Wa’alaikum Salam.
Prophet (peace and blessings of Allaah be upon him) said: “If a dog licks the vessel of any one of you, let him throw away whatever was in it and wash it seven times (one of the 7 with earth).”
Firstly, as Muslims, we believe strongly that most of the things that the Prophet (saw) said or did were inspired by God, the All-knowing, hence we trust his judgement. Secondly, it is said that there are scientific reasons to it as well. However, I apologize because I didnt manage to find an elaboration on this. If there are anyone out there who are in the know, do share with us via the “Share!” tab.
Nonetheless, there are so many things which mankind has yet to discover, and theories, too, change over time. But being Muslims, alhamdulillah, we put our trust fully on the Quran and Sunnah, for they have yet to be proven false! In the contrary, they have led mankind to many discoveries and even debunked many false theories! Shall talk about this some other time, okay? :D
Things to note here is the difference between Najaasah (impurity) and Haram (sinful deed). Najaasah is only Haram when you consume it. In other words, it is prohibited for us to consume impurities (the ones highlighted by the deen). It is also impermissible for you to perform Solah in the state of impurity - ie. najees on your clothings and so on. Other than that, it is not a sin if you come in contact with any types of najaasah, including dogs and pigs. What one needs to do is to just cleanse himself afterwards (according to ways thought depending on the degree of najaasah), for his/her solah to be valid.
To answer the question, I would say that, dogs saliva is najis (impure) as identified by Rasul (saw). When in contact, one needs cleanse the affected area. The specific (scientific) reason to it, sadly, I am not in the know. It is haram (sinful) when consumed. And Solah in the state of “impurity”, is not valid.
Wallahu Ta’ala A’lam. :)