Thank you again,
Now when we are in a situation where we can rule out infectious agents, we do have to consider alternative causes for lower bowel based diarrhea. In regards ***** ***** issues, we'd need to consider dietary sensitivities, inflammatory GI disease (ie IBD), intermittently shed protozoa like Giardia, and disturbances to the natural GI microflora (good gut bacteria that are responsible for normal digestion and play a role in stool formation).
With all this in mind, we do have some supportive care measures that would be indicated at this stage. If Panacur wasn't used, it would be ideal to speak to his vet about a trial on this. The reason is because it can be helpful with mild giardia cases (which as I noted tend to take a few fecals to truly rule out due to intermittent shedding in the feces). As well, they could potentially trial him on a course of Metronidazole if this was a suspect despite the lack luster results. And I would just note that while you are discussing this with his vet, it is worthwhile to see what the stool sample was actually tested for (since some tests just check for parasites and don't culture for bacteria or test for viral agents). That way we can be thorough and double check that infectious agents are not our trigger for his signs.
Otherwise, diet trials are a good option. Now if he got worse, then you were right to stop the diet when you did. Though I would note that when we are testing for food allergies/sensitivities we tend to need to feed the diet for 4-6 weeks to appreciate its full affect. And I would note that if you did, be wary of any of the ingredients in that grain-free diet (ie soy, the protein used, etc), as they may be something he is sensitive to. Otherwise, if you are feeing chicken/rice, do make sure to have this pup on a dog specific vitamin/mineral to ensure you aren't causing any complications with trace deficiencies. And to bulk this diet up to help his stools, you can also add in a spoonful of tinned pumpkin, a 1/2 teaspoon of Benefiber, or even some Weetabix as all are fiber rich and can help firm up stool.
Otherwise, since these signs are common with disturbance in the GI microflora, I would note that it is worth supporting the good GI bacteria. To do so, there are a range of dog probiotics and GI supplements on the market. Examples of this include Protexin Enterogenic or Fortiflora. As well, since his stools are loose, you could even use Propectalin, Fast Balance, Canikur, or Protexin Prokolin. These contain the GI microflora support we want but also has a dog safe anti-diarrhea (kaolin) to slow the diarrhea and prevent secondary nutrient or hydration loss. And all of these are OTC at vets, pet stores, and again on Amazon.
Overall, with the testing done so far infectious agents are less likely to blame here. Therefore, we want to double check that worming and what tests were done; but otherwise consider the above supportive care to start trying to rule out diet induced and digestion based issues while trying to normalize his stools. If we can settle him, we are happy. But if this lingers, then we'd have to start considering those other concerns and would have to speak to his vet about further testing (ie bloods, endoscopy of the gut).
I hope this information is helpful.
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