QUESTION:
I have a 3 year old ferret (Cosmo) who recently stopped eating. The pet store where I usually buy his food ran out and suggested I try a different brand. Well needless to say he didn't like it. I then was able to purchase his regular brand, however he is still not eating. His bowel movements have decreased considerably and have changed - they now look very similar to tar. It is not diarrhea, nor has he been vomiting. He has lost a considerable amount of weight and just isn't his same old self.

ANSWER:
How long did he go without food? Unfortunately ferrets do not adapt quickly to a new food and it has to be introduced slowly at the side of the original food, you cannot switch 'cold turkey' as it causes all sorts of gastric problems. It sounds as if he might have some ulcers, which means you will need to get him to a ferret knowledgeable vet ASAP!!! He will need meds to get better. However until you get to a vet, PLEASE buy him some GERBER'S BABY FOOD (blue label) CHICKEN/TURKEY.

Start to feed him immediately, at least 2-3 times a day about 3-4 teaspoonful (add a teaspoonful of water) this will help him gain weight and is easy for him to digest. If he will not lick it from your finger, then use an eye dropper and dribble it across his front teeth, once he enjoys the taste then he can eat from a shallow bowl. My concern at this point is he may have hypoglycemia (low blood sugar) either from not eating, or from tumors on the pancreas   (INSULINOMA) which is very common in ferrets, it can be treated with meds. You will need to go to the vet to do a 'blood fasting glucose' on him to determine if this is the problem, even so right now the baby food will benefit him greatly as it will keep his glucose leveled off and he will feel more like eating. If he has ulcers he will need to ONLY eat baby food for about a month 4 times a day and if it is low blood sugar you will still continue to feed him baby food twice a day with his medicine for the rest of his life.