The MLs have similar activity to the older broad spectrum groups of drugs like the benzimidazole and levamisole drenches. Moxidectin is also registered to have a persistent effect against Haemonchus contortus and Ostertagia species. This will provide protection against reinfection with these worms for a minimum of two weeks after drenching, and may be a useful control feature. Ivermectin has a rapidly diminishing effect against ingested larvae for a few days after treatment. MLs are active against some parasitic insects.
MLs have no registered claim for activity against blowflies, mites or lice on sheep.
Although generally active against worms that are resistant to the other drench groups, strains of Ostertagia circumcincta resistant to MLs have been found on a small number of farms in recent years. The long-term effectiveness of MLs will depend on how they are used, and it is important that this group is included in drench resistance tests. Group 3 - MLs Many farms have worms which are resistant to the benzimidazoles, levamisole and combinations of these drench groups. As MLs are unrelated to these drench groups, they are usually effective against worms which are resistant to Group 1 and Group 2. It is important to have a drench resistance test done to identify which of these groups is effective in your sheep before they are used.