Hello & welcome to Just Answer/Pearl. I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today. At Liony's age, with her activity level, and her being otherwise well; it is quite possible that she has suffered a superficial trauma to this wee area of skin that has then becoming infected by opportunisitic skin bacteria. When this happens we can see inflammation, moist dermatitis (+/- scabbing or ulceration) or even form a little lump with an abscess. That said, we do have another concern for lesions in this area. Specifically, we'd have to consider whether this is cheilitis (Example). Cheilitis is a infection of tissues at the junction between normal skin and mucosa (ie mouth, nose, etc) and can be caused by bacteria as well as be triggered by fungal agents (like yeast) and viruses (ie poxvirus). Since this has just appeared (unless you feel it looks just like the example of cheilitis), you can try to settle this at home. To do so, we can often treat them topically with a mild, safe antiseptic like salt water (1tbsp salt to a pint warm water). This should tackle mild bacterial infections. For cases that have deeply established infections, we sometimes need to use guinea pig safe antibiotics given via injection or oral medication. We don't often use topical antibiotics for these cases since there is the risk that they will lick them off. And this risk is two fold, since it will impede treatment success but also many topical antibiotics are not safe for the cavy GI (and ingestion can lead to serious diarrhea that can be fatal in some cases). Furthermore, if the viral or fungal causes are suspected, these can be diagnosed by swabs or scrapes of the lesion being either examined under the microscope or cultured by a vet lab. If any of these agents are found, then treatment will be altered to target them specifically. Overall, the most common cause for a sudden lesion like this would be trauma induced from her getting up to mischief. And if that is the case, then salt water bathing the lesion will likely settle it for you. But if you feel that she does have a brewing cheilitis or this doesn't appear to be settling in the next few days with your supportive care, then do consider following up with her vet at that stage. They can confirm your suspicions, dispense guinea pig safe antibiotics, and guide you on any further testing if required. If you don’t already have an exotics vet, you can find one near you at http://www.aemv.org/vetlist.cfm or HERE (http://www.guinealynx.info/vetlist.html) . If you are struggling also check here (http://www.rabbit.org/vets/vets.html), as rabbit vets often see our wee pocket pets as well.
I hope this information is helpful.
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Liony's nose got better in three days, so she clearly just bashed it running around.
You are very welcome and thank you for the update on Liony.That is great news. I would say that since she has demonstrated that she is prone to injury when dashing about, do consider double checking her cage to ensure that there are no bits that she could catch herself on in the future. As well, if she is racing through the cage, consider placing her toys/cage components in a manner to prevent long straight/clear paths to deter her from building up too much speed before she bumps into the cage edges to prevent any future injury.All the best with wee Liony,Dr. B.