Does my hopper mouse have a tumor?

Question
Hi, I am very worried about my male hopper mouse, I have been noticing that he has a large black bulge on or around his anus, right in between his testes, and I am worried that it may be a tumor, I am planning to call my local pet store tomorrow to ask them as well, but I wanted to try and do a little research on it first, and I came across this site and decided to ask you as well, can you please help me with any information you may have on this, and should I be worried about him?

Answer
Hi Sam,

How large is the bulge?  It could be a tumor, but since he is so young, I would be a little surprised.  Usually when a lump pops up on a mouse there are two main possibilities - a tumor, or an abscess.  An abscess could occur if he was injured (are there any older males with him that could have bitten him, or have he and his siblings been fighting at all?) and the injury became infected.  It could then swell and discolor.  The only person who can reliably determine the difference between an abscess that needs antibiotic treatment and a tumor that does not is a professional veterinarian.  An abscess could go away by itself, but it could also rupture and make the mouse extremely sick, so it would require immediate attention.  A tumor is not always treatable in mice but doesn't always mean a death sentence either - if it is not impeding his movement or growing very quickly he could still live happily.

If you can send me a followup with a photo I can give you a little more information, but if it's definitely looking like a problem a vet visit might be your best bet to correct it.

Best of luck!
-Tam