Between 2016 and 2018, about 40 Canadian and US diplomats and their families stationed in the Cuban capital mysteriously became sick. The so-called “Havana Syndrome” caused headaches, dizziness, loss of balance, nausea and difficulty concentrating and falling asleep.

The initial fear was that the Cubans were triggering the symptoms with some kind of sonic weapon.

Research conducted by Dr. Alon Friedman of Ben-Gurion University and colleagues at the Dalhousie University Brain Repair Center in Nova Scotia, Canada, points to a different culprit: overexposure to pesticides — in particular, fumigation to stop the spread of the Zika virus prominent around that time.

They found a correlation between the individuals most affected and the number of fumigations that were performed at their residences. Both the Cuban and Canadian authorities were fumigating more frequently than usual, according to records from the Canadian embassy.

By conducting brain scans on the affected individuals using MRI and magnetoencephalography (a functional neuroimaging technique for mapping brain activity), the multidisciplinary research team of 15, led by Friedman in Israel, concluded that exposure to cholinesterase inhibitors in organophosphorus insecticides was the probable cause. Cholinesterase is an enzyme required for proper nervous-system functioning.

The researchers detected different levels of damage in an area of the brain that is susceptible to neurotoxins and that can cause the kind of symptoms reported by the diplomats and their families.

The report says that the results “support the diagnosis of acquired brain injury.”

The report also compares the symptoms in Cuba with other neurotoxic attacks over history, including the Tokyo subway sarin gas attack in 1995 and the death of the half-brother of North Korean leader Kim Jong-un in Kuala Lumpur in 2017. Those cases resulted from high-dose exposure to neurotoxins, whereas the Cuban incidents were consistent with low-dose exposure.

The researchers will now focus their attention on examining the effects of cholinesterase (ChE) blockers in other commercial products. Certain classes of pesticides work by inhibiting ChE and tropical countries like Cuba regularly spray a wide variety of pesticides to kill insects that carry infectious diseases.

“The study validates the need for us to continue to learn more about the use of pesticides and other toxins,” Friedman said. “It is a global health issue that reminds us how much we still have to learn about the impact that toxins have on our health.”