmintic resistance, with an adequate parasitic refugium reducing the rate of development of resistance. Parasites in refugia compete with the resistant parasites and provide a pool of drug-sensitive genes to dilute the resistance genes. They include parasites in untreated animals, free-living parasites (e.g. on pasture) and encysted parasites (which are not affected when the animal is dosed with most anthelmintics). Avoiding worming / drenching when pasture refugia levels are low and in the case of horses, avoiding the treatment of low egg shedding horses at all worming sessions, will help maintain refugia.

Once an individual property has a large population of resistant worms, these resistant worms may readily be spread geographically when farm animals and horses are moved from property to property.