Q: My gerbil had some kind of tumor around the genital area. Today it sort of popped, and blood was all over the cage. He’s very weak and looks to be in pain. What happened? What can I do?
A: It is impossible to say exactly what happened but my guess is that your gerbil developed a cyst, tumor or abscess near his genital area, and it ruptured or ulcerated. It may be that an abscess or tumor enlarged in that area and a blood vessel or vessels was feeding that mass. When the abscess or tumor or cyst ruptured, the blood vessel ruptured right along with it.
Gerbils cannot lose very much blood before they become anemic. So your observation about weakness is probably very accurate. He may also be in pain, as you state.
You could put a warm compress (not hot) along the area that ulcerated or ruptured but that will only be a temporary solution for this problem. Keep the area as clean as possible. You may want to get your gerbil off any wood shavings or bedding and replace that with paper towels in his cage. If there are other gerbils, separate him from the others. The best thing you can do to help him is to take him to your veterinarian as soon as possible.