There are several reasons why your dog may smell bad, and you should try to isolate the source of the odour. Once identified, examine the area carefully and, if necessary, seek advice from your veterinarian.
In older dogs or those fed a soft diet, poor dental hygiene allows tooth decay and gum recession, creating sites for putrefying bacteria to multiply and create the odour. Your dog may require dental treatment and antibiotics.
Bacterial or yeast infections of the ear may produce cheesy odours and cause your dog to shake her head or scratch her ears. She will need to have her ears cleaned and be treated with ear drops or oral medications.
Rolling in wild animal scents will make your dog smell bad and is usually solved by bathing her. Some skin conditions such as dermatitis may cause greasy secretions onto the coat which then become infected and smelly, requiring medicated shampoo or antibiotics. Infected skin wounds may smell putrid and require medication or surgery.
Elderly or neutered female dogs may develop urinary incontinence and leak urine onto the hair round the vulva, creating an acrid ammonia-like aroma. Medication can tighten the urinary muscles and pelvic floor, preventing further leakage.
Female dogs can develop a womb infection or pyometra which may cause them to leak pus from their vulva. The pus can smell putrid and your dog will require emergency treatment from your veterinarian.
The dog has two small glands in the anal muscle ring which usually empty each time the dog defecates normally. Diarrhoea or constipation may allow the sacs to overfill, creating a characteristic fishy smell. The sacs should be emptied by a veterinary technician and you may be advised to change your dog's diet.
Some causes of diarrhoea may create a particularly bad odour and should be treated by a veterinarian.