Skin Lumps and Bumps -- What Are They?
Lumps may be abscesses, fatty lipomas, cysts, or tumors. Examine your guinea pig at least once a week for lumps and bumps, and see a vet for an evaluation.
Your vet will perform a thorough visual examination. He/she may also recommend:
A fine-needle aspiration biopsy (FNAB)
A wedge biopsy (more material collected, more accuracy)
Cleaning and installation of a drain (abscess)
Cavy-safe antibiotics (abscess)
Collected material would be examined by microscope in-house, sometimes sent to a pathologist for interpretation.
For more information, read :
Abscess - due to the cheesy nature of guinea pig pus, abscesses require complete removal or cleaning and daily flushing
Cervical Lymphadenitis (CL) or "Lumps" (See also Tracis' Links thread in the reference forum)
Cyst -- some cysts may require surgical removal to prevent recurrence
Lipoma -- these fatty deposits are usually benign
Mammary Tumor -- both male and female guinea pigs can develop mammary tumors
Thyroid adenoma -- see post in reference forum
Another guinea pig with lumps was described in that thread with a diagnosis of lymphoma.
Trichofolliculoma -- these common tumors are usually not cancerous