As we learn in the second cour of the first season of Durarara!!, Niekawa Haruna is the Saika-wielder primarily responsible for the slashing incidents that have been plaguing Ikebukuro for a while.
In episode 17, when Anri reveals herself to be the true bearer of Saika, Niekawa realizes that it must have been Anri that slashed and "infected" her with Saika.
But, according to Shinra, Saika had been dormant for some five years prior - starting when Anri took possession of Saika after her parents' death, and ending presumably when the current series of slashing incidents began.
When did Anri slash Haruna? And why? I would think that if Anri had slashed her during Saika's supposed period of dormancy, somebody would have found out about it, at least.