Darkest Hour (Heartland, #13)

Heartland is a horse farm nestled in the hills of Virginia, but it's much more than that. Heartland is like no other place - it's a place where the scars of the past can be healed, a place where frightened and abused horses learn to trust again.

An equine flue is infecting Heartland's horses. Despite their desperate efforts, Amy and Ty cannot keep it from spreading. They try to isolate the oldest and youngest horses - the ones with weaker immune systems - but more fall ill. To make matters worse, the late summer heat is intense, Ty and Ben are at odds, and Amy feels caught in the middle. Then, just as some horses start to get better and the weather is about to break, things spin out of control. When the clouds clear away, nothing seems the same.