Friend, it seems like her butt is itchy (anal pruritis) and as a result she is causing damage to the skin. A very common cause for anal pruritis is an underlying allergy (food or environmental). It is also very important that you stop her from licking. You see, the licking itself causes inflammation and will delay healing. This is what you can try:
1- Change her diet to a prescription food called Hill's z/d or a grain free diet with a different source of protein then what she normally has had in the past. For example: If she seldom ate fish then put her on a grain free food that has fish. When on this diet she should not eat anything else or she will not improve. Needs to be on this diet for 6-8 weeks before you see significant improvement. For treats you can give her veggies like broccoli, baby carrots, cauliflower, etc.
2- Start her on Benadryl (diphenhydramine) 25mg 4 tablets every 12 hours. You can buy benadryl over the counter in any pharmacy.
3- Supplement with Eicosaderm (omega 3 fatty acids); you can buy this over the counter in any pet store or online. Give at label dose.
4- Need to stop her from licking or she is going to delay healing. Pick up an ecollar (cone) in your local pet store to stop her from licking/gnawing at affected area.
5- Pick up in your local pharmacy "Hibiclens" antimicrobial soap or chlorhexiderm scrub. Wash area with this soap 2-3 times a day. If you do not find the above soap use over the counter antibacterial soap.
6- After washing affected area as above apply a topical cortisone or hydrocortisone. Do this 2-3 times a day for the next 7-10 days. You can buy this over the counter in any pharmacy.
4- If no significant improvement after doing the above I would look into a consult with a board certified dermatologist.
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