Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your wee one’s situation, and wanted to help.
If Lucy's signs are worsening, we need to tread with care. With her history, we'd mainly suspect weakness associated with the dehydration and nutrient/energy loss that diarrhea causes, but we also need to be wary of non-related issues. In this case, as long as she can walk normally (though weak) and hasn't had any trauma, then we'd want to be proactive and get this diarrhea under control while addressing her nutrient/fluid losses.
Now I am glad to hear that you have started a light diet. The aim with these is that they are easier to digest even with a compromised gut. So, she can get more in and will lose less in her stools. Of course, do make sure to offer this in small meals to again reduce the workload of the gut.
Further to this, since dehydration is a real concern here, do check her hydration status now. To do so, we want to check her gum moisture (if its dry/sticky, that is a sign of dehydration). As well, we need to make sure her eyes are not looking sunken and that she doesn’t have a"skin tent" when you lift the skin. To see how to check these signs for dehydration, you can find a good video HERE (http://www.ehow.com/video_12232503_dog-dehydrated.html). If you are seeing any signs of dehydration already, then that would be our cue to have her seen before this becomes an additional issue for her (especially as it is often dehydration that makes them feel unwell and again can cause weakness). Just to note, as long as she isn't vomiting, you can start syringing her with electrolytes (ie Lectaid, Resorb, Pedialyte). If you do so, aim for 48ml per kilogram of her body weight plus add in an amount to match the volume of fluid she is losing in her diarrhea (you can of course subtract anything she drinks on her own). This will help her fight the dehydration and the electrolytes in these are important for restoring her electrolyte balance (another potential weakness cause).
Finally, as long 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 Kaolin/Kaopectate (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/kaolin-and-pectin-kapectolin-k-p)).This is available OTC at most pharmacies. Otherwise, Propectalin, Canikur, FastBalance, and Protexin Pro-Fiber (all 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 her upset GI.
Overall, we need to tread with care. Diarrhea in dogs can be related to infectious agents (ie bacteria, viruses, parasites, protozoa, etc), inflammatory gut issues, and ingestion of harmful items. But the worry here is that this is now getting to a stage where her electrolytes, fluid levels, and nutrition are all being tapped out by the diarrhea. Therefore, we need to help keep her stable to see if we can get her settled. Or if this doesn't or she worsens, then we'd want a check with her vet to diagnose the root cause for her diarrhea and make sure there is nothing more amiss with those back legs.
I hope this information is helpful.
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All the best,
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