A new peer-reviewed scientific study has found no evidence of a mystery brain disease in theCanadianprovince of New Brunswick, suggesting instead a troubling combination of “misdiagnosis and misinformation”.
The research comes as the Maritime province prepares its own assessment of more than 220 suspected cases, in the hope of giving families some answers toa medical mystery that has gripped the region for years.
The report published on Wednesday in the Journal of the American Medical Association, known as Jama, is an independent reassessment of 25 cases diagnosed by Moncton-based neurologist Alier Marrero from within the broader sample of 222 patients.
The authors, affiliated with the University of Toronto, New Brunswick’s Horizon health network and other Canadian institutions, found that those within the cohort were misdiagnosed and had common neurodegenerative diseases, such as Alzheimer’s and Parkinson’s, functional neurological disorder, traumatic brain injury and metastatic cancer.
The findings, however, are unlikely to tamp down speculation from families that the report once again ignores their suspicions of environmental contamination in the region.
Health officials in New Brunswick first warned in 2021 that more than 40 residents were suffering from a possible unknown neurological syndrome, with symptoms similar to those of the degenerative brain disorder Creutzfeldt-Jakob disease.
Those cases were referred to Marrero after the initial physicians were initially stumped by the symptoms displayed by their parents. A year later, however, an independent oversight committee created by the provincedetermined that the group of patients had most likely been misdiagnosedand were suffering from known illnesses such as cancer and dementia.
A final report from the committee, which concluded there was no “cluster” of people suffering from an unknown brain syndrome, signalled the end of the province’s investigation.
But earlier that year, the Guardian reported that a top federal scientistworriedthere was “something real going on” in New Brunswick. Anothersaid the investigation“was shut down” and that caseloads were higher than officially acknowledged. “I don’t think it is helpful to suggest or point to who or why – suffice to say that we were prepared to marshal both financial and human scientific resources to tackle the mystery, but they were declined,” the scientist wrote.
According to a February 2025 letter Marrero wrote to federal and provincial officials, seen by the Guardian, the number of patients suffering from unexplained neurological symptoms has since jumped to 507 across New Brunswick, Nova Scotia, Prince Edward Island Newfoundland, Ontario, Quebec and Alberta. Several under the age of 45.
The new study found that “well-known conditions were identified in all 25 cases, including common neurodegenerative diseases, functional neurological disorder, traumatic brain injury and metastatic cancer,” adding that a study of 11 autopsy cases led to the conclustion that “a new disease was extremely unlikely, with a probability less than .001”.
Patient advocacy groups have rejected the idea of a new “mystery illness”, instead arguing that the cases are linked by environmental contaminants. But the researchers say they do not believe the patients were made ill by exposure to something in the environment, such as herbicides or heavy metals like mercury.
They determined that “misdiagnosis and misinformation become inextricably entwined and amplify harm exponentially”, noting that more than half of the people eligible for the study declined to participate.
They also blamed “inaccurate clinical assessments and an over-reliance on ancillary testing” for perpetuating the idea of a “mystery illness” and said they found “meaningful discrepancies” in case histories.
Marrero said in a statement he is “in profound disagreement with the study conclusions” and has “many questions regarding the methods and the content”, adding that he was “appalled” that an investigation with a “small number” of patients has been conducted without his knowledge.
Susan Holt, the province’s premier,pledged a fresh and “thorough” investigation after taking office last last year.
“I think we need to be doing everything we can to shed some light on this and find a way to stop what’s making people sick,” she said at the time.
New Brunswick’s chief medical officer of health Dr. Yves Léger, said in a statement the Jama study doesn’t change his office’s intention to complete its own investigation into the 222 cases, which Marrero has officially referred to the province.
Updates will be posted on the province’snew website, and a public report with recommendations is expected in the coming months.