Now I share your concern that this is more than a hairball. Usually that isn't something we'd see affect 2 cats at once and that doesn't explain the diarrhea. Instead, we'd be more wary of an infectious issue (ie bacterial, viral, protozoal, or parasitic) or a share exposure (thankfully less likely here).
With this all in mind, since they are at high risk of dehydration, nutrition loss (which can make them weak, lethargic and lose weight) plus liver compromise (since cats off food can develop hepatic lipidosis), we'd want to be proactive here. In fact, it'd be ideal to have them reassessed +/- make sure that stool sample checked for all of the above not just worms as is often all that is checked in the practice. We'd want their vet to start symptomatic treatment (ie injectable anti-nausea medication, antibiotics, +/- appetite stimulants and fluids). That way we can counter these signs and get them settled before they take a toll.
Any delay and we'd at least need to consider some home supportive care. To start, provided they've not just vomited (since otherwise we’d need to rest their stomachs for a few hours first), 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 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 double check with your vet if either has a known health issues or is on any medications you haven't mentioned. Though if they cannot keep this down due to nausea that is usually a red flag that we need to bypass their mouth with injectable anti-vomiting medication from your vet.
Though if we can settle their stomachs, we can plan to follow this with a light diet (ie boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), or meat baby food (onion/garlic free). There are also OTC vet diets that can be used (ie Hill’s I/D or Royal Canin’s sensitivity) 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 reduce diarrhea. We can also add fiber (ie canned pumpkin) +/- OTC feline specific probiotics (ie Foriflora, Benebac) to try to restore normal gut microflora. And if they are very runny, you can even consider treating with OTC 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 but avoid Pepto Bimsol, Imodium or salicylic acid containing products since those aren't kitten friendly. Otherwise, Propectalin, Canikur, Fast Balance, and Protexin Pro-Fiber (all OTC at vets, pet stores, and even Amazon) would be another option since they combine Kaolin with probiotics.
Since dehydration is a risk for kittens with these signs, we need to keep an eye on hydration. To check this and ensure they aren't dehydrating there are a few parameters you can look at. Further to checking for gum moisture, you will want to make sure neither has sunken eyes nor 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 them back to their vet for reassessment and those aforementioned treatments.
Overall, there are a wide range of agents could trigger this GI upset we are seeing with Bruno and Layla. With these signs persisting, causing diarrhea and affecting them both, a hairball isn't likely to blame. So, we'd be best to have them reassessed or have another vet locally examine them. Any delay and you can use the above but if they appear dehydrated already, or do not respond to the above within a few hours; then we'd want a recheck to ensure this doesn't take a toll on them. And if we have them rechecked, we'd hope to use those above noted treatments to get them both settled and back to feeling like themselves.
Please take care,
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