The boil can be caused by an infected hair follicle, a grass seed, a sebaceous cyst, dietary insufficiency or toxins building up in the circulatory system and parking themselves under the skin as a boil...
Skin issues are the result of a build-up of toxins under the skin seeking release through a local infective process. This is one avenue that the immune system uses, to rid the body of toxins from the blood. Blood toxins resulting from chemicals in the diet, infection, over-sensitivity in the nervous system or some other sort of metabolic stress, will set up conditions where such impurities are deposited just under the skin which then becomes locally unhealthy, sensitive and irritated.
Diet
I recommend that you place your dog on a diet of raw meat and bones i.e. beef/mutton bones with meat on them or raw chicken necks and wings, mince, definitely no processed food and snacks. You can also add some freshly cooked vegetables and rice. If your dog is not already on this type of diet, then I suggest that you introduce it slowly.
McDowells recommendations
Skin problems in dogs, hotspots & skin sores are most effectively treated with Blood cleansing herbs.
Blood cleansing herbs are employed to eliminate or neutralize toxins in the major organs of the body. Including the liver, kidneys, colon, lungs, lymph and skin.
This blood cleansing mix contains alterative herbs which are high in iron and sulphur to detoxify the blood. The Herbs restore the proper function of the body, usually through altering metabolism by improving the tissues' ab
Dab castor oil over the boil daily to draw out the infection. You can cover with a light bandage. Alternate castor oil with colloidal silver on to the boil to treat the infection topically. It should burst and release its contents. This is a good thing. Keep applying the castor oil and colloidal silver until it completely clears.
Also add 2 dessertspoons of crushed White millet and linseed to the diet once daily to improve tissue quality and repair.