TL;DR: A soft swelling on a cat's shoulder is often an abscess or cyst and typically requires veterinary attention within 24–48 hours, especially if accompanied by fever or lethargy.
What causes a large soft swelling on my cat's shoulder?
A soft, squishy swelling on a cat's shoulder is most commonly an abscess—a pocket of pus caused by a bacterial infection, often from a bite or scratch. Other common possibilities include a cyst, which is a fluid-filled sac, or a lipoma, which is a benign fatty tumor. In some cases, it could be a reaction to a recent injection if your cat was vaccinated lately. Because the shoulder is a common site for both scuffles with other animals and medical injections, it is a frequent spot for these types of masses to appear.
Is a soft swelling on my cat's shoulder a medical emergency?
- Urgency Level: Medium. While rarely an immediate life-threatening emergency, it can be very painful and prone to rupturing.
- Seek Care: You should schedule veterinary care within 24 to 48 hours.
- Immediate Danger Signs: Contact an emergency clinic sooner if your cat is extremely lethargic, has a high fever, refuses to eat, or if the swelling is rapidly expanding and hot to the touch.
- Rupture Care: If the swelling bursts, it will need professional cleaning and a course of antibiotics to heal properly.
How can a photo help my veterinarian triage the swelling on my cat's shoulder?
- Diagnostic Tool: A clear, well-lit photo allows the triage team to see the scale of the swelling and the condition of the overlying skin.
- Identifying Marks: Images can reveal telltale signs like a scab or puncture wound that may not be visible at first glance.
- Determining Urgency: This visual information helps the clinic determine how quickly your cat needs to be seen.
- Procedure Preparation: Photos help the staff prepare for necessary treatments, such as a localized drainage procedure, before you arrive.
Clinical Context (Merck Veterinary Manual)
Large, soft swellings on a cat's shoulder may be caused by vaccine-induced fibrosarcomas, which appear as nodules or plaques between the shoulder blades. These tumors are associated with rabies and feline leukemia virus vaccinations, with aluminum hydroxide (commonly used in adjuvants) implicated in their development. Alternatively, the swelling could be indicative of lymphosarcoma, potentially accompanied by a cervical swelling, dyspnea, jugular distention, tachycardia, anterior edema, and fever. Fracture of the supraglenoid tuberosity of the scapula, mid to distal scapula, or proximal humerus due to trauma should also be considered.
Chapter: Oncology, Immunology
Source: The Merck Veterinary Manual, 11th Edition (Page 957)