In most sheep districts of Australia, knowing the drench resistance status of your property is essential if you are going to be able to effectively manage worms. If you do not have any drench resistance information, you should seek professional advice on how to have it done as soon as possible.

Theoretically, drench resistance occurs once a population of a species of worm can survive a dose of a drench that would have previously killed it. Worms killed by a drench are said to be susceptible to the treatment.

The currently accepted industry definition of drench resistance, as measured in a Faecal Egg Count Reduction test (FECRT), is a reduction in worm egg count of less than 95 percent.

This definition is important to understand, because at this level of efficacy drench resistance would most likely not be causing clinical worm problems, such as scouring and obvious weight loss. However it could be associated with loss of sheep production. By the time that obvious drench failure occurs then resistance is very well established within the worm population. This is one of the reasons why it is unlikely that resistance can be reversed, even if the drench group is not used for a long period of time. Preventing or reducing the onset of drench resistance is the best option.

Drench resistance is genetic (controlled by genes). Resistance to each group of drenches is controlled by different genes, meaning that resistance develops to each drench group separately. Being a genetic trait, drench resistance is also heritable (can be passed on from one generation of worms to the next).

Initially, resistant worms are rare in a population of worms. When a sheep is treated the resistant worms survive and, if they find a mate, can reproduce. The resultant offspring are resistant and if they survive as larvae on the pasture and infect another sheep they will make up a greater proportion of the worm population than their parents did. Over time, and with continued treatment, the overall resistance level to the treatment within the worm population increases.

The rate of development of drench resistance can be influenced by many factors such as:

• The chemical group and persistency of the product involved
• The frequency of treatments
• The worm species involved
• Environmental factors such as climate