kitten with diarrhea and possible ear mites

Question
We took in a pregnant stray a couple of months ago in the hopes that we could find a good home for the kittens (once they were born) and the mama. Mama was very underweight and had ear mites. Several days after we took her in, she gave birth...so the kittens were around for at least a few days while mama was still being treated for the ear mites.

Mama also had severe diarrhea the entire time she was here, but no other symptoms. Ate and acted normally. When the kittens were old enough, we took her to the vet and had her tested for Leukemia/FIV and also had her feces examined for parasites. Everything came back clear and the vet told me to try some insanely expensive canned food, which I declined.

When the kittens were six weeks old, we were forced to take mama to a no-kill shelter since we couldn't find a home for her and could no longer deal with all the diarrhea. We DID find homes for all of the babies, but ended up keeping one. He is eight weeks now and has been perfectly healthy except now I'm seeing that he's developed diarrhea as well. His stools were perfectly normal until a week ago but now they're just like mama's were.

What could possibly be causing this? The ONLY thing I can even think of is that the kitten shares a room with our two pet rats and it is possible that he could be consuming the occasional rat feces that drops from the cage before we get a chance to sweep it up. Mama was kept in the same room as well....do you think that could be it? Would cats even eat something like that?

Or is there something else that could be happening? The kitten might have gotten ear mites from his mama. If so, it hasn't gotten bad enough for me to tell yet. Is that something that could be causing the diarrhea? I would take him to the vet, but I doubt they could give me any more answers than they gave me for the mama....which was nothing. Any advice would surely be appreciated.

Answer
There are many causes for diarrhea.  In kittens, one very common reason is a parasite called coccidia. If you can collect some of the kitten's feces, you can take that sample to the vet and have it checked for that and other intestinal worms. Usually the mother's stool sample will not show any worm eggs, even if there are parasites because they are in the tissues and the milk rather than in the feces.  Anytime a kitten has diarrhea, it is important to make sure that he stays hydrated because kittens have a very delicate water balance.  Too much diarrhea and not enough water taken in will make a kitten much worse.  You can give a few drops of liquid immodium (loperamide) several times daily to the kitten to help firm up the stool.