Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
First, I would note that black widow venom is a neurotoxin that needs to be injected. If she ate the spider, its likely her stomach acid would have just digested this. And this means the spider isn't likely the cause of her signs. So, while it has to be considered, more common issues like bacterial or viral gastroenteritis, pancreatitis, parasites/protozoa infections, general dietary indiscretions, and ingestion of harmful items (ie toxins, plants, non-edible items).
With this all in mind, as long as Shasta can keep water down, we can try some home supportive care to see if we can settle her stomach. To start, if she hasn’t just vomited (since otherwise we’d need to rest her stomach for a few hours first), then you can try an OTC pet safe antacid like Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid), Zantac (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/ranitidine-hcl-zantac), or Tagamet (More Info/Dose Here @ http://www.petplace.com/article/drug-library/library/over-the-counter/cimetidine-hcl-tagamet). Whichever you choose, we’d give this 20 minutes before offering food to allow absorption. Of course, do check with your vet before use if she has any known health issues or is on any medications you didn’t mention. As well, if you try this and find the nausea just too severe to keep it down, then that is usually a red flag that we need the local vet to bypass her mouth with injectable anti-vomiting medication.
After that has had time to absorb, we can start small meals of a light/easily digestible diet. Examples you can use are cooked white rice with boiled chicken, boiled white fish, cottage cheese, scrambled eggs, or meat baby food (as long as its garlic/onion free). There are also OTC vet diets that can be used (ie Hill’s I/D or Royal Canin’s sensitivity) too. When you offer these meals, give her 30 minutes after to settle. If she keeps the food down, you can give a bit more and so on. 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. As long as improvement is being seen, I usually advise that the diet be continued until her signs are settled, and that they are then slowly weaned back to their normal diet.
Since dehydration is a risk, we need to keep a close eye on her hydration. To check this and ensure she’s not becoming dehydrated, there are a few things you can test. Further to checking for gum moisture, you will want 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 parameters for dehydration, you can find a good video HERE (http://www.ehow.com/video_12232503_dog-dehydrated.html). If you do find these dehydration signs, then that would be our cue to have her seen before this becomes an additional issue (especially as it is often dehydration that makes them feel unwell).
Finally, as long as you have not seen 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 if this is infectious; 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, we most often use 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 (all 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 this upset GI.
Overall, a wide range of agents could trigger the signs we are seeing but a black widow spider isn't likely to cause what you are seeing Therefore, we’d want to start supportive care to settle her stomach. If she cannot keep that or water down at any point, appears dehydrated already, or doesn’t respond to the above within 12-24 hours; then we'd want to get her vet involved. They can 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, your vet can treat her with injectable anti-vomiting medication, fluids, +/- antibiotics to get her back feeling like herself.
Please take care,
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