There are presently only three drugs licensed by FDA for use in sheep. If resistance to one of these dewormers is present, resistance to albendazole may already be present in the flock. All available dewormers in the US today are members of one of these three classes of drugs. It is important for shepherds to know whether each class is effective in their flock before finding out otherwise during an episode of clinical parasitism. If the flock is large enough, it is possible to test all three chemical classes in a summer by using groups of sheep.
Lambs are usually the easiest to use for resistance testing.
Figure 8 - Collecting samples from enough animals to represent the flock's status is important.