Now at his age, worms usually won't cause dramatic diarrhea. Usually that is a younger dog situation. That said, if he hasn't been wormed in the past month, then it is worth covering that base. Though we have to be aware that this is more likely due to a bacterial, viral or protozoal agent. As well, with that belly tensing, there is a risk of this being due to a foreign body but at his age that tends to be less common. Still we do want to keep an eye on that just in case.
Now since he is otherwise well, we can try supportive care just now. To start, you can plan to try small meals of a light/easily digestible diet. Examples you can use are cooked white rice with boiled chicken, boiled white fish, or scrambled eggs, or meat baby food (garlic/onion free only) There are also OTC vet diets (ie Hill’s I/D or Royal Canin’s sensitivity) that can be used too. The aim of these diets is that they will be better tolerated/absorbed by the compromised gut. Therefore, it should get more nutrients in and result in less GI upset and less diarrhea. You can also add fiber (ie canned pumpkin, all bran) and an OTC canine probiotic (ie Fortiflora, Benebac) to these meals to firm the stools quicker and support the gut.. As long as improvement is being seen, I usually advise continuing this until the signs are settled, and then weaning slowly back to what you normally feed.
Since dehydration is a risk with explosive diarrhea cases, we need to keep an eye on his hydration. To check this and make sure dehydration isn’t an issue, there are a few parameters you can test. Further to checking for gum moisture, you will want to make sure his eyes are not looking sunken and that he doesn’t have a "skin tent" when you lift the skin. To see how to check these, you can find a good video HERE (http://www.ehow.com/video_12232503_dog-dehydrated.html). If you do see any of these signs already, then that would be our cue to have him seen before this becomes an additional issue (especially since its often dehydration that makes them feel unwell).
Finally, as there is no blood in those stools, you can consider trying a pet safe anti-diarrheal. As I am sure you appreciate, these would not be a cure for infectious issues; but it can still be of benefit. It will reduce diarrhea load, allow the body to absorb more water/nutrients, and soothe the upset gut. In regards ***** ***** options, the one we most commonly use is OTC Kaolin/Kaopectate (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/kaolin-and-pectin-kapectolin-k-p)). Otherwise, Propectalin, Canikur, Fast Balance, and Protexin Pro-Fiber (which is available OTC at vets, pet stores, and even Amazon) would be another option. All will slow diarrhea and those last ones have the added bonus of providing support to the delicate good GI bacteria. So, these can be used as a short-term means of soothing his upset GI.
Overall, a wide range of agents could trigger the signs we are seeing. Worms aren't likely but we can address them as we support him to see if we can get him settled. Of course, if he doesn’t respond to the above within 12-24 hours; then we'd want to get his vet involved. They can assess his hydration, test a stool sample, ensure nothing in his stomach that shouldn't be, or any sinister viruses present. Depending on their findings, his vet can treat with injectable antibiotics +/- fluids to settle his gut and get him back feeling like himself.
Please take care,
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