During battle, again, it's up to the player to determine the outcome of the bite. However, I would say that the toxin, in normal IC posts, would be slow acting and possibly have an effect for the remainder of the thread and following 2-3 threads (considering that time is liquid).
Symptoms would be disorientation, dizziness, headaches, blurry vision, obvious inflammation and pain at the site of the bite, but the character would be able to heal on their own to prevent the companion from being overpowered. I would say that these would also be the same symptoms if a character got bitten very harshly by a bear, wolf, or larger animal and lost alot of blood.