Excerpted from "What Do Dogs Know?":
Dogs can sense more than just oncoming seizures. Richard Simmons, a research associate working on a project supported in part by the U.S. National Institutes of Health, told me a story about Marilyn Zuckerman of New York and her Shetland sheepdog, Tricia.
Tricia had developed the annoying habit of sniffing or nuzzling Marilyn's lower back whenever she sat down. Marilyn's husband looked and noticed that there was a dark mole in the location that Tricia seemed to be interested in. It seemed odd that the dog cared about this mole, but since it caused no discomfort, Marilyn just ignored it. One spring day, though, Marilyn was lying face down on her balcony in a bathing suit, simply enjoying the sunshine when suddenly she felt teeth on her back. It was Tricia, who apparently was trying to remove the mole.
Marilyn's husband suggested that there must be something odd about the mole if it was bothering the dog that much. More out of curiosity than anything else, Marilyn showed it to her doctor. Before the day was out, Marilyn was at the Cornell Medical Center; where the mole was diagnosed as skin cancer -- actually a virulent and potentially fatal form of melanoma. Tricia's early warning probably saved Marilyn's life.
As Simmons told me: "it was because of stories like Marilyn's that we began testing dogs' diagnostic abilities. Our preliminary data suggests that dogs can detect melanomas and several other types of cancer well before there is any other indication of a problem. Some dogs will show agitation the moment a person with cancer enters the room. It may well be that someday in the future, inspection by a dog may become a routine part of cancer screening."