Thank you again,
Now I am glad that we can rule out issues that could cause continuous blood flow from the rectum. In that case, we'd have to be most wary of these signs being infectious in nature or related to toxicity. Specifically, we can see upper/lower GI signs like this with viral infections (ie Parvo, Hemorrhagic gastronenteritis), bacteria (like Salmonella, Campylobacter, etc), protozoa (ie Giardia, Coccidia, Cryptosporidia) and ingestion of harmful items (ie toxins, plants, non-edible items). As well, while a minor sign, I'd note that mucus she sees when she scoots is likely from secondary inflammation of the colon but could also be related to the anal glands being inflamed (though usually those won't cause vomiting as well).
With this all in mind and with that volume of blood, we'd want to plan to have her seen as soon as her vet is open. In then meantime, we'd want to start supportive care to lessen her signs. Since she can keep water down, you can consider treating with an antacid. Common OTC pet safe options would be: Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid), or Zantac (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/ranitidine-hcl-zantac). Whichever is chosen, we’d give this 20 minutes before food to allow absorption. Of course, do double check with your vet if she has a known health issues or is on any medications you haven't mentioned. Though if you give this and she cannot keep it down due to nausea that is usually a red flag that we need to bypass her mouth with injectable anti-vomiting medication from her vet.
Afterwards, we can consider starting her on a easily digestible diet like cooked white rice with boiled chicken, boiled white fish, cottage cheese, scrambled eggs, or meat baby food (as long as its garlic/onion free). 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. Fiber (ie canned pumpkin, all bran) can be added to this to firm her stools. Also OTC canine probiotics (ie Benebac) can be used to support the gut. And I would note that we need to avoid any anti-diarrheal medications at this point since those above viruses/bacteria can actually cause more harm if allowed to linger in the gut.
Since dehydration is a risk here, we need to keep an eye on her hydration. To check that she isn't dehydrated, there are a few things you can test. Further to checking for gum moisture, do make sure she doesn’t have sunken eyes and that she doesn’t have a "skin tent" when you lift the skin. To see how to check these parameters 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).
Finally, since we have some discomfort around the anus, while we will want her vet to double check her anal glands, we can gently salt water bathe the skin to counter any fecal scalding. OTC hydrocortisone cream can be used here to reduce skin inflammation if needed as well.
Overall, there are a wide range of agents could trigger these signs. So, we need to tread with care and will want her seen as soon as her vet is open. Though for the moment, we’d want to start supportive care to settle her stomach and then keep her eating and drinking to prevent dehydration and energy loss. Once her vet is open, then assess her hydration, rule out fever, make sure there is nothing in her stomach that shouldn't be there or any sinister viruses present. Depending on their findings, her vet can treat her with injectable anti-vomiting medication, fluids, +/- antibiotics and anti-diarrheals (once those agents are excluded) to get her settled before this can take a toll on her.
All the best,
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