Eucoleus aerophilus (or Capillaria aerophila) is an airway-dwelling nematode that infects primarily foxes and occasionally other free-ranging carnivores, dogs, cats and people.  It occurs worldwide, including in Canada, and can be especially prevalent in animals raised on fur farms. It has been reported in cats in Ontario and Saskatchewan.  Adult male E. aerophilus are approximately 24mm and females 32mm in length.  The oesophagus occupies 1/3 to 1/2 of the body length.  Eggs resemble Trichuris spp. eggs with bipolar plugs, but are less colorful, have more parallel sides and have characteristic anastomosing ridges on the outer Eggs measure approximately 65 µm by 30 µm.

Eucoleus aerophilus can have either a direct or an indirect life cycle.  Adults live in the trachea and bronchi (and occasionally upper airways) of the definitive host.  In the nasal and frontal cavities of a dog or wild canid, a differential diagnosis is E. boehmi, a close relative.  Female nematodes produce eggs which are coughed up, swallowed and passed with the feces.  Development of the infective first-stage larva in the egg takes 5-6 weeks and eggs can survive for months in the environment.  In the direct life cycle, a definitive host ingests the infective larvated egg from the environment, the larva hatches and travels through the small intestine wall, enters the lymphatics or bloodstream to the lungs, breaks out into the airways and completes its development to adult.  In the indirect life cycle the eggs are ingested by an earthworm intermediate host in which the eggs hatch to release the infective first stage larva.  A definitive host then ingests the earthworm and the larva follows the same path as in the direct life cycle.  The prepatent period is 6 weeks.

Light infestations with E. aerophilus are usually asymptomatic.  Moderate to severe infections can result in bronchitis, nasal discharge, wheezing cough, sneezing, dyspnea, bronchopneumonia, abscesses in the lungs, emphysema, or secondary bacterial infections which can be fatal in younger animals. This parasite is more likely to be associated with respiratory disease in dogs and cats with chronic disease or immunocompromise.  The larva and adult nematodes can cause irritation of the respiratory mucosa and constricted air passages.  Diagnosis is usually made by detecting E. aerophilus eggs in a broncho-alveolar lavage or in the feces.  No products are labelled for this parasite within Canada, but fenbendazole and macrocyclic lactones, especially the newer products (moxidectin, milbemycin, eprinomectin), should be effective.