Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your wee one’s situation, and wanted to help.
First, I do want to make note of your mention of tea tree oil; as this is something you need to use with care. If you are using this, you do need to make sure that Tisha cannot lick it off where you applying it as it can be toxic in large enough doses.
Now it is important to appreciate that the brown discoloration is a side effect. The brown on her fur is saliva staining. This is due to the amylase in her saliva sitting on her fur after she has licked these irritated areas. The way to remove it is to tackle what is making her lick these areas and stop her from doing so. Because once you do, with time and new hair growth, the brown will start to fade.
Now as I noted the saliva staining tells us that she is licking these areas because they are irritated. And as you noted allergies can certainly cause that diffuse irritation. Furthermore, it is possible to see this due to both food allergies and pollens in the grass. In fact, we can see allergies to anything dogs come in contact with. So, this can be a challenge to address for them and we may need to consider allergy testing (which is helpful for pollens and grasses), food trials (if a protein was though to be to blame), and supportive treatments (ie antihistamines, steroids, immunomodulating drugs, immunotherapy vaccination, etc). With the right combination, we can reduce her irritation and therefore her constant need to lick her skin.
This all said, I am a bit concerned about your mention of her not walking. With this, I would advise a good look at the areas between her toes and under pads on the bottom of her feet. I'd be concerned that you may find moistness, redness, and infection. Often when dogs constantly lick their feet, they will contaminate these poorly ventilated crevices with bacteria from their mouth. This leads to a moist bacterial dermatitis in this area, which can be very sore and even more itchy (making them chew/lick even more). If you find a mild dermatitis, then sometimes we can settle this by preventing licking (ie placing an ecollar) and topically treating by bathing the feet with a mild antiseptic (ie salt water, dilute chlorohexidine) or a good anti-bacterial/yeast shampoo for his feet like Malaseb (example) or wipes like Malaket (example) . For severe cases, we sometimes need to treat orally/systemically with antibiotics and steroids. So, depending on the state of the undersides of her feet, this may need to be tackled before allergies are addressed.
Finally, if we can rule out any secondary infection, then I would just note that you may want to consider starting your mission to rule out and address allergies. As I noted, there are a lot of treatment options, but if her signs are mild and you haven't yet tried antihistamines then this may be a good starting place here. When treating dogs with antihistamines, we most commonly will treat with Benadryl (More Info/Dose Here). A low dose (ie. 0.5mg per pound of her body weight twice daily) can just be enough to break that allergy cycle, and give her skin the chance in needs to settle. We like to keep the dose low in dogs, as they can have drowsiness with this medication (just like people). And, of course, if she is only any other medication or has a pre-existing condition, then you do want to speak to your vet first before using these. So, this would be a good starting place for Tisha, but of course if her signs are severe then your vet can dispense one of are alternative medication options that I mentioned before.
Overall, an allergy is quite likely but with your history of not walking I'd be concerned whether there is more going on just now. So, we'd need to address that and then focus on allergies. In doing so, we can hopefully settle that irritation and thus reduce her need to chew her feet. And if we can halt that behavior then we should see the brown staining fade with time.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
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