Kitten ate spicy food, now has diarrhea. When should I see a vet?

I suspect the sauce is causing the diarrhea, and not likely the onions. There likely wasn't enough onions to cause issues. I can give you a home remedy for Maru to see if it helps. Withhold food for 6 hours, then feed a bland diet of boiled white meat chicken (no bones) in small amounts several times a day for a couple of days. Then slowly add back in his normal diet. If the diarrhea continues on the bland diet, he starts vomiting food, stops drinking water, starts vomiting water, or becomes very lethargic, he will need to see a vet right away. Best of luck.

If she is otherwise acting ok then you could withhold food for the rest of the day and offer her a small amount of a bland diet tonight. You could buy a gastrointestinal diet or try a small amount of boiled chicken. If the diarrhea persists, she starts vomiting, or becomes lethargic then she needs to be examined by a vet for treatment. Hope she feels better soon.

Poor kitten! I think you're in the right for trying to get in sooner to have her checked out by your vet. If she is bleeding a lot or is having severe diarrhea or vomiting, then seeking out care from another vet or even an emergency clinic may be a good idea. Keeping her bottom clean and dry can help, and feeding a bland meal of some boiled chicken may help as well until you can be seen.

Diarrhea is a common problem with cats and is usually self-limiting (will go away on its own). Usually it's not serious, but it's always possible that it could be the start of something serious. If she has any of the following, then she should see a vet: 1) Blood in diarrhea 2) Very watery diarrhea 3) Starts acting very depressed 4) Stops eating or drinking for longer than 24 hours. 5) Diarrhea lasts longer than three days. You can help her get through this by feeding bland food (i.e. boneless cooked chicken). Feed small portions (1/4 cup or less) at meal time. You can also add probiotics to the food (can get this at the pet store), which has demonstrated some efficacy for improving diarrhea. Ensure fresh, clean water is always available. I hope this helps and Sphynxy gets better soon.

Poor Ralph! If he is eating, drinking, and playing, then I would not consider this to be an emergency - it's never wrong to take him to an emergency vet if you're worried, but from what you are describing, I don't think it's necessary right now. If he had an intestinal blockage or other serious problem related to the paper he ingested, I would expect him to be vomiting, not eating, and acting lethargic. Since he's acting normal apart from his diarrhea, it sounds like he just has a mild upset stomach at this point - which is quite common in kittens who have eaten things they shouldn't :) As long as he's feeling fine, you can try feeding him a bland, easily digestible diet of plain boiled chicken or chicken-flavored baby food for the next 2-3 days to see if this helps to resolve the diarrhea now that he has pooped out the piece of paper. If this doesn't help, or if he develops any of the symptoms of an intestinal blockage that I listed above, then I would definitely recommend having him seen by a vet at that point as he may need x-rays or other testing to determine what's going on.

Withhold food for so long in such a small kitten is not recommended. Diarrhea is a signs referring to a intestinal problem. It could be due to parasitic infestation (very common in kittens) dietary indiscretion, inflammatory disease, bacterial diseases or viral diseases. You could provide bland food (boiled breast chicken or white fish or gastrointestinal commercial diet) for a 24-36 but, if she continues to have diarrhea, might get dehydrated and therefore need to be checked by a veterinarian. Also if you can spot blood in the faces or if she get lethargic and not eating, or if she start vomiting, I would suggest to have her checked by your veterinarian as soon as possible. 
Good luck and post again on the forum if you need further advices. I hope my answer was helpful.

It could be that Ferdinand is suffering from intestinal worms. Those are very common in kittens, and if he is not up to date on his deworming treatments, I would pick one up for him. You will have to deworming him again in two weeks to make sure that all the worms are killed. If he is up to date on his deworming treatments, it could be he ate something rotten outside that upset his stomach when you weren't looking. In that case, I will give you a home remedy to try to see if that helps. Withhold food for 8-12 hours, then feed a bland diet of boiled white meat chicken (no bones) in small amounts several times a day for a couple of days. Then slowly add back in his normal diet. If the diarrhea continues on the bland diet, he starts vomiting food, stops drinking water, starts vomiting water, or becomes very lethargic, he will need to see a vet right away.