No evidence of mystery brain disease in New Brunswick, says government report

A new report from New Brunswick’s top doctor has found no evidence of a mysterious brain disease that has allegedly been impacting hundreds of patients across the province.

Dr. Yves Léger’s report, released Friday, says most patients did not have elevated levels of herbicides or metals like arsenic in their systems that could have made them sick. It says the nine autopsy reports his office studied do not suggest the existence of a new neurological condition and that many patients suffered from other illnesses, such as Alzheimer’s disease or Lewy body dementia.

“I can’t imagine what it must be like to live with an illness, and especially not knowing what’s causing that illness,” Léger said during a news conference in Fredericton on Friday.

”You’ve been reported to us as having an undiagnosed illness. However, the findings from this, and previous work, is suggesting that it’s likely that you actually have an illness that can be diagnosed.”

Nearly 400 New Brunswick residents — mostly in the Acadian Peninsula and Moncton areas — have reported symptoms of what the province’s Health Department has previously called a “neurological syndrome of unknown cause.” The matter has baffled experts for years, made national and international headlines and prompted criticism from patients and advocates who say the health system has not been addressing their concerns.

Most of the 399 reported cases, including all of the 222 studied in Friday’s report, have been referred by a single neurologist, Dr. Alier Marrero, who could not immediately be reached for comment Friday. Beginning in 2020, the Moncton-based doctor suggested there were elevated levels of herbicides and metals in patients and theorized the substances may be playing a role in the mystery illness. He referred his findings to Léger’s office for help.

Symptoms included memory problems, balance issues, behavioural changes, muscle spasms and bursts of intense pain.

A 2022 Health Department report under the former Progressive Conservative government examined 48 cases, 46 of which were from Marrero’s office. It said there was no evidence of a cluster of unknown disease cases, and that most could be attributed to other known illnesses. A 2025 study published in the scientific journal JAMA came to similar conclusions.

During the 2024 provincial election, Premier Susan Holt’s Liberals campaigned on a pledge to conduct another investigation. The Liberals won a majority and the new investigation kicked off in March 2025 with the results published Friday.

Léger said certain limitations impacted the new report. His office looked only at specific data and test results, not the patients’ full medical files. Some testing methods were less useful than others for detecting certain substances — for example, in some cases a urine test was used when a blood sample would have been more appropriate. While it has nine autopsy reports for deceased patients, the office doesn’t know if any living patients have been diagnosed with another condition such as Alzheimer’s disease or Lewy body dementia.

Followup testing when contaminants were found were often not completed. For example, 20 of 201 patients tested for the herbicide Glyphosate had levels that were higher than normal. However, only three of those 20 patients were retested, with all of the second tests coming back below acceptable levels. One recommendation from the report is for better followup testing.

“We were faced with a number of limitations that did affect our ability to provide definitive answers to some questions,” said Léger. “However, we are still able to conclude that our investigation does not find evidence of a widespread issue among this patient group. We don’t think that exposure to these substances is a contributing factor to most patients’ illness.”

Kat Lanteigne is the former executive director of BloodWatch, a non-profit focused on a safe blood supply, and has been advocating for affected patients since 2021. She says patients have been pushing for an independent scientific investigation by a federal panel of neurologists for years and that still hasn’t happened. She said such a review would include a full look at all patient files.

“It is the responsibility of (Minister of Health John Doran) and Premier Holt, who promised these patients an investigation,” Lanteigne said in an interview Friday.

“In fact, Premier Holt campaigned on the backs of their suffering. And to this date, they have not ensured that process has happened. At no fault to any of those patients.”

The Public Health Agency of Canada will conduct a separate review of all the data collected as part of Léger’s investigation. Lanteigne said that’s still not acceptable and falls short of a full independent investigation.

The report makes a series of recommendations, including establishing a formal reassessment process. The report notes 89 of the 222 patients it studied only consulted with one neurologist, Dr. Marrero, and recommends two specialist sign-offs before suspected “neurological syndromes of unknown cause” are referred to Léger’s office in the future.

Léger referred questions of potential disciplinary measures against Marrero to the Vitalité Health Network and the College of Physicians and Surgeons of New Brunswick. Both organizations said they were reviewing the report to determine next steps.

This report by The Canadian Press was first published Jan. 23, 2026.

Devin Stevens, The Canadian Press