Thank you for the additional information.
First, since cavies are a prey species and instinctively will hide illness from us until it is very advanced, I would always say that being proactive and having them checked by their vet sooner rather then later is always the wisest course of action. That way, you are able to identify what is triggering her signs, treat them, and give her the best chance of recovering as quickly as possible.
Now what particulalry concerns me here isn't actually her skin. I will discuss what we need to consider regarding her skin in a moment, but unless she is itching constantly such that she isn't making time to eat (and therefore losing weight) or if the sores are leaking serum (which would cause protein loss for the body), then it is quite possible that this skin issue is actually a secondary problem. Specifically, it could be an opportunistic infection that is taking avantage of her as her body deals with whatever is causing the weight loss.
And the weight loss despite a normal appetite is quite a concerning sign. If she were showing any appetite decline, then we'd be in an emergency situation for her since this is a species not designed to go off their food. But if she is eating and showing no changes to her motions, then we have to ask ourselves where the weight (and nutrition) is going. This would raise concerns for us of potential organ troubles (ie liver, kidney, GI, etc) as well as infectious causes. Furthermore, if she isn't a very young cavy, then we'd also have to consider that we can see tumors appear with little sign other then weight loss (as they steal nutrition from them). And it is because of this a physcial examination by her vet would be ideal at this stage.
Now turning to the skin signs you have noted, when guinea pigs show scaly skin, we do need to consider a few issues. The main ones that we need to rule out are the non-pruritic mites (ie Cheyletiella), fungi (ie "ringworm") and nutritional causes. Each can give the skin a dry/scaling appearance with little else in the way of signs.
Cheyletiella, also known as ‘walking dandruff' is a surface mite that lives on the skin and causes skin irritation for the pocket pets. Over time the mite spreads over the body causing a thick scaling and may also cause an associated hair loss. To diagnose this mite’s presence, a vet can check a sample of scale/crust under the microscope. If present or suspected, this mite tends to respond to treatment with topical mite medication (ie ivermectin based, Xeno, etc).
Fungal skin disease is another common cause for scaling of the skin in the guinea pig (example) We can see the ringworm can cause initially circular lesions but often it doesn’t appear that clearly. Rather it starts as a region of scale that can appear crusty or raised and spreads along the skin. Some animals will shed the hair as the fungus spreads, where others might lose hair in crusty tufts leaving dry and scaly patches of skin. Diagnosing this infection requires a hair pluck to be sent for culture of the fungus to confirm its presence.
Finally, dietary concerns must be ruled out for your cavy. If they have dietary deficiencies (either from poor diet or selective grazing on their part), the skin often suffers (since the body will be feeding the more critical organs first). Dietary-wise, we do need to double check her diet's protein and Vitamin C levels as deficiencies in both these components can lead to issues of this nature for the skin and coat. If you are supplementing his Vitamin C directly, then you need to make sure she is getting at least 50mg/kg daily. If you are relying on the pellets to supplement his Vitamin C, then check that they are meeting this requirement (and be aware that if the pellets are old or been open for a while that the vitamin will break down).
Protein-wise, we usually associate these signs with a diet with less than 15% protein. Or a secondary dietary problem is overfeeding grains and cereals, such as corn. This is easily remedied by increasing fruits and vegetables in the diet or by feeding half the dry grain/seed mix with boiled rice or puffed rice cereal. You can also support the skin and make sure it has adequate essentials fatty acids by supplementing her with 1-2 drops of cod-liver oil daily for a few days (then drop to weekly).
Overall, I am concerned that the skin signs may actually be a secondary issue to something more sinister causing her weight loss. Therefore, it would be ideal to consider getting your vet involved at this stage. If she is eating well, then you may choose to wait until Monday morning to have her seen. (Though any appetite decline would warrant having her seen sooner). If you do, then in the meantime, you do want to review her diet and make sure it is balanced and she is actually eating all parts. And while we can often treat against those aforementioned parasites using an over the counter ivermectin product, I would advise holding off until she has been checked and she is cleared for any organ based issues (since we don't want to over stress her). While the vet checks her for systemic issues, they will also be able to diagose the skin signs for you and apply proper treatment to address this as well. Depending on their overall findings, you will be able to determine what is the culprit for her lost weight and you will be in the best position to treat her effectively.
And just to note if you do want to have her seen tonight, I wanted to mention that most veterinary practices here do have contingency plans for emergency care for their patients. This means that if you ring the practice, they will likely have a message to direct you on how to contact their emergency service. And if you don't have a vet you can find one local to you, you can check RCVS register (LINK). or you can check here to find your local Vets Now (LINK) who are open all nights/weekends. And if you don't already have an exotics vet, and wish to find one near you, via the RCVS register (LINK). As well, a lot of the vet schools (ie Edinburgh, Bristol, RVC, etc.) will either have an exotics vet on site or will have ties to one that they can refer you to (ie. Glasgow). As well, you can check here http://www.aemv.org/vetlist.cfm .
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
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