A trichofolliculoma is a benign skin tumor of guinea pigs arising from hair follicle cells — the most common skin tumor in guinea pigs, typically presenting as a single, cystic lump filled with keratin and hair fragments.
Most trichofolliculomas are benign and curable with surgical excision, but they can become secondarily infected and grow large enough to impair movement if left untreated.
Last reviewed: June 2026
What Is a Trichofolliculoma in Guinea Pigs?
A trichofolliculoma is a benign hamartoma (a disorganized growth of normal tissue elements) arising from a hair follicle. It consists of a primary hair follicle surrounded by multiple secondary follicles, all enclosed in a cystic structure filled with laminated keratin, sebaceous material, and hair shaft fragments. As described in Quesenberry & Carpenter's Ferrets, Rabbits & Rodents, trichofolliculomas are the most frequently reported skin neoplasm in guinea pigs and are generally considered hamartomatous (developmental abnormality) rather than truly neoplastic in behavior — they do not metastasize.
Trichofolliculomas are most commonly found in middle-aged to older guinea pigs (typically 3–6 years old). They tend to appear on the dorsal back, rump area, or flank. The lesion begins as a small, smooth nodule under the skin and may grow slowly over months to years, eventually developing a central pore or opening through which waxy, caseous (cheese-like) material or hair can be expressed. Secondary bacterial infection of the cyst contents is a common complication that causes the mass to become inflamed, painful, and malodorous.
Recognizing the Signs
The typical trichofolliculoma presents as:
- A round to oval, firm to fluctuant (slightly soft) lump under the skin
- Located on the back, rump, or flank — rarely on the face or limbs
- A visible central pore (opening) from which cheesy white-to-gray material can sometimes be expressed
- Gradual growth over weeks to months
- Strong, unpleasant odor if secondarily infected
- Alopecia (hair loss) overlying the mass
- Pain, redness, or warmth if infected
Unlike malignant skin tumors (which tend to be rapidly growing, irregular, ulcerated, or fixed), trichofolliculomas are usually smooth, well-circumscribed, and mobile. However, secondary infection can make them appear more aggressive clinically. As described in Carpenter's Exotic Animal Formulary, any new skin mass in a guinea pig should be examined by a veterinarian — while trichofolliculomas are benign, lipomas, sebaceous cysts, fibrosarcomas, and mammary tumors also occur and cannot be reliably distinguished by appearance alone.
Diagnosis and Treatment
Diagnosis is made by veterinary physical examination, often confirmed by fine-needle aspirate (FNA) cytology — which typically reveals keratin lamellae, hair shaft fragments, and sebaceous debris — or by histopathology of the excised mass. Surgical excision is the treatment of choice and is curative in most cases. The entire cyst wall must be removed to prevent recurrence; incomplete excision commonly leads to regrowth.
Anesthesia for small mammal surgery (isoflurane gas anesthesia) is generally well-tolerated in healthy guinea pigs. Pre-operative blood work (glucose, PCV) is recommended to identify concurrent disease in older animals. Post-operative pain management (meloxicam) and a protective collar (e-collar) to prevent self-trauma are standard. As noted in the AEMV Pet Care Guides, 2024, guinea pigs have some unique anesthetic considerations, making an experienced exotic veterinarian preferable for these procedures.
Veterinary exam and FNA cytology typically cost $100–250; surgical excision under anesthesia costs $300–700 at most exotic clinics. If secondary infection is present, antibiotics (trimethoprim-sulfa, enrofloxacin) are prescribed pre-operatively to reduce bacterial load before surgery.
When to See a Vet
Call your vet today if:
- You notice a new lump on your guinea pig's back, rump, or flank
- An existing lump is growing rapidly or has become painful
- The lump has an opening and is discharging foul-smelling material
- Hair loss over the lump, redness, or warmth
Go to the ER immediately if:
- The lump ruptures and there is bleeding or extensive discharge
- The guinea pig stops eating, is extremely lethargic, or appears to be in severe pain
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Frequently Asked Questions
Is a trichofolliculoma dangerous for my guinea pig? Trichofolliculomas themselves are benign and do not spread. However, they can become secondarily infected, which is painful and risks spreading bacteria locally. Large tumors can interfere with movement. Surgical removal is simple, curative, and recommended before complications develop — early removal is easier and less expensive than managing an infected or large mass.
Can I pop or squeeze a trichofolliculoma? Do not attempt to express or drain the cyst contents at home. Squeezing a trichofolliculoma ruptures the cyst wall and forces infected material into surrounding tissue, causing cellulitis and potentially abscess formation. A veterinarian should perform any drainage under sterile conditions, followed by surgical excision.
How much does trichofolliculoma removal cost? A veterinary exam and FNA cytology run $100–250. Surgical excision under anesthesia at an exotic vet costs $300–700 depending on the size and location of the mass and whether antibiotics are needed pre-operatively. Histopathology of the removed mass adds $100–200 to confirm the diagnosis.
How can I tell a trichofolliculoma from an abscess in my guinea pig? Both present as lumps under the skin. Abscesses are typically more acutely painful, softer (fluctuant), warmer, and more rapidly developing. Trichofolliculomas grow slowly and may have a visible central pore from which waxy material can be seen. Any skin mass of uncertain origin should be evaluated by a veterinarian — fine-needle aspiration (a quick, minimally invasive procedure) can usually distinguish them.
Do trichofolliculomas come back after surgery? Recurrence is uncommon after complete surgical excision. The key is removing the entire cyst wall — if any wall remains, the cyst can regrow from residual follicular epithelium. Your veterinarian should confirm complete margins on histopathology to minimize recurrence risk.
Still Not Sure if Your Guinea Pig Needs a Vet?
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