Is she eating any grass?
Have they checked a stool sample?
Does she have access to outside water like a pond or stream?
Try keeping her out of the river for a week and see if it changes the vomiting.
It is possible there is an intermittent contaminant in the river water that she is ingesting (say an overflow from a pipe carrying waste from a home or factory up stream) that is leading to digestive upset.
If you have not tried a broad spectrum wormer such as Panacur (fenbendazole) you might want to try that as sometimes a hidden worm infection doesn't show in the stool or a protozoan parasite such as giardia or coccidia might not show if the stool checked wasn't extremely fresh.
If the vet has not run blood work that might be another good test to run to check liver and kidney function etc.
You can also consider doing an endoscopic exam of the upper gastrointestinal tract so the vet can look to see if there is anything that coud be the problem such as a small ulcer, a splinter of wood stuck somewhere that was too small to show on the x-rays, a calcification of the stomach valve can be seen using that etc.
There are health issues such as Addisons which you can read about here which can cause intermittent vomiting
http://www.marvistavet.com/html/addison_s_disease.html
Hope this helps you!
Actually vomiting in the morning is really common in dogs due to the stomach being empty and irritation from stomach acids that flow in anticipation of breakfast can cause vomiting.
Sometimes a high fiber snack before bed can help keep that from happening.
Check which wormers were used - Panacur just happens to clear a lot of things while others such as Nemex might not.
Toxins can travel a long way.
Blue green algae can be an issue. Usually though its right after drinking water with that in it
http://www.whole-dog-journal.com/news/toxin_warning_for_dogs_who_swim.html
I'm presuming your vet checked liver, pancreas and gallbladder while doing x-rays but if not those are other areas that can trigger vomiting.
I have heard of some dogs not doing well on Z/D due to the corn component in it. If you see no improvement after doing a trial with that you might try a grain free food.
With your dog on ZD you can ask your vet about an appropriate treat but plain canned pumpkin, a sweet potato, or cooked winter squash mixed with some ZD might work for you. Or even feeding the second meal later in the day.
I'd still try a week of no river swims to see if that resolves this.
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