Tumors
ASF's are prone to spontaneous, fast growing warty tumors. From my readings in various papers, the warty and ulcer like tumors known as keratoacanthomas and squamous carcinomas, are associated with a papillomavirus (MnPV). The tumors can be seen in the vaginal region as a clustered mass, along the tail as bumps, on the back and hind quarters as warty black growths, or on the face of the animal. (Pictures can be found here)
They will develop warty tumors, patchy hairloss, and osteoarthritis as they age past approx 9 months, although it can be younger, or delayed as much as 14 months, or not at all.
So far, it would seem that the tumors develop after 9 -12months of age, although not all will get this condition.I have at least one Amber colored female well over 14 months of age that still has not developed any skin tumors or growths.
The tumor development can be very fast and spontaneous. I've had one female just recently (6/1) who within days developed fast growing tumors all over her body, under the skin and on the skin surface. I should have taken a picture, but I didn't remember to before she was euthanized and destroyed. It was disturbing to see the alarming rate that the tumors grew, she had just reached approximately 8-9 months of age when these tumors developed.
I suggest keeping rats for at least 9 months of age and hold back animals from colonies that are less susceptible to the tumors.
They are also prone to stomach cancer, which is one of the main reasons these animals are used in research.
You can google "squamous cell carcinoma mastomys natalensis" and read many articles (some you have to pay for) that discuss the virus and the cause of these tumors. (I have linked to many articles at the bottom of this entry)