Macrocyclic lactones (MLs) revolutionized parasite control in horses and other animals. They are unique in that they are effective against arthropods and nematodes. The first of the widely used avermectins was ivermectin. In 1983, it was marketed for use in horses as an injectable formulation but was withdrawn in 1984 after about a year and half on the market because of adverse problems. It was replaced by a paste formulation and an oral/stomach tube liquid formulation. Ivermectin is highly active on bots, ascarids, large and small strongyles, pinworms, strongyloides, stomach worms, and some other internal parasite species. Another ML, moxidectin, became available in 1997 as a gel formulation for oral administration. The parasiticidal activity of this compound is similar to ivermectin except efficacy is less on the common bot (Gastrophilus intestinalis) but high on encysted small strongyles. Recently however lower than initial activity on ascarids and small strongyles has been found for both ivermectin and moxidectin.