Lungworm is a lower respiratory tract infection affecting horses caused by the parasitic organism, Dictyocaulus arnfieldi. D. arnfieldi is commonly found in donkeys, who serve as a natural host (carrier). Donkeys will shed D. arnfieldi eggs in their feces, which rapidly grow into infective larvae. The larvae can survive 6 to 7 weeks in pastures under favorable environmental conditions, and is more likely to thrive in damp and/or shady areas where manure was deposited from donkeys. Horses become infected by eating the larvae in the pasture. Once ingested by the horse, larvae will make their way to the lungs.

Horses develop clinical signs of infection as a result of the presence of the larvae in their lungs, usually associated with chronic coughing. This cough appears very similar to that of horses with heaves with horses observed with a prolonged expiratory phase of respiration.

Lungworm is fairly simple to treat, consisting of a single dose of ivermectin given orally by mouth. A few days following treatment, horses may temporarily show worsening clinical signs that than rapidly improve.