Acquired resistance is strong, and infections in mature horses are quite uncommon.
Coughing, accompanied by a copious mucoid nasal discharge is a common clinical sign in infected foals, and this is due to lung migrations of P. equorum larvae. In uncomplicated cases, respiratory disease is usually transient. The presence of immature and mature adults in the small intestine is usually asymptomatic. However, heavy infections may result in rupture of the small intestine at the mesenteric attachment where blood vessels enter and where the musculature is weakest. Surgical intervention is often necessary to relieve the obstruction and may necessitate multiple enterotomies to remove the parasites.