This is a parasite which usually infects donkeys (and usually the donkey has no problems with it). The donkeys can serve as a source of pasture infection. Adult horses (which are not the usual host for this parasite) pick up the parasite while grazing a pasture with donkeys. The life cycle of the parasite cannot be completed in the adult horse, and the horse ends up with lungworm larvae in its airways. The result is airway inflammation and a cough. On the plus side, this parasite is easily killed with ivermectin or moxidectin, and we see problems with it incredibly infrequently.