Canine juvenile bacterial vaginosis, often referred to as puppy vaginitis, can develop in puppies as young as 6 weeks of age. While the discharge might be heavy in some puppies, in others it's barely noticeable. Other than the mess aspect, it's usually not something to worry about.
Any female puppy can develop vaginitis -- there's no particular breed predisposition. It can go away for a time, then flare up again. Even if it's a consistent problem during puppyhood, it should disappear by the time your dog goes into her first heat cycle, between the ages of 6 and 8 months. If it continues once she's an adult or after she's spayed, she may have more serious issues. In that case, your vet might conduct a vaginoscopy, looking for growth of particular organisms causing infection in the vaginal canal.
While many puppies with juvenile bacterial vaginosis remain asymptomatic other than a slight discharge, some puppies lick their privates so much they develop a skin condition called perivulvar dermatitis. Her vulva area becomes red and itchy. You might have to place the "cone of shame," also called the Elizabethan collar, around her neck to stop her from licking the rash. She might also pee frequently, and the hair around her privates may appear soiled and crusty.
Most cases of puppy vaginitis don't require treatment. However, since symptoms mimic those of more serious diseases, your vet wants to make sure it is indeed puppy vaginitis causing the discharge. Bladder infections and congenital abnormalities can result in discharge. Your vet might conduct blood tests and urinalysis, and may test the discharge sample, to determine it's just a case of puppy vaginitis.
Use baby wipes or moist tissues to keep your puppy clean. If her discharge is particularly thick, use a cleanser without alcohol. Give her genital area a good wipe-down twice a day until the condition clears up. Don't use a douche or other method for vaginal cleansing -- it's overkill for your puppy's situation. If your puppy develops dermatitis, your vet might prescribe antibiotics to fight infection. If your puppy emits a heavy discharge, your vet might suggest spaying before she goes into heat for the first time.