Praise be to Allah.
Touching dogs or their saliva does not nullify wudoo because if tahaarah is done according to the rules of shareeah, nothing can nullify it except things for which there is evidence in shareeah. There is no such evidence that touching dogs or their saliva have this effect. It was not mentioned by the scholars among the things that nullify wudoo.
Ibn Qudaamah mentioned in al-Mughni (1/264) things that nullify wudoo, and he did not mention touching dogs or their saliva. Then he said: These are all the things that nullify tahaarah, and it cannot be nullified by anything else according to the view of the majority of scholars
But there is no doubt that dogs saliva is very dirty and extremely naajis (impure), and it can only be removed by washing seven times, one of them with earth. There is a difference between this and something which nullifies tahaarah. And Allaah knows best.