Swellings in cats are quite common and can be found on almost any part of the body. A swollen chin may have a number of causes, but the most common would have to be a condition known as "feline acne". This is characterised by the presence of blackheads on your cat's chin and lips. The area may become inflamed, resulting in localised swelling. While not serious to your cat, it can cause some discomfort. The exact cause of feline acne isn't known but several factors may play a role including the use of plastic food and water bowls which are more likely to harbour bacteria, poor grooming, overactive sebaceous glands and possibly hormones.
The treatment of feline acne is fairly straightforward, switch to glass or ceramic food bowls and wash frequently. Clean the chin area with an antibacterial soap or iodine (Betadine), which should be diluted to the colour of weak tea. More severe cases, cleansing the skin with an ointment or gel containing benzoyl peroxide (such as OxyDex) or chlorhexidine may be of use. Topical steroids may be recommended to reduce inflammation and oral antibiotics if the area has become infected.
Other causes of chin swelling in cats includes:
Insect bite or sting - Ant bites, spider bites (I know, a spider isn't an insect) and bee stings are the most common bites and stings in cats causing localised swelling, itching and pain. In most cases, this will resolve itself within a few hours, but you may choose to give your cat some Benadryl (diphenhydramine hydrochloride). This can help to reduce inflammation and itching.
Abscess - An abscess is a localised collection of pus usually caused by a cat bite. They can occur on any part of the body, but the head, limbs and back, near the base of the tail are the most common areas, including the chin. An abscess will form a firm, painful lump which feels hot to the touch. Other symptoms may include fever and loss of appetite. Eventually, this will open up, with a foul smelling liquid discharge coming from the wound. Abscesses should be seen by a veterinarian, if it has not opened, it will need to be lanced, once open, it will be drained and cleaned. A course of antibiotics will be prescribed.
Oral cancer - Oral cancer is usually caused by a type of cancer known as a squamous cell carcinoma. This cancer is particularly invasive, often spreading to surrounding bones and tissues. Other symptoms of oral cancer include a lump in the mouth, loose teeth, bad breath, difficulty eating and drooling. Oral cancer is often hard to treat due to the speed in which it invades surrounding tissue, and an early diagnosis offers the best prospect. Treatment depends on the location of the cancer, if it is in the lower front jaw, then surgical removal may be possible, chemotherapy and radiotherapy may also be recommended.
Dental abscess - Similar to an abscess, a dental abscess occurs when bacteria invade the gums or tooth roots. Common symptoms are reluctance to eat, swelling, loose teeth and fever. Treatment of dental abscesses may include removal of the affected tooth, cleaning the abscess and antibiotics.
Allergic reaction (rodent ulcer) - Almost anything can cause an allergic reaction in cats which can result in swelling on the chin and lip. Flea bites, food bowls and food being common causes. Finding and eliminating the cause and if a food allergy is the reason, switching to a different "type" of food should resolve the issue.