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A lump or growth is hard to diagnose even with an office visit, over the internet it is even harder as we can not even see the growth. A lump may indicate cancer, but many such growths are harmless. Many lumps are not painful or bothersome. It may be a fatty tissue deposit called Lipomas or a wart or a hematoma, but to be positive your vet will need to test the lump to be sure. An abscess can form when a dog has a puncture wound that heals on the outside while the bacteria is multiplying under the skin. It might also be a cyst that is becoming infected and painful.
Any lump found on your animal should be tested to determine if it is a cancerous or benign lump. Your vet will want to perform a fine-needle aspiration or other appropriate test. It is performed quickly and allows some of the cells of the lump to be evaluated by the veterinary pathologist. This test will allow the vet to determine the nature of the lump and take the necessary steps to remove it. Some vets will leave it alone if it is not serious. If it is an abscess, he may just drain it and prescribe antibiotics. Lumps that are solid feeling, feel attached and fast growing should be checked as soon as possible as these are the ones that are more likely to be serious.
Here are a few sites for additional information to allow you to get an idea based on the physical characteristics..
Now dogs have anal glands at the base of the tail but they are generally below the rectum. These can become swollen, overfilled and painful. Dogs often self express them by dragging their rear on the ground. Excellent site on anal glands
Dog's have sebaceous glands and perianal glands at the base of their tails. Sometimes, these glands can become clogged and swollen. A warm compress on the spot and cleaning with Benzyl peroxide gel may help with the problem. I would also use some Neosporin or triple antibiotic cream on the area as well. Dogs with this swelling of the sebaceous gland may have a problem with hypothyroidism or a hormonal imbalance, so I would suggest you have your dog seen by your vet if you do not see improvement.
The way you describe it as a bump that is painful at times lead me to believe it is likely a lump that needs to be biopsied by your vet and possibly removed since it frequently causes him pain.
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