The irritation caused by the worms stimulates the lungs to produce great quantities of mucus. This mucus becomes foamy and blocks the air passages when the calf tries to force air into its lungs. The adult worms themselves cause mechanical blockage of air passages. Death of the animal may occur through suffociation if the infection is severe enough. Signs of this disease become apparent during the late summer and fall.
Infected calves develop a cough which is usually noticed after mild exertion.
Affected calves usually have a rough hair coat and lower weaning weights. The major economic loss resulting from lungworm infection is due to poor weight gain. Calves experiencing an acute syndrome show difficult breathing with the mouth open and the tongue extended. Some of the acutely ill animals may respond to a corticosteroid-antibiotic injection if they are treated gently. Adults are more resistant to lungworms than calves.