In the novel The Pale Horse, Sheila is one of the dogs belonging to John and Rhoda Despard at the village of Much Deeping. Mark Easterbrook comes across Rhoda treating Sheila with an ointment. Rhoda comments that the vet had diagnosed that Sheila has ringworm. She says the ointment makes hair fall off.
This last remark should have served as a significant clue, but surprisingly, although Mark Easterbrook comes to the right conclusion at the end, he doesn't actually cite this incident as one of the steps leading him up to his inference. In the 2011 ITV adaptation of the novel where Miss Marple is inserted as a detective, Miss Marple comes across Captain Cottam medicating his dog the same way and this time the clue proves significant.