The child was practicing hiragana with zeal.
My heart was struck by how he practiced skipping enthusiastically every morning in front of the station.
Tofugu’s new employees were all keen to show their enthusiasm and worked even when they could have been sleeping.
When your heart is feverish and hot about something, it is obviously something you have a lot of enthusiasm about. Think about the things you've enthusiasm for. Then, imagine your heart beating and getting all hot from thinking about it. Why else would people get so upset when talking about religion and politics. It's their feverish heart! Their enthusiasm!
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Be careful of the ねっ portion (normally is ねつ).