The mysterious high-pitched buzzing sound associated with the so-called "sonic attacks" on nearly 40 U.S. and Canadian diplomats living in Cuba may have been caused by a neurotoxic agent used to kill mosquitoes, a new study commissioned by the Canadian government claims.

...There have been various theories floated ‐ everything from crickets to microwave weapons ‐ as to the cause of the headaches, dizziness and other concussion-like symptoms.

Global Affairs Canada commissioned a clinical study to get to the bottom of the case.

"The working hypothesis actually came only after we had most of the results," Alon Friedman, the study’s lead author, told Canada’s CBC News.

Canadian diplomats were affected by similar reactions to US counterparts ‐ though the study noted that the symptoms of the Canadians were more gradual than the "acute, directional... auditory stimulus" in some of the US cases.

The study notes that tests carried out on 28 participants ‐ seven of whom were tested both before and after being posted to Havana ‐ support a diagnosis of brain injury acquired by diplomats and their families while in Cuba.

The patterns of brain injury "all raise the hypothesis of recurrent, low-dose exposure to neurotoxins", the report said.

Specifically, the results were "highly suggestive" of something called cholinesterase inhibitor intoxication.

Cholinesterase is an important enzyme in the human nervous system, and blocking it through an inhibitor can lead to death. The chemical weapon, Sarin, is an example of a potent cholinesterase inhibitor, as is VX, which was used in the killing of Kim Jong-nam, the half-brother of North Korea’s leader.

But the low, consistent doses the researchers believe were delivered are consistent with exposure to commercial pesticides, the study’s authors said.

Some pesticides work by inhibiting cholinesterase, the report said, and during the 2016-2018 period when diplomats became ill normal fumigation in Cuba was stepped up due to the Zika epidemic in the Caribbean.

The report said the diplomats’ illnesses coincided with increased fumigation in and around residences where they lived. One of the authors of the study, the professor Alon Friedman, clarified in an email to Reuters that both Canadian and Cuban authorities were fumigating.