The Canadian government has confirmed that one of its embassy workers in Cuba has been affected by a health ailment with “unusual symptoms”.

This is the 13th case since 2017 of a mystery illness affecting Canadian diplomats and their dependents in Cuba, Global Affairs Canada said in a statement on Wednesday. The Canadian government has been unable to identify the cause of the illness so far.

A delegation of senior Canadian officials will visit Cuba next week. The government, meanwhile, said staff posted in the island nation can return home. In April, Ottawa had ordered the families of its diplomatic staff in Havana to return home. Afterwards, it designated the embassy in Cuba as an unaccompanied post, which meant that diplomats posted there would not be able to take their families along.

Last year, the United States had asked its diplomats to leave the Caribbean nation after its staff began to suffer unexplained health problems. At least 21 embassy workers and their families reported injuries, some as serious as a traumatic brain injury and permanent hearing loss. US investigators still do not know who is behind the attacks. A report in The New York Times in September said microwave strikes might be responsible.

US diplomats stationed in China also reported such attacks earlier this year, forcing Washington to evacuate several of its staff members in June.

The symptoms of the illnesses vary from person to person. In addition to hearing loss and concussions, some experienced nausea, headaches and ear-ringing, and some now struggle with concentration and word recall.