Hello & welcome. I do apologize that my colleagues could not aid you earlier (as I wasn't available when you sent your question). Still I would like to help you with your wee one today.
It is understandable that you don't want to completely stop Fluffy's veggies access, but I would say that lettuce has very little nutrition (its practically solid water). So, if she is normally hydrated, you could try just keeping her on the peppers +/- a bit of something else (ie carrot, carrot tops, radish tops, mustard greens, etc). Or if we really think the loose stools are due to her veggie's high water content (though less likely an issue unless you had just added loads of veggies to her diet prior to this) you could try her off the veggies but replacing them with a Vitamin C supplement. That way you can rule out the water laden veggies but not risk scurvy.
Otherwise, if removing the high fluid containing foods from her diet doesn't change her situation, then we do have to think about the more common infectious processes that could cause these signs like a bacterial infection, viruses, parasites, or protozoa. In the case of these, you will often need to get your vet involved for specific treatment (ie antibiotics, anti-parasitics, etc) and this may be the next step if her diarrhea doesn't respond to your diet changes.
Further to all of this, I do want to outline some supportive care steps you can take for Fluffy, Now it sounds like she is eating for you, which is good. Poorly cavies often go off their food and when they do it can lead to emergency gastric stasis issues. So, do monitor her appetite to make sure she keeps eating. If it wanes, then you may need to start hand or even syringe feeding to keep nutrition and fluids getting into her and avoiding stasis.
Now since her gut is obviously under attack and the diarrhea is profuse, you may want to consider treating her with a fiber supplement/GI micro flora support agent. Fiber would help bulk up the feces and slow the diarrhea. The micro flora support would aid in supporting the GI's good bacteria which are likely under threat. The two products that you might want to consider (and are available OTC at the vets) are the paste formulation called Protexin Fibreplex (link) or the pelleted fiber product called Protexin Pro-Fibre (link). (Do note that while these products are labeled for rabbits, it is the same one we'd use for small rodents like cavies, example). Both have that fiber bolus we would want to give as well as a pro-biotic element to support that delicate GI micro flora in this time of GI upset.
To battle fluid loss from her diarrhea, you do need to monitor her drinking +/- offer fluids. If you do need to provide support, consider offering electrolyte solutions (since electrolytes will be lost in diarrhea as well). There are a range of options on the market like Lectaid (available OTC at the vets and some pet stores) or you can choose to offer/syringe Pedialyte (fruit flavours seem to be best tolerated). In a pinch, we can use diluted Gatorade (50% diluted with water) though this isn't ideal for the longer term. These will help replenish electrolytes and get some glucose into her system. A typical dose for animals is 4.8mls per 100 grams of body weight per day. This would give us her daily requirement, though it wouldn't include losses in diarrhea. Therefore, in these situations, we want to calculate her maintenance dose but then add in a volume equivalent to her diarrhea losses. This total volume is then divided into multiple offerings over the day to ensure that she receives her required fluid intake without her diarrhea losses overpowering her and causing dehydration.
As well, just to note, you think she might be chilled, you can make a safe warmer for your cavy from a clean sock filled 2/3rd full with uncooked white rice. Tie it closed and microwave (approx 1-1.5 min). Make sure to shake it before adding it to the cage, to allow the heat to distribute. Make sure its not too hot (as we don’t want to burn her. If it cools, you can re-warm as required.
Finally, in regards ***** ***** dirty backside, I would advise keeping a close eye on the skin in the area to make sure she doesn't develop any signs fecal scalding. If you do see this, I do want to warn not to be tempted to put any creams on this area. The problem with creams in this species is that they often will lick off anything you put on. And this is a major problem if you were to use a cream containing any antibiotics (since so many on the market are not safe for this species). As well, even antibiotic-free barrier creams can be less the ideal for these wee ones since even those that are not overtly toxic tend to lock in the moisture (and bacteria) present, which is the opposite of what we'd want with a moist dermatitis secondary to diarrhea. Therefore, if you do see any sores or inflammation, then you can salt water bathe the area (using a tbsp of salt in a pint of warm water) and making sure no feces is left to irritate the peri-rectal skin. This will help break the cycle of fecal scalding. If there is a severe moist dermatitis, then we'd have to think about systemic/oral treatments (which any antibiotic used for her diarrhea may help with this as well) or using topical treatments while she wore an e-collar (less then ideal since e-collars can also make cavies reluctant to eat). So, do keep a close eye on her backside and make sure it stays as clean and dry as possible.
Overall, too much water in their diet via the veggies can be an issue, but often it starts when someone has introduced a significant volume of new food stuffs. So, you can try stopping the veggies and using a Vitamin C supplement if you think this if your culprit. Otherwise, you will have to consider our infectious concerns. And in that case, we want to be proactive and aggressive here. Therefore, it would be ideal to have her vet check her over and start her on treatment for infectious causes of diarrhea. If you do have her seen, aim to collect a sample of this diarrhea for them to examine. They will be able to identify the cause of her diarrhea and help get her back on track as quickly as possible.
Just to note in case your vet isn't a guinea pig specific vet, or you wanted to see someone who specializes in guinea pigs, you can find one near you at HERE or HERE . If you are struggling also check here the rabbit database (http://rabbit.org/vet-listings/) as rabbit vets often see our wee pocket pets as well. You can also check RCVS register (LINK). using the advanced search option and clicking on the correct specialty.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
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