Does Akismet work retroactively?

I have a client that has a blog, that currently has over 40,000 comments. When this blog was created the settings allowed all comments to be posted automatically without any moderation. I now have 40,000 comments to moderate, as some are genuine and others are spam.

I am going to activate the Akismet plugin, and comment moderation to combat the new spam comments. The question I have is “does Akismet work retroactively?” So if I install it now, will it go back and reassess all of the past comments, or will I have to go through them one by one to separate the genuine from the spam.

If this is not the case is their another way of addressing this issue (that does not involve deleting all of the previous comments from the database)?