All these sentences come from MACMILLAN, LANGMAN AND OXFORD:
1. He sent word by an army messenger that they had arrived in Jakarta.
2. Word of the incident spread quickly.
3. Word/ Rumour/ Legend has it that her husband is not the father of the child.
4. The word is that the two companies are planning a merger.
5. spread/ pass the word: Health officials are encouraging people to spread the word about the benefits of exercise.
6. send/ bring word: The mayor sent word he'd be late.
i'm not sure if there is a hard and fast rule. The meaning is nearly the same so it is more about using in context.