The louse is a parasite that flourishes in poor hygiene conditions.
A canary suffering from louse shows signs of nervousness, irritation and scratches constantly.
The first thing you should do is remove the canary to the cage and proceed to a deep cleaning (not just of this one, but of any cage that has come in contact with it). Consult with the veterinary for a disinfecting product recommendation for the canary, and if necessary, the cages. In addition, you should consult about the need to prevent louse on the rest of the canaries even if they do not notice signs of disease.