The American diplomats suffered a "variety of physical symptoms" - some so serious that they necessitated a return to the USA, according to State Department spokeswoman Heather Nauert.
The US says two Cuban diplomats have been expelled in response to "incidents" in Cuba, but the details remain mysterious.
It said the decision to expel two Cuban diplomats from Washington had been "unjustified and unsubstantiated".
It was suggested that the hearing loss could have been caused by some kind of sonic device placed outside the diplomats' homes, emitting inaudible sound waves that can cause deafness.
Global Affairs Canada spokeswoman Brianne Maxwell said Thursday that agency officials "are aware of unusual symptoms affecting Canadian and USA diplomatic personnel and their families in Havana".
The U.S. first learned of the medical issues towards the end of 2016, more than a year after the U.S. reopened its embassy in Havana. "We're taking this incident very seriously, and it's under investigation right now". "We asked two officials who were accredited at the Embassy of Cuba in the United States to depart the United States".
"Some U.S. government personnel who were working at our embassy in Havana, Cuba, on official duty. reported some incidents which have caused a variety of physical symptoms", Nauert said.
The Associated Press (AP) reported on Wednesday that the American diplomats suffered severe hearing loss attributed to a covert sonic device.
She added: "We had to bring some Americans home or some Americans chose to come home as a result of that". The Cuban government said it launched an investigation when it was made aware of the incident in February. I'm not going to be able to give you a ton of information about this today, but I'll tell you what we do have that we can provide so far.
USA officials told The Associated Press that about five diplomats, several with spouses, had been affected and that no children had been involved.
At this point, all that is for certain is that an "incident" occurred that has gone unreported for months and suddenly diplomatic relations with Cuba are on the rocks.
Relations between the United States and Cuba were unfrozen in 2015 by USA then-President Barack Obama and his Cuban counterpart, Raul Castro. All of facets of the Cuban embargo that had been loosened by Barack Obama, including a ban on tourism, would continue to be enforced, he said. She added a reminder that "the Cuban government has a responsibility and obligation to protect our diplomats".