Guinea pig lump on back and open sore

Question
QUESTION: I have 2 guinea pigs, no idea on the breeds, but both seem to be in good health. One of them, Navi, has a lump on her back for at least 6 months now. I checked it daily, and it was just there not getting any bigger of smaller. It felt hard so I just assumed it was a growth and didn't seem to cause her any pain or discomfort so I just left it. Last night when I was petting them I noticed her lump felt different, so I took a closer look and noticed its an open dry sore now. The edges are hard and crusty, and still feels like a hard lump. I cleaned it with soap and water, and put her back. My other guinea pig, Naru is fine, she has no lumps on her or sores that I can find. I am planning on talking to the vet across the street if he can at least look at it since he doesn't do guinea pigs, otherwise its a bit of a trip to get to a vet just to be told to keep cleansing it. I'm at school right now so pictures will be income when I get home. Thank you.

ANSWER: It sounds from your description like a sebaceous cyst. They are fairly common on guinea pigs and are almost always on their back. The sebaceous glands are an oil gland that is supposed to secrete oils to the skin to help keep them soft and pliable. Even in humans they will sometimes plug up. The result is that the body continues to make this sebaceous matter but it builds up and forms a lump because it can't get out.

The sebaceous material inside is very thick and is usually the color of the skin. If it happens on a pig with black hair the skin is also black. If the pig is patched with different colors the sebaceous material will be the color of the skin on which the lump has formed.
The substance inside often has a sour smell. Sometimes they come back because as the hole closes the pores are still closed in the sac causing it to continue to build up.

If that's what this is it is a benign (non cancerous) cyst that will not cause the pig any harm or pain. Malignant cysts or tumors tend to continue to grow whereas sebaceous cysts will grow to a certain size then either just stay there or they will open on their own and the material gets out. Sometimes they don't ever open and the patient isn't bothered by them at all.  

An abcess is an infection caused by a bite or an injury and is filled with pus from bacteria.  They generally burst very soon after they've started. They are painful to the animal just as they are in humans and the pus inside is typically staph. It is a foul smelly odor and most often requires antibiotics. Once it has opened the pain is gone, much like having a pimple on your face. It feels better when it's popped.

If that's the case then the vet may want to give you antibiotics for her. But if it's sebaceous it won't need any further attention. Cleansing it is the big thing for abcesses. Keep it flushed with water and soap. If this wound is not showing any signs of pus I would suspect it is nothing more than the sebaceous gland that has opened on its own.

Pictures will help. Don't get too close to the animal when you're shooting the picture otherwise it's blurry and out of focus. I'll look for the pictures and let's hope this is just a passing thing that will correct itself.

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Navi's Back   QUESTION: Just for completeness here is the picture of her back, hopefully you can see it clearly enough. When I squeezed it a black waxy discharge came out, there was some white in it. No blood, looks like what comes out of a pimple like you said. It did smell bad so I'm taking her to the vet tomorrow, along with her sister for a checkup. She doesn't seem bothered by it apart from when I was squeezing it she didn't seem too happy. Thank you again.

Answer
That black waxy goo was the sebaceous material indicative of a sebaceous cyst. This kind of thing doesn't require medication or even flushing of the wound.  It's not a bacterial infection and will close up on its own.   

Now that you've seen and experienced this you'll be able to identify another if you see it.

Thank you for the picture, you did a good job.