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.
This resistance develops partly from older animals having prior exposure to the parasite, and partly from having larger air passages which are less likely to become blocked by worms and mucus. Larvae eaten by resistant animals usually do not mature to the adult stage. Lungworms may be brought into the herd by carrier animals or the herd may become infected by grazing on a contaminated pasture. Low lying pastures, drainage ditches, and swampy areas provide ideal conditions for the development of lungworm larvae. Larvae survive for only a few days under hot, dry conditions, but may live for several months during mild moist periods.