Ohtani takes leave of absence
According to a post on the Pokeani message board, Ikue Ohtani, the voice of Ash's Pikachu, is ill and has taken sick leave from work. It is still unknown what will be done about Pikachu's voice, but two possibilities exist.
The first would re-use clips of Ohtani as Pikachu from old episodes and splice it into the new episode audio.
The second would cast a temporary replacement actor in Pikachu's role. It is unknown who this actor would be, but the person is nearly guaranteed to be female so as to stay in the same range as Ohtani.
This is not the first time a main character's actress has temporarily left the series. Megumi Hayashibara, the voice actress for Musashi, took maternity leave from AG085 to AG092. During this time, Musashi was voiced by Akiko Hiramatsu.