Derquantel is a so-called spiroindole, is obtained from fermentation extracts of Penicillium simplicissimum, a particular species of soil fungi. This drug is used in veterinary under brand name Startect, in combination with abamectin against several gastrointestinal roundworms that affect sheep. At the recommended therapeutic dose derquantel is highly effective against Haemonchus spp, Cooperia spp, and Trichostrongylus spp, including those resistant to most classic anthelmintics (benzimidazoles, levamisole, macrocyclic lactones). However it has a lower and variable efficacy against Ostertagia spp (= Teladorsagia), and is basically ineffective against Oesophagostomum spp and Trichuris spp, other important gastrointestinal worms of sheep. It is also ineffective against lungworms and other roundworms that infect sheep outside the gastrointestinal system. Derquantel is a nicotinic cholinergic antagonist: it blocks cholinergic neuromuscular transmission. The effect on the worms is that they are paralyzed and die or are expelled.