Guinea pig mastitis is a bacterial infection of the mammary glands that occurs most commonly in lactating or recently-weaned sows. It causes hot, swollen, discolored mammary tissue, and untreated cases can rapidly progress to septicemia — making early recognition critical.
Last reviewed: June 2026
What Is Guinea Pig Mastitis?
Mastitis — infection and inflammation of the mammary glands — occurs in guinea pig sows, most often during or shortly after nursing a litter. The most common causative bacteria are Streptococcus species and Staphylococcus species, which gain entry through small skin abrasions, bite wounds from nursing pups, or hematogenous spread. As described in Quesenberry & Carpenter's Ferrets, Rabbits & Rodents: Clinical Medicine and Surgery, cases also occasionally develop in non-lactating females with cystic mammary disease or in animals housed on abrasive bedding that causes repeated minor trauma to the ventral skin. Because guinea pigs have multiple mammary glands along their ventral abdomen, more than one gland may be affected simultaneously.
Recognizing the Signs of Mastitis
Early mastitis is easy to miss because the signs may be subtle and the glands are covered by fur. Running a gentle hand along the underside of a sow, particularly after she has had pups, is the best way to catch it early.
Mammary gland signs:
- One or more glands that feel hard, lumpy, warm, or painful to the touch
- Visibly swollen, reddened, or purplish-blue discoloration of the skin over the gland (blue discoloration suggests severe infection or abscess)
- Discharge from the nipple — may be thickened, bloody, or yellow-green (pus) rather than normal milk
- Skin ulceration or open sores over a severely affected gland
Systemic signs (indicating serious illness):
- Reluctance to nurse pups or abnormal pup behavior (constant crying, poor weight gain)
- Loss of appetite and lethargy
- Hunched posture, reluctance to move
- Elevated temperature or feeling hot to the touch
- Rapid deterioration over 24–48 hours
Sows with blue-black discoloration of a mammary gland are experiencing gangrenous mastitis, which carries a guarded prognosis and requires immediate veterinary care. If pups are present and the sow cannot nurse safely, supplemental or hand-feeding of the litter may be necessary (AEMV Pet Care Guides, 2024).
Treatment and What to Expect
Mastitis in guinea pigs requires prompt veterinary treatment. A vet will assess the severity of the infection and determine whether abscess drainage is needed.
Standard treatment components:
- Antibiotics — systemic antibiotic therapy is essential; trimethoprim-sulfamethoxazole (TMS) is commonly used in guinea pigs because many other antibiotics are unsafe. Culture and sensitivity testing guides selection in severe or non-responsive cases.
- Anti-inflammatory pain relief — meloxicam reduces pain and inflammation and is safe for guinea pigs.
- Abscess drainage — if a mammary abscess has formed, surgical lancing and flushing may be necessary.
- Supportive care — syringe feeding and fluid support for anorexic animals.
- Pup management — pups may need to be weaned early or supplemented if the sow cannot nurse safely.
As described in Mitchell & Tully's Manual of Exotic Pet Practice, early-stage mastitis treated promptly typically responds well to antibiotics. Gangrenous mastitis or septicemia carries a more guarded prognosis and may require hospitalization.
When to See a Vet
Call your vet today if:
- You feel a hard, warm, or painful lump along your guinea pig's underside
- A mammary gland appears reddened, swollen, or has any discharge
- Your sow has recently had pups and is now refusing to eat or seems lethargic
- Pups are crying constantly or appear to be losing weight, suggesting the mother's milk supply is affected
- Your guinea pig has a non-healing sore or ulcer on her abdomen
Go to the ER immediately if:
- Any mammary gland has turned blue, purple, or black — this is gangrenous mastitis, a life-threatening emergency
- Your guinea pig is unresponsive, cold, barely moving, or in severe pain
- Your sow has stopped eating entirely and appears to be deteriorating within hours
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Frequently Asked Questions
How can I tell if my guinea pig has mastitis vs. a normal lump? Normal mammary tissue in a non-lactating guinea pig feels soft and flat. A mastitis-affected gland feels distinctly hard, lumpy, warm, or tender compared to surrounding tissue. Visible redness, skin color changes (especially blue-purple), or any discharge from the nipple are abnormal and always warrant a vet visit. In a lactating sow, one-sided swelling or a gland that feels much harder and hotter than the others is the key sign.
Can guinea pig mastitis be prevented? Good husbandry reduces risk: keep bedding soft and dry to prevent skin abrasion, monitor pups for overly aggressive nursing, and check your sow's mammary glands daily during lactation. Ensure pups are weaned at the appropriate age (around three to four weeks) to reduce prolonged mammary stress. Any sow that has had mastitis previously should be checked more frequently in subsequent pregnancies.
Is mastitis in guinea pigs contagious to other guinea pigs? The causative bacteria can potentially spread through direct contact, shared bedding, or contaminated wounds, but mastitis itself is not considered highly contagious under normal housing conditions. Separating an affected animal from others while treating is still prudent, especially if there are open wounds or discharge.
How much does treating mastitis in a guinea pig cost? An exotic vet exam typically costs $75–180. Antibiotics and pain medication for a straightforward case add $40–80. If abscess drainage, cultures, or supportive hospitalization are needed, total costs may reach $300–700. Cases requiring surgery or intensive care for septicemia can exceed $800–1,500. Catching mastitis early — before abscess formation — dramatically reduces both cost and risk.
Can mastitis recur in guinea pigs? Yes. Sows that have had mastitis once are at higher risk with subsequent pregnancies. If a sow repeatedly develops mastitis, spaying (ovariohysterectomy) may be discussed with your vet to eliminate future risk, particularly if she is no longer being intentionally bred.
Still Not Sure if Your Guinea Pig Needs a Vet?
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