Many thousands of these larvae are coughed up and swallowed. Once swallowed, they are carried through the intestine and pass in the manure where they develop for three to seven days. During this time, they pass through three larval stages to the final free-living stage which is infective to cattle. Manure pats may contain thousands of larvae which contaminate pasture, feed, or water of cattle. Calves become infected when they eat contaminated food.
Infective larvae are relatively inactive, so little migration occurs from well formed manure pats.
Once swallowed, the infective larvae penetrate the wall of the intestine and are carried to the lungs in the blood. Once they reach the lungs, the larvae leave the blood and develop into adults in the air passages in approximately seven days. Adults may live for 50 to 70 days before being expelled. The infection gradually disappears in the absence of reinfection. Cattle containing a few worms may appear healthy, while more heavily infected animals, especially calves, experience difficulty breathing and may even die.