The active ingredient in Quest is moxidectin and is not effective against lungworms. Wormers with active ingredient ivermectin are much more effective at eliminating lungworms. Fortunately in horses, adult lungworms (Dictyocaulus arnfieldi) seldom mature. Horses are abnormal hosts, and the lungworms only partially develop. Adult lungworms are seldom present to lay eggs which would hatch into larvae to pass out in the horse’s feces. Therefore, larvae are rarely found in horse feces. The stunted larval stages in the lungs cause pneumonia in horses with clinical signs including coughing and abnormal lung sounds. Definitive diagnosis involves finding larvae in tracheal washes.
Treatment consists of dosing with Ivermectin which is highly effective on all stages of lungworms.
Donkeys are normal hosts for lungworms (rarely do donkeys have clinical problems with lungworms), so maturation takes place in their lungs and larvae are shed in their feces. Donkeys are the source for infections in horses. Before Ivermectin, it was traditional to recommend that horses not graze pasture with or where there had been donkeys, since donkeys were carriers or disseminators of these parasites for horses. Donkeys periodically given Ivermectin cease being carriers of lungworms for horses.