As Phil explained to Patches while they watered their horses, the screwworm is the larva of a blowfly bred in sores on living animals.

The unhealed wounds of the branding iron made the calves by far the most numerous among the sufferers, and were the afflicted animals not treated the loss during the season would amount to considerable.
"Look here, Patches," said the cowboy, as his practiced eyes noted the number needing attention.

"I'll tell you what we'll do.

We'll just run this hospital bunch into the corral, and you can limber up that riata of yours." And so Patches learned not only the unpleasant work of cleaning the worm-infested sores with chloroform, but received his first lesson in the use of the cowboy's indispensable tool, the riata.
"What next ?" asked Patches, as the last calf escaped through the gate which he had just opened, and ran to find the waiting and anxious mother.
Phil looked at his companion, and laughed.

Honorable Patches showed the effect of his strenuous and bungling efforts to learn the rudiments of the apparently simple trick of roping a calf.