Indeed, knowing where you live would be nice.
Around here we can have different supplies then you have.

It is a little bit difficult to see from a small picture,
but if i may guess, i would say it has something to do with the environment.
I´ve seen quit similair wounds, at least if i see it right in the picture

Best thing i should advice is checking the conditions in the viv like humidity and temperature.
The wound should be treated to make sure it doesn't get infected,
if it isn't already.
Good solutions around here are Bactyfec, ampicilinne powder or solution or Sulfanilamidum.

Altough it is best to let a vet check to be sure,
also the amount of solution you need to use and the period are things a vet can deside better then we from a distance