Severe Dandruff & Possiblly Flea Infestation

Question
QUESTION: Hello, Dr. Gotthelf! I have 4 cats (parents & their babies) and all four of them are indoor-only cats. Of those four, one has longer hair than the others so I've always tried to brush him at least once a day.

I noticed that he always had a severe dandruff problem, a long time ago. Especially along his spine and tail base, I guess where he can't easily groom everyday. I thought maybe he is not drinking enough water or maybe the brush is scrapping his skin, causing it to flake off. I do see him drink water but not as much as the others. And he seems to like it when I brush him hard so I'm not too worried about that. But recently, as I was brushing the problem area one day, I noticed these black sand or dirt looking things in the brush and I can feel them deep down in his fur. Come to find out, those are flea dirt!! I had no idea indoor-only cats can get fleas so I was in shock...!

So immediately, I started searching the adult flea and wanted to start flea control in the house. (As I finally found the cause of the mystery bites I've been getting on my legs!) But...still to this day, I haven't seen any fleas. My boyfriend always had cats growing up so he knows what they look like and he tells me that if the adult fleas are not on the cat, that means they had left him so there's no need for flea control medications. I feel that is not necessarily true because I just got 3 new bites on my legs and the cat still has flea dirt, although the amounts decreased dramatically.

The other 3 cats have no flea dirt but I'm worried that its only a matter of time...Is it the length of his hair that made him a better candidate than the others? The same cat also has a "poopy butt" problem which is self-explanatory but it is a very smelly problem which a cat is walking around with a poop hanging from his butt, almost everyday. Do you think that has something to do with it?  

I did start on the flea control just as a precaution since I still see the flea dirt. I purchased the diatomaceous earth and sprinkled it on the cat and the carpet.

ANSWER: If you think it is flea dirt, put some on a white piece of paper and add a small amount of water.  If it is flea dirt, it will turn red after a few minutes.  That is because the flea poop is the cat's blood.  Cats are very good at eating the fleas off their body and that may be why you are not seeing any adult fleas.  That cleaning process can lead to ingestion of tapeworm eggs from ingested fleas, so watch the fresh poops for any sign of tapeworms, like small rice sized segments or these dried segments attached to the hair around the poopie butt.  For that condition, I would advise you to have the rear end shaved into what is known as a potty patch.

---------- FOLLOW-UP ----------

QUESTION: I've tested the flea dirt already to make sure it was flea before purchasinng the datomaceous earth so I know it was flea. After sprinkling the DE, I see much fewer flea dirt so I think it's working. So I guess it's possible for only one cat amoung the four to have fleas?  I'll keep checking his poop and get him a potty patch! Thank you for your advice!

Answer
For some uinknown, unexplained reason, fleas have apreference for something on the cat or dog skin.  I wish I knew if there was something they don't like (then I could synthesize it and have a flea repellant) or there is something that they really like on the cat's skin.  So it is possible to have fleas on one animal but not he others.