A mysterious illness causing fatigue, headaches and memory loss amongst dozens of Canadian and American diplomats in Cuba may be the result of intensive pest fumigation, not sonic “attacks,” a new Canadian health study suggests.

The study of Canadian diplomats in Havana and their families by researchers at Dalhousie University found their debilitating symptoms may be the result of exposure to neurotoxins; embassy records from the time show a “significant increase in fumigation in recent years” with weekly doses of pesticides in many diplomats’ residences.

Exposure to a toxin used to kill mosquitos following the Carribean’s Zika outbreak is a “plausible explanation” for brain injuries acquired by government staff, the report found.

U.S. and Canadian diplomats in Cuba began reporting mysterious neurological issues with concussion-like symptoms in the fall of 2016. Some victims of so-called“Havana Syndrome” reported memory loss, troubled sleep, and hearing strange buzzing noises, leading to unsubstantiated claims by U.S. officials that the illness was caused by an “attack” by Cuban intelligence officials.

“I do believe Cuba’s responsible. I do believe that,” U.S. president Donald Trump said of Havana Syndrome in 2017. “And it’s a very unusual attack, as you know.”

Canadian authorities have not made similar allegations.

Earlier this year, the Canadian government slashed its embassy staff and programming in Havana in half after 15 staff members and their families exhibited symptoms associated with the syndrome. Some services such as visa and passport services were restored this summer.

The Dalhousie study, first obtained by the French-language news program Enquete, was commissioned by Global Affairs Canada and co-authored by neuroscience professor Alon Friedman and psychiatry professor Cindy Calkin. It examined 28 participants, including a control group, individuals who had recently returned from Havana, and a post-mortem of a dog who was exposed while living in Cuba with his owners.

Symptoms, researchers found, were consistent with low levels of frequent exposure to a toxin used in pesticides that can also inhibit the enzymes needed for humans’ nervous system to work properly.

“While other causes cannot be ruled out, our findings point to an environmental risk with immediate implications for prevention, screening, and follow up,” the study says.

Both Canadian and Cuban authorities conducted fumigation around diplomatic residences due to Zika fears, Friedman told Enquete.

Some former Canadian diplomats launched a $28-million lawsuit against federal government earlier this year, claiming it failed to take adequate measures to protect them in Cuba. A Global Affairs Canada spokesman has said all affected employees are receiving appropriate medical attention and support.

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Havana Syndrome has also prompted brain injury research in the United States, although the Dalhousie study notes that Canadian and American diplomats did not report identical symptoms. Most Canadian diplomats did not report hearing unexplained sounds and developed symptoms more gradually over their time in Havana, the study says.

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“It will be important to review fumigation procedures, types of agents being used, and environmental levels of exposure,” the study concludes.

“Alternative protective methods for the prevention of mosquito-borne diseases should be considered.”