Oral, periodontal disease is the most common cause, where plaque, unhealthy tissues, decomposing food particles retained delivers the sour milk odor.
Small breeds are more prone to oral disease because the teeth are closer together, the smaller animals live longer, and their owners are more prone to feed softer food.
Older age more predisposed.
In most cases there are no clinical signs other than the malodor. Daily home care (brushing) aids in the removal of plaque, helps control dental disease and odor.