According to WebMD, swelling of the left side of the face or head in cats is a common symptom of an allergic reaction to allergens or snake bites. Abscesses can also cause one side of the face to appear lopsided. Treatment consists of antibiotics and antihistamines.
PetMD explains that facial swelling in cats is a symptom of swollen salivary glands, also referred to as salivary mucocele. Salivary mucoceles are caused by trauma to the face or head, animal bites, foreign objects lodged in the mouth and ear canal surgery. Symptoms include facial swelling, oral pain and difficulty swallowing. Mucoceles can occur on the left or right side of the face.
According to Brightwood Animal Hospital, puncture wounds that occur during cat fights can cause swelling and abscesses at the point of entry. Puncture wound symptoms often take several days to develop into abscesses because punctured skin quickly seals, leaving the infection to fester underneath the skin. Abscesses typically develop on the sides of the face, neck and rump. Symptoms include pain, swelling and pus at the point of entry. Treatment for puncture wound abscesses consists of drainage, antibiotic injections and oral medication. If treated in time, puncture wounds typically heal within five days.