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There are a few different reasons for a dog to chew at the are you indicate. Your vet is correct that the most common reason is a flea problem. Some dogs are allergic to fleas so even one bite can set uff a reactioncausing itchiness and make a dog gnaw at their rear. Fleas pick that area because it is difficult for a dog to get to them when they are around the tail region. You might not actually see a flea as they are pretty quick but it might still be fleas. You need to have your dog on a good flea preventative. In addition, you have to ensure the environment is also treated. A good flea spray containing an insect growth regulator will help get thiem out of your carpets and furniture.
Cutting a piece from a flea collar and adding it to your vacuum bag will kill any fleas that you vacuum up and prevent them from escaping back into the house. Vacuum every day for at least a month to get all the fleas and eggs out of the carpets. You will also want to empty the bag every day outside so no fleas escape back into the house.
You can also treat the yard if you wish with food grade Diatomaceous earth, which helps kill them in the yard without poisons. This can also be used in carpets as well. You can read more about that here.
Fleas do not like a wet environment so frequent watering of the lawn can also make it inhospitable to fleas as well. There are a couple of other reasons as well. One is a tapeworm infestation. Tapeworms release segments that exit the anus and spread tapeworm eggs into the environment. They can cause severe itching in some dogs. They can usually be seen in the stool and are small about 1/2 inch moving things. If the stool has dried, they look like rice in the stool or stuck in the hair around the anus. Drontal plus is also a prescription product and is used for the treatment of Tapeworms, though dewormers with praziquantel work well against tapeworms. Your vet should have done a fecal exam to check for parasites.
Now if the anal glands are bothering a dog either because they are over filled or an infection is starting, that might also cause itching around the rectum. Most vets will empty the glands if they are fill, so I would think your vet would have caught this as well. . The anal glands are 2 sacs on either side of the anus at about the 4' o'clock and 8 o'clock positions. If your dog is continually licking their anus, it could indicate that they are full. This problem can usually be resolved by emptying out the glands. To empty them or express them, you will want to cover the area with a tissue and press your finger on the dog's anal glands with an upward motion. A foul smelling liquid should come out. If your vet didn't attempt this, you might give it a try to see if the glands are an issue. Excellent site on anal glands
A Perianal Adenoma is also a possibility. You can read about this here:
So start with the most likely issue and be sure her flea preventative is working and rid the environment of them. Check her anal glands and go ahead and get her dewormed to be on the safe side. If nothing seems to help, get a second opinion from another vet.
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