Hello,

The hair loss/alopecia your dog is experiencing could be down to any number of causes.

It could be an allergic reaction to something though probably not food as you say there's been no change in diet.

Mites or mange might be behind it. If no irritation it could even be some sort of fungal infection.

Without wishing to state the obvious in any way - a visit to the vet is the best thing to do. The problem can then be diagnosed and treatment advised.

A pretty informative site with some general information on hair loss in dogs is Dog Hair Loss - Canine Hair Loss - Dog Skin Problems

Hope you get the bottom of it soon and your dog makes a full recovery.

Reg.