What you can see without forcing the mouth open
Most of mouth rot's early evidence is visible from outside. A healthy reptile's lips meet in a clean, even line; early mouth rot disturbs that line — one side sitting slightly proud, a corner that no longer closes, a subtle swelling along the jaw. Look for redness or discoloration along the lip margin, dried discharge crusted at the corners of the mouth, and saliva that has turned thick and ropy, stringing between the jaws when the animal yawns or gapes rather than staying thin and clear. The animal may rub its face on enclosure furniture, a sign of irritation, or hold its head slightly tilted. Never pry the mouth open to investigate further: a sore mouth, forced, means pain, a stressed patient, and a real risk of damaging a weakened jaw.
What the vet's examination typically reveals
Behind those external hints, the condition — infectious stomatitis — is an infection of the mouth's lining, usually where tissue was first damaged. Examined properly, the gums are inflamed and red, sometimes with small bleeds, and patches of white, yellow or grey cheesy material cling to the gum line and the roof of the mouth — dead and infected tissue rather than anything that wipes away. Teeth may loosen in neglected cases, and the infection can track into the jaw bone itself. This is why the condition carries the blunt name it does, and why it belongs in a clinic: assessing how deep it goes, and clearing infected tissue safely, is examination-under-control work, not something a cotton bud at home can replicate. What you are watching for at home is the outer evidence that starts that appointment.
Eating and behaviour changes
A mouth that hurts changes how a reptile feeds, and feeding is where many owners first notice. The animal approaches food, strikes or reaches, then drops it or gives up — interested but unable. Meals get slower, then refused. Weight begins to slip in an animal that was previously holding condition, and the general demeanour dulls: less basking, less activity, more time hidden away. In snakes, a shed may go badly around the head, and the mouth area can look asymmetrical in photographs taken weeks apart. Keep those feeding notes dated, because 'he went off his food' becomes more useful as 'he last ate properly three weeks ago, and has dropped food at every attempt since'.
Why it starts: injury, stress and the setup
Mouth rot is rarely a primary event; it is infection exploiting an opening. The opening is often mechanical — a mouth injured by live prey that fought back, by striking glass during feeding excitement, or by relentless nose-rubbing against enclosure walls in a stressed animal. Behind the injury sits the immune picture: temperatures running too cool for the species, chronic stress, poor nutrition and dirty conditions all lower the defences that would normally keep mouth bacteria in their place. That is why a good vet visit for mouth rot covers the enclosure as well as the mouth — basking temperatures as measured, not as set; feeding method; recent stressors such as a move or new cage mate. Treating the mouth while leaving the cause in place invites a second round.
The threshold for the vet, and what not to do
Book promptly — within days, not weeks — if you see cheesy patches, bleeding, a mouth that will not close evenly, or a reptile that has stopped eating with any mouth sign alongside. Treat it as urgent if the jaw is swelling, discharge is heavy, or the animal is lethargic as well. At home, restrict yourself to keeping the enclosure clean, the water fresh, and stress low; do not scrape patches, swab the mouth with household antiseptics, or offer leftover antibiotics from another animal, each of which can worsen tissue damage or mask the picture the vet needs to see. Transport the reptile warm and secure. Afterwards, the findings and recheck dates belong in the reptile's KinsLe Pet Passport, so the gum line at each recheck is compared against the first visit rather than against hope.