Many worm control products also have activity against other parasites for which the treatment is not primarily intended. Most notably, products containing macrocyclic lactone compounds, affect both internal and external parasites. Use of these chemicals to control cattle worms can also unintentionally select for resistance in external parasites. It is important to read the label to determine which parasites will be controlled. Resistance is a significant issue in ticks, buffalo flies and cattle worms and when choosing a chemical for worm control consider the possible effects of increasing selection for resistance in non-targeted parasites.
When managing worms, one approach is to leave some animals in the mob untreated, so that unexposed worms remain in ‘refugia’ to dilute the resistant worms. This helps to reduce the build-up of resistant larvae on pastures. The opposite strategy is used to manage a herd severely affected by ticks, where treatment may be required for all groups of cattle in the herd. In this instance ‘tick only’ and ‘worm only’ treatments offer a better management strategy to reduce the unintentional build-up of resistance in the non-target parasite population.
Figure 1. Consider if the product you are using to target one parasite might also be affecting other parasites such as worms, ticks flies or lice. Images courtesy of AR Walker Wikimedia CC (worm), Lex Turner (ticks) and Jess Morgan (flies and lice)