Thank you for the further information on this wee one.
First, just to note, please don't think I was suspecting fleas. I had just been trying to get an idea of whether the "black spots" were a skin change or something on the skin. But I do thank you for the photos, since they are genuinely worth a thousand words. The black spots you are seeing develop are patches of hyperpigmentation. They are the skin's defensive reaction to chronic irritation (akin to how people tan with sun exposure). So, seeing this is not surprising in a case of such chronic skin disease but unfortunately doesn't tell us what is triggering his signs.
That means we still have to consider a range potential causes behind his chronic skin irritation. When we are dealing with itchy hamsters, the common differentials we need to rule out are bacterial infection, fungal, parasites, allergies, nutritional issues, hormonal disease and even organ failure. All of these can make a hamster itchy and manifest in this manner.
If we start with his environment, then we do have to rule out whether he could be having a diffuse allergic reaction to his bedding. Bedding issues are not an uncommon reason for hamster skin disease, as bedding can often be a route of mite exposure and we can see hamsters have skin reaction to particular bedding materials. Therefore, especially if you are may be using a new type or batch, you might want to try him on paper based bedding or even shredded newspaper for the short term to make sure his signs are not due to secondary to allergic disease.
Once we have ruled out bedding and environmental allergens, then parasites tend to be the issue to address. That said, I appreciate you have treated him for mange via shampooing. As well, in the photos his skin actually looks quite settled and does not look typical of a traditional mite infestation. That said, we would still have to consider one mite called demodex. When we see demodectic mange, we can see flair ups and hair loss as well but this mite infestation tends to be secondary to underlying diseases that are weakening the immune system. In all cases where mites are suspected, we do tend to diagnose them by taking a scale/crust/scraping and examining it under the microscope. In any case, if we wanted to officially rule out mites, then we could also consider just treating with a OTC topical mite medication (ie ivermectin based, Xeno, etc).
Next, the bacteria. Now since he doesn't have signs of pimples/pustules, then this would be lower on our list of concerns. That said, we often will see this secondary to chronic scratching but in Patches 's case I do suspect your bathing has kept this from gaining a foot hold here.
Now his diet does sound quite diverse; so we do need to be careful here. Dietary issues can easily be a problem for these wee ones especially when they have a lot of feed options. This isn't because the diet isn't balanced when we provide it but rather we can see some hamsters selectively graze their food options (due to preference for the unhealthy bits or dental disease) When they happens, we can see itchy skin and hair loss in hamsters who have a protein deficiency (ie if he has been overeating the seeds, corn and skipping his pellets). As well, protein deficiency is also known to cause changes to the skin quality (drying/scaling which is itchy too). In addition, we can see diet causing skin issues if they are being overfed grains and cereals (ie corn). This is easily remedied by offering more fruits and vegetables in the diet or by feeding half the dry grain/seed mix with boiled rice or puffed rice cereal.
Just a tiny note in regards XXXXX XXXXX since it can be seen though is not often found to be overly itchy for them. When we do have cases of ringworm, they often can appear with patches of subtle scaling and hair loss. Common places to see it is on the head/face, back, and sometimes belly. Therefore we do always want to keep it in the back of our minds with hamster skin disease. And if it is a suspect (since I cannot really appreciate the scaling in the photo), then you'd need your vet to take a hair pluck or scrape sample and have the lab culture the fungus.
Finally, systemic disease (metabolic, cancerous, organ based) can also cause manifestations of skin disease. As crazy as it will feel, we do have to consider these with Patches because he is already getting to middle age. In these cases, we often will have other hints (ie increased appetite, thirst, weight loss, hair loss that is symmetrical on each side of the body, etc) but not always. And typically if we cannot rule out the above, then we have to consider these agents. Often a good physical exam +/- a check of bloods or urine can tell us if any of these are an issue and the underlying cause for his skin disease.
As I am sure you can appreciate, there are a number of conditions we have to consider for wee Patches. Though it sounds like we can put parasitic and bacterial agents at the bottom of our list of concerns to start. Therefore, from here, you want to ideally try to determine the trigger for his signs with a step-by-step approach. First, you should review the diet you are feeding and what he is actually eating (Try to make sure he is getting >16% protein ad possibly up his fruit/veggies). Then try him on paper-based bedding to rule out allergic skin disease. If the scaling is severe, then fungal issues need to be ruled out. And if you think he is really drinking more and this flank hair loss looks symmetrical on the other side of his body, then we'd want to consider getting his vet involved since those two signs are quite suspicious of potential hormonal or organ based skin disease.
I hope this information is helpful.
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