The Roar (The Roar #1)2009

In an overpopulated world where all signs of nature have been obliterated and a wall has been erected to keep out plague-ridden animals, twelve-year-old Mika refuses to believe that his twin sister Ellie was killed after being abducted, and continues to search for her in spite of the dangers he faces in doing so. Meanwhile, when Ellie comes to suspect the truth behind her captivity, she's ready to risk exposure to the elements and answer the call of the wild.