The anthelmintic activity of tetramisole, a racemic mixture, resides in the l-isomer, levamisole. It is commonly used in cattle, sheep, pigs, goats, and poultry to treat nematode infections; it has no activity against flukes and tapeworms. It is normally administered PO or SC, and efficacy is generally considered equivalent with either route. Topical preparations for cattle have been developed.
Levamisole acts on the roundworm nervous system and is not ovicidal. Its broad spectrum of activity, ease of use (being water soluble), reasonable safety margin, and lack of teratogenic effects have allowed it to be used successfully. Because of its mechanism of action, the peak blood concentration is more relevant to its antiparasitic activity than the duration of concentration. Levamisole resistance appears to be associated with a loss of cholinergic receptors. Levamisole has immunostimulant effects at dosage rates higher than those used for anthelmintic activity, and it has been used in people and to a limited extent in other animals in several diseases.
In ruminants, levamisole (pour-on, injectable, and oral formulations) is highly effective against the common adult GI nematodes and lungworms and many larval stages. It lacks efficacy against arrested larvae, such as those of Ostertagia ostertagi. Levamisole slow-release boluses are available in some countries and contain 22.05 mg levamisole. They release 2.5 mg during the first 24 hr and the remainder over a 60-day period.