White spots in the back of the mouth can be sialolites, spots due to herpes infections, yeast problems, or spots caused by tricho infection. Sialolites can be normal, it doesn't do any harm to the pigeons and is not at all related with any kind of disease. When the spots are caused by Herpes infection, it is possible that it will go away after a few weeks, but sometimes the pigeons have at the same time some problems with the respiratory system, so it is likely to treat against respiratory diseases also. Against the virus himself, you can't do anything at all... When the problem is caused by a tricho infection, treating against tricho is advisible fe with Berimax. It is possible that you had at the same time a yeast problem, but may this was only a secundary infection, and the spots were not caused by this yeast. It is possible that the problem is solved, but at this moment you still can see some spots due to sialolites. Do you see at this moment that your pigeons show any sign from illness?