Praise be to Allah.
Touching dogs or their saliva does not nullify wudu because if taharah is done according to the rules of Shari`ah, nothing can nullify it except things for which there is evidence in Shari`ah.
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 wudu.
Ibn Qudamah mentioned in al-Mughni (1/264) things that nullify wudu, and he did not mention touching dogs or their saliva. Then he said: These are all the things that nullify taharah, 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 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 taharah.
For more, please see this category: Nullifiers of Wudu.
And Allah knows best.