Follicular cysts do not spread beyond the skin but can become infected.
Epidermal and Hair Follicle Tumors in Animals - Integumentary System - Merck Veterinary Manual
Foreign body reaction to ruptured follicular cysts in dogs - Kim - 2021 - Veterinary Medicine and Science - Wiley Online Library
What's the difference between a sebaceous cyst and a bump?
Cytology of Lumps and Bumps: The Common Stuff - Veterinary Medicine at Illinois
Cushing's Disease in Dogs: Symptoms, Causes & Treatment
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Interdigital Furunculosis in Dogs - Integumentary System - Merck Veterinary Manual
Pancreatitis in Dogs - Veterinary Partner - VIN
Hygroma in Dogs - Integumentary System - Merck Veterinary Manual