Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your in-law's pup is miserably itchy and I understand that you want to help.
Allergies are the most common cause of itchy skin and can give her a skin crawling, all over itchy type effect which many dogs find irritating and makes them scratch and lick so much they lose hair and get secondary bacterial skin infections, which make them itchier. I'll give you an allergy rundown of likely causes for skin troubles. She may have more than one allergy given how symptomatic she is now. Dogs that have one allergy often develop several with time. The effect of multiple allergies aren't additive, they actually compound one another. Please understand too that allergies are not something we treat and cure, but rather something we battle with consistently for the pup's entire life. The goal is to decrease exposure to allergens, interfere with the reaction chain, and thus keep them as comfortable as possible. That explains why the injection helped somewhat, but if continued therapy wasn't done then symptoms returned.
Even if you don't see fleas I do recommend using protection. Flea bite allergy is the most common allergen and it only takes one bite a month to make an allergic dog scratch so I recommend using flea prevention even if you never see one again. Frontline Plus, Advantage II or Advantix are excellent topicals and the new Seresto flea and tick collar is as well, with the benefit of lasting for at least 6 months and in some cases as long as 8-9 months. Don't use over the counter products, especially Hartz or Sargents, as most are ineffective if not toxic. Unfortunately even after the fleas are gone the allergic reaction can continue for weeks and I will discuss controlling that reaction below.
Other allergens can be inhaled (like grass pollen, dust mites or molds). These type of reaction is called atopy.
You can use a combination of antihistamines and high doses of omega-3 fatty acids to help with atopy (they also help with the symptoms of flea allergy). In combination fatty acids and antihistamines work synergistically, much better than either one alone. If her symptoms worsen seasonally I would think that inhaled allergens are a part of her problem.
You can try:
1)Benadryl (diphenhydramine only don't use the combination products with acetaminophen or decongestants as they can be toxic for dogs) at a dose of 1mg to 2mg per pound of body weight or one 25mg capsule per 15 to 25 pounds of body weight orally every 8 hours.
OR 2)Claritin (loratidine) at 5mg per 25 pounds of body weight once or twice daily.
OR 3)Hydroxyzine at 1mg per pound of body weight orally every 8 hours.
OR 4) Chlorpheniramine at 4mg to 8mg per dog once or twice daily.
OR 5) Zyrtec (Cetirizine hydrochloride) at 1/2 mg per pound of body weight orally every 24 hours. That would be one 10mg tablet per 20 pounds of body weight. Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because dogs cannot tolerate decongestants.
Some dogs do better on one antihistamine rather than another. Give the one you pick a week trial and if it isn't working try another. Be aware antihistamines can cause sleepiness or hyperactivity in some dogs. These side effects do wear off with repeated use.
Omega-3 fatty acids are fish oil products. 3V by DVM or Derm Caps ES are good brand name products. Use the high end of the dosing schedule for your pup's weight. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give her 20mg of EPA per pound of body weight per day. For example a 30 pound dog could take 600mg of EPA per day.
Cool water baths with an oatmeal shampoo or chlorhexiderm shampoo (which is antibacterial and antifungal) and a conditioner with an antihistamine may help. The water rinses off allergens and the cool temperature soothes itchy skin. Do not bathe your girl for 2 to 3 days before or after applying flea control products or the bath will interfere with the product's efficacy. If she has a long coat cutting it short helps decrease allergens sticking to her coat, and this decreases exposure. It may help to wipe her off with a damp rag when she comes indoors to remove allergens from her coat and feet if she has been outdoors a lot.
With large areas of the body affected topical sprays are unlikely to be helpful.
Food allergy is very possible with her as well if she seems to be itchy all year round. Dogs can develop allergies to any protein or carbohydrate so even if she is only fed one thing that can be what she is allergic to. Dogs with food allergies tend to lick and scratch their paws, face and ears the most, but any of the "allergy reactive areas" can be affected.
I understand that they have tried different foods but they need to make sure that the food that you put her on is a true hypoallergenic diet. The trouble with "limited ingredient" or "low allergy" pet store brands is that the same machinery is used on multiple lots of food without sterilization cleaning in between. So for example even if a food says it has salmon and rice if the previous batch had beef and corn then you will get traces of those ingredients in your bag of food. Not a big deal if your dog isn't allergic but a waste of money thinking that the food was hypoallergenic and not good for your dog if those happen to be allergens for your dog. The veterinary brand true hypoallergenic foods are more expensive because it isn't cheap to thoroughly remove all traces of a previous food mixture from the machines used to process food. They also change proteins (hydrolyze them) so the pup's body doesn't recognize the allergen or have very unusual ingredients (kangaroo, for example).
Generally what I recommend is trying to clear the skin and then adding one food item (chicken, beef, corn wheat etc) every month to see what they react to. Then we can find a regular food to try.
As far as permanent diets I do tend to stick with Purina Pro Plan brands or Nature's Recipe as I find those rarely if ever have cross contamination. Purina Pro Plan Turkey and Barley or Nature's Recipe Vegetarian or Venison are pretty good products. I know that this isn't easy from personal experience (my dog is allergic to wheat) and it is time consuming, but worth it.
There are no magic proteins or carbohydrates that no dog will be allergic to, any protein or carbohydrate can stimulate an allergy if a dog has had a previous exposure, especially if they were exposed to the ingredient within the first year of life (when most food allergies develop).
If you choose to try testing/treating her for a food allergy I recommend that you try a true hypoallergenic diet like Hills z/d or Purina Veterinary Diets HA. No treats, flavored medication or bones while on the diet and it must be used for a least 12 to 16 weeks to see the full effects. You should see some improvement in 6 to 8 weeks.
Another option if the antihistamines and omega 3's aren't enough is a product called Atopica. It suppresses the immune system a bit so it decreases allergic symptoms but it doesn't have as many harmful side effects as systemic steroids. Another option is a brand new drug called Apoquel (generic name oclacitinib) which interferes with the allergic pathway. It works very quickly to stop the symptoms of an allergy. Most dogs are reported to be much more comfortable in a day or so. Unfortunately it is on backorder now due to overwhelming demand, but it is something to keep in mind for the future if she continues to have trouble.
If they are interested they will need to discuss these medications with their veterinarian as they are prescription products, but I wanted you to be aware that there is more that can be done.
Another option is trying immunotherapy. She would need to be tested to determine exactly what he is allergic to, and then she is given small amounts of the allergen to build up her tolerance to it, increasing the amount of allergen in the injection incrementally so that her immune system no longer responds to it. This isn't a quick fix, it takes time to slowly build up their tolerance and as she develops new allergens things may need to be added, but it is an option.
If it's been a while since her last exam parasites like cheyletiella, demodex or sarcoptes mites should be looked for by their veterinarian as well if she isn't improving as they can lead to very itchy skin. She may also need oral antibiotics if she has scratched enough to create raw areas, so she may need to see a veterinarian no matter what you do at home, depending upon her skin condition.
Please let me know if you have any further questions.