While that's not the culprit in Onyx's case - the Sergeant's collar Crandall bought is specifically for cats - Gicking suggests talking to a vet about flea and tick remedies before buying something at the pet store.
Erika Loftin, a critical care specialist with DoveLewis Emergency Animal Hospital, doesn't recommend flea collars at all.
"They tend to have a very local affect," she tells The Dodo. "In other words, they might control the fleas on the head or the neck but if some of the fleas come running down the animal and bite them some place else."
Tragically, it's also possible for a pet to die from the wrong treatment.
"I think, in general, the way they work is there are chemicals associated with the collar, embedded in the collar, that are toxic to fleas," she explains. "Fleas are more sensitive to that than mammals because they have differences in their nervous systems.
"But I think there is always a chance that you'll have a particularly sensitive animal or perhaps a collar that was intended for one type of animal put onto another type of animal."