Bart the Boston terrier is having trouble defecating. He strains quite a bit, according to Allison his owner, and there is some swelling on occasion to the left of his tail base.
It sounds as if Bart has a perineal hernia. Hernia describes an opening in a body wall, usually within or between muscles. In the case of a perineal hernia, it is an opening in the funnel-like muscle structure that closes off the back end of the abdominal cavity with the rectum and urogenital tissue passing through to the outside. Perineal hernias occur in male dogs, and predominately those that have not been castrated. There are many theories as to why males are at a higher incidence for this problem, though none have been proven and it is likely there are multiple factors involved. One possibility is an enlarged prostate, which can push against the perineal muscle wall when the dog defecates; this extra pressure can weaken the wall and eventually lead to rupture.
Once the perineal wall is compromised, further straining can cause widening of the hernia and protrusion of the organs. Incidentally, it is quite common for perineal hernias to occur on both sides. Structures that can be found within a perineal hernia include abdominal fat, sections of the colon, the bladder and, in males, the prostate itself. Depending on what structure or structures are involved, symptoms can include straining to defecate, straining to urinate and, in severe cases, the inability to defecate or urinate, the latter being a medical emergency.