A few of my male Rag 2 -/- BALB/c mice have gotten infections - bilobed masses on their hindquarters. Many of the mice do get rectal prolapses, but these infections have been occurring in mice without obvious prolapses, so I'm unsure if these are rectal infections. They are not in the testicles. Has anyone else experienced something similar? I have not seen these in any of my female mice or any of my WT Rag 2 +/+ C57BL6 mice (all kept in the same room).
Has your colony been screened for pinworm? Although, you usually can't tell if they have pinworms by just looking at them.
Start by confirming with your animal facility manager or veterinarian that the colony is indeed pinworm free. Pinworm commonly causes rectal prolapse and can easily be spread from cage to cage so be very careful when handling your mice!
Another possibility is that you're seeing abscessed fighting wounds. We commonly have to separate our Balb/C males due to increased aggression are they age. They do tend to bite and scratch each other around the genitals and tail, and often at night when you can't observe them.
It could be heavy pinworm infestation. Rectal prolapse is one of the signs of pinworm infestation. Please consult your veterinarian.
Balb/C male has fighting tendendcy with other male in same cage and injury could be due to this. If under experiment , you need to seggregate them and apply antiseptic on affected part.