bald patches on dog

Question
I have a one year old American pit bull terrier that has bald patches all over now.  It started a month ago on her back leg and on one of her ears.  I took her to the vet, they did skin scraping (negative for mites) and a ringworm test (again negative).  While waiting for the ringworm results, she was on antibiotics for a week.  I bathe her about once a week and she is inside during the mornings (kitchen) and outside the rest of the time.  I walk her every night.  I wash her blankets and house about every 2 weeks.  We live in Okinawa Japan at a military base here and it's very hot and humid right now.  I have another appointment next week, but would like to know what other things could be wrong.  I changed her food from Eukanuba Natural Large Breed Lamb and Rice to Nutro Chicken Meal, Rice, and Oatmeal (supposedly for sensitive stomachs) about 2 weeks ago.  So, I think that's everything so far...Any help that you can give me about possible things wrong with her or other things I can do for her would be greatly appreciated.  Thank you, Tricia
p.s. - I forgot...she's up to date on all shots, not spayed (yet), and the bald patches don't seem to bother her (no scratching or licking).

Answer
Tricia -

It is difficult top determine why your dog is losing hair.  All the necessary testing has been done; it seems as though your pet must have something that is not common or hormonal in nature.  

Alopecia areata is believed to be a  disease of the immune system that causes hair loss due to destruction of the hair follicles.  At this point we have not yet determined the inciting cause of the hair follicle destruction, but we do know that it causes the hair to fall out and grow back from time to time.

Sometimes hormones associated with reproduction organs can cause hair loss in certain breeds. If you are not planning on breeding your pet, consider spaying her; often spaying will cause an interruption in the hormones and the hair will grow back. This occurs more often in Pomeranians than the terriers.

You are feeding an excellent diet; perhaps the weekly bathing may be drying the skin out.  But this just an assumption - I cannot say without actually examining your dog.   

Ask the veterinarian if nutritional supplements such as vitamins with EFA (essential fatty acids) would be helpful.  EFAs are useful in improving skin and coat health.

Good Luck -

Charlotte Sherrell, DVM