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Hot spots on his back near his tail almost sound like a possible flea allergy. So make good and sure that there are not fleas present initiating it and making it worse. Try using a flea comb to check, if possible (may be difficult with certain types of long, thick, or curly hair). If you see dark sand grain-sized particles that when placed on a paper towel and wetted turn red, those are flea droppings. If you do find fleas, at the bottom of the post I'll include how to deal with them. If you do not find fleas, hot spots can be initiated with just an insect bite such as a tick or fly bite or can be due to a dog's coat being damp and not having dried quickly enough, or can be caused by ear problems or anal gland problems (depending on where the hot spot is located), or allergies, or skin irritants, or can be basically idiopathic (no discernable cause). My golden got a really bad hot spot last year after I used a new shampoo on her (the shampoo was something I had won in a raffle and it had the scent of coconut). My dog had an adverse reaction to the shampoo, whence the hot spots. Needless to say, I'm not using that shampoo again.
Hot spots are basically a bacterial skin infection, possibly allergy induced and progressing through self-mutilation. Unfortunately, I've discovered that the only way to convince a dog to not bother a hot spot is to totally make it impossible to reach. Meaning use an Elizabethan collar for awhile. On top of that use medications to try to clear it up.
The benadryl was a good suggestion and probably helped some, but not enough to convince your dog not to bite and scratch. If the hot spots are not getting cleared up by the time your vet returns, you may need to get your vet to supply oral antibiotics and steroids (Due to the side effects of steroids, I always try to clear a hot spot up myself before giving up and going the steroid route).
If you find no fleas - that is good. As previously mentioned, hot spots tend to have an infection component to them, so an antibiotic ointment (or topical antibiotic-steroid cream) is a good thing to start with. Another problem with hot spots, which are also called "Acute Moist Dermatitis", is that they, as the name implies, are moist and exude pus. They can appear and enlarge quite rapidly and the dog's licking and scratching does not help at all. So while an antibiotic ointment can be helpful at first, it will not help the moist puss to dry up. So the next step is to work on getting the hot spot to dry up and heal. The Elizabethan collar is still a necessity at this stage. I had almost gotten my golden's hot spot to heal - it was completely scabbed over so I took off her Elizabethan collar and she immediately re-opened it up due to the scab being itchy and I had to start the treatment all over again.
It is very important to completely clip the hair and undercoat away from the hot spot and even clear the hair from around the edges of the area. Make sure that your dog is brushed out and not shedding so that no dead moist hair is trapped next to the skin.
Things that may help:
1. gently clean the area with a surgical soap or dilute hydrogen peroxide (remember that hot spots can be incredibly painful for your dog so be careful not to traumatize your dog by cleaning the area - plus be careful not to get bitten if your dog is in pain).
2. A product called Sulfadene can be obtained at pet stores and may work well.
3. Other things I've tried on hot spots are Bag Balm (it is antiseptic, but keeps the area moist and thus not good for getting it to dry out and scab up), Petrelief medicated anti-itch spray (helps, but you still need an Elizabethan collar), ChlorhexiDerm Flush (a topical antimicrobial, cleansing/drying solution - sounds like it should "work miracles" - it is helpful, but not miraculous!), medicated Gold Bond Powder (relieves pain and itch and can help dry it up, but can get a bit crusty on the area so I've ended up having to clean it off sometimes), Wonder Dust wound powder (a horse product for open wounds - some people swear by it, can help dry up the hot spot, also can get a bit crusty and have to be cleaned off the area).
Finally, remember to deal with any underlying skin or allergy problems.
Hope this was helpful. GOOD LUCK --- Hot spots are truly a pain!
Below is info on how to deal with fleas, should you find any:
Besides a flea bath, vacuuming and spraying the house, you need to treat areas outside in which your dog occupies. Also, you need to wash all bedding that your dog lies on (it helps to spread out white towels for your dog to sleep on since these can be easily washed. If your dog has short or straight enough hair, using a flea comb is also a good idea.
Make sure that the insecticide that you use contains an insect growth regulator (an insect growth regulator prevents flea larvae from maturing).
If you have other pets such as cats, they must be treated also (however, be very cautious that the products are designed to be used on cats since cats can be poisoned by products designed for dogs).
After you have done all of this, due to the fleas' life cycle of eggs -larvae - cocoons - adult, with only 1% of the flea population being in the adult stage at any one time, you will need to repeat the ENTIRE treatment - bathing with flea shampoo, vacuuming, spraying, washing bedding, treating outside areas, etc. two weeks after the initial treatment, and possibly do a third treatment in two more weeks.
As noted above, key to controlling the fleas is to interrupt their life cycle at the immature stages so they do not develop into adults. A flea's entire life cycle can be as short as two weeks (and this is what would be expected in a warm and humid environment like that which occurs during the summer).
Because the adult fleas that we see are only a small fraction of the total population of fleas present, you must control the fleas in their immature stages to contend with constantly developing new adult fleas.
Following is detail info on a flea's life cycle:
The egg stage: flea eggs are small, while, and oval and just visible to the naked eye. They can be laid on or off the host animal. Plus they fall off the animal easily and get spread around. They hatch after a period of a few days to a few weeks.
The larval stage: The eggs hatch into a white tiny worm-like larvae. They can only be seen with the aid of a magnifying glass. This stage usually lasts from one to two weeks in a warm, humid environment. Larvae feed on dust, flea droppings, and skin scales. They prefer a dark environment and will burrow into any available material such as carpeting, cracks in floors, or furniture fabric. They often occur near a pet's resting or sleeping areas. Direct exposure to water is lethal to larvae. Also, a spray containing an insect growth regulator prevents them from developing.
The cocoon or pupal stage: The larvae spin small whitish cocoons called pupae to complete development into the adult flea. The adult may emerge from its cocoon after only one week or may not emerge for over a year. Good environmental conditions, vibrations of the host animal walking, and the warm temperature and pressure of a sleeping animal on the cocoon itself will cause a flea to emerge from its cocoon.
Eggs, larvae, and cocoons are all killed when temperatures surpass 95 degrees F.
The adult stage: This is the flea that is familiar to people. Once it emerges from its cocoon, it looks for a suitable host for a blood source for food. Flea infestations happen due to a female flea being able to lay as many as 30 eggs per day for several months or more.
Good luck in getting rid of the fleas. Treating every two weeks for a month and a half should get rid of them (unless they are being brought in from outside - such as on an outside cat).