Lungworm is an infection of the lower respiratory tract in horses, usually resulting in bronchitis or pneumonia, caused by the parasitic roundworm Dictyocaulus arnfieldi. The infection can cause severe coughing in horses and can be difficult to distinguish from other respiratory diseases.

Donkeys, which usually show few signs of the infection, are the prime source of pasture contamination for horses. Horses that share pasture with donkeys or follow them into grazing used by donkeys within a few months are most likely to become infected. Adult female worms in the lungs of infected donkeys (and less commonly horses) lay eggs that hatch in host feces after being coughed up and swallowed. After a short period, the larvae become infective in feces on pasture; they may remain infective unless killed by drought or very cold conditions. The severity of disease is related to the number of larvae ingested. Once infected, adults generally become immune to further disease, but some will contract very mild infections. Such animals can act as a source of further larval contamination, although infected horses do not produce many infective larvae. Previously infected adults can become reinfected if they have not been exposed to the lungworm larvae for over a year (and therefore have lost some of their natural resistance).

Signs of lungworm infection range from moderate coughing with slightly increased respiratory rates to severe, persistent coughing and respiratory distress or failure. Infections with no visible signs can occur. Diagnosis is based on these signs, known transmission patterns, and the presence of first-stage larvae in feces. Examination of the airways with an endoscope and x-rays can be helpful tools. Bronchoscopy can be used to collect washings from the trachea to examine for eggs, larvae, and white blood cells. It can be a challenge for a veterinarian to diagnose lungworm because infected animals do not always pass the larvae in their feces, and when they do, they may be few in number. Because of the relative infrequency of infection in horses, diagnosis may be made only after failure of antibiotic therapy to improve the condition.

Several antiparasitic drugs, especially moxidectin and ivermectin, are very effective for treatment of lungworms. Horses at pasture should be moved inside for treatment, and supportive care may be needed for complications that can arise in all species. Sporadic infections can be controlled more easily by management, such as avoidance of grazing horses with donkeys.

Last full review/revision July 2011 by Bonnie R. Rush, DVM, MS, DACVIM; Neil W. Dyer, DVM, MS, DACVP; Joe Hauptman, DVM, MS, DACVS; Ned F. Kuehn, DVM, MS, DACVIM; Stuart M. Taylor, PhD, BVMS, MRCVS, DECVP; Wendy E. Vaala, VMD, DACVIM; Maureen H. Milne, BVMS, MVM, DCHP, MRCVS