Recurrent hives can be tricky to figure out the cause, but well worth trying to work through some common underlying causes. They are an acute hypersensitivity reaction. I assume your veterinarian has done skin scrapings to look for mites (demodex and sarcoptes) and bacteria and yeast. If not, then this is a starting point. If bacteria, yeast or mites are found then treating this may help resolve the problem. I also assume you have done a good bath with her weekly since these starting occurring in case this is a topical irritation.
Drugs, bugs and food allergies are the most common causes of hives. That's a wide list and tough to narrow down. If on any drugs, then stopping this is a good idea. You can also discuss doing a hypoallergenic food trial with your vet. This is not as simple as changing brands, and most dogs are not allergic to gluten or grains (that is a marketing ploy); they are commonly allergic to proteins such as beef, chicken or milk. So, a hypoallergenic food trial involves feeding a diet either with a new ingredient she's never seen or a hydrolyzed diet for 8-12 weeks. I would also change the class of heart worm preventive, just in case this is an atypical reaction.
As well, lab work is very important to look for internal causes or immune system changes. Any tumor/masses in the body (sometimes mast cell tumor can trigger these type of reactions but can be hiding internally in the spleen)? Has anything changed in the yard or the household (new cleaners, sprays carpets, etc)? If so, then this could be the culprit. Most people want to blame this on food, but this is unlikely to be the cause.
If these continue to recur and we cannot figure out the cause, then I recommend you have your veterinarian refer you to your closest veterinary dermatologist, who may want to do intradermal skin testing to try to figure out the cause (bee/wasp hypersensitivity?).
These cases are tough and can take weeks to figure out. AS long as it responds to antihistamine +/- steroids then we can at least control her itching.
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