Dogs have four salivary glands--- two under the tongue and two at the base of the jaw. (Cats have the same number and placement). Sometimes, these glands suffer trauma from injury or get infected, and the symptoms will include drooling, or the development of something that looks like a goiter under your dog's jaw, a swollen area that jostles like a water balloon.
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Salivary Cysts
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These types of infections are caused by parasites and bacteria, and are most common in the mandibular salivary glands under the jaw. The usual result is that there's an unnoticed infection that causes the tissue around the gland to swell, and eventually rupture, letting the saliva pool backwards down the neck.
Bacterial Infection
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While the salivary glands are highly resistant to bacteria (one of the functions of saliva is to clean bacteria off of food, and to soften the food being eaten), they can get infected. The number and types of bacterial infections for canine salivary gland diseases are fairly extensive, with staph infections being the likeliest outcome.
Autoimmune Diseases
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In older dogs, the most common cause of salivary gland problems isn't infection from outside agents, but an autoimmune response, where the dog's own immune system is attacking the salivary glands. Symptoms include swelling under the jaw, and 'dry mouth'.
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