New Brunswick has no mystery neurological disease, scientific study concludes

A new scientific study has found no evidence of an undiagnosed mystery brain disease in New Brunswick, says a report published Wednesday in the Journal of the American Medical Association, known as JAMA.

Instead, an independent reassessment of 25 of 222 patients diagnosed by Moncton neurologist Alier Marrero as having a "neurological syndrome of unknown cause" concluded that all of the cases were attributable to well-known conditions.

These include common neurodegenerative diseases, such as Alzheimer's and Parkinson's, functional neurological disorder, traumatic brain injury, and metastatic cancer, says the report.

The researchers also do not believe exposure to something in the environment, such as the herbicide glyphosate or heavy metals, made the patients ill.

"The broad range of very different, well-recognized diseases diagnosed is also strong evidence against a single environmental toxin causing the patients' symptoms," wrote the 13 co-authors. Six neurologists who conducted second, independent clinical evaluations at hospitals in New Brunswick and Ontario are among them.

The researchers contend that what they found to be misdiagnoses were mainly due to "inaccurate clinical assessments" and an "overreliance on or misinterpretation" of supplementary testing.

They said any patient diagnosed with a neurological syndrome of unknown cause should get a second opinion, which many of Marrero's patients have refused to do.

"Education, reassurance, and mental health support should also be prioritized for patients and families who have been profoundly impacted by claims that a potentially fatal mystery disease continues to affect them."

Marrero could not immediately be reached for comment.

The report comes after the New Brunswick government launched the "data analysis phase" of its own investigation into undiagnosed neurological symptoms Marrero says are now affecting hundreds of people in seven provinces and whether environmental substances are a factor.

A public report with recommendations is expected this summer.

An epidemiological study in the fall of 2021 found no specific behaviours, foods or environmental exposures that linked the patients together and could pose a risk.

In addition, an oversight committee made up of six neurologists in February 2022 found "potential alternative diagnoses" for 41 of an original cluster of 48 patients, including Alzheimer's disease, Lewy body dementia, post-concussion syndrome and cancer.

"No such syndrome exists," Dr. Jennifer Russell, the then-chief medical officer of health, said at the time.

The authors of the JAMA report are the latest to weigh in on the mystery illness controversy, which dates back to 2019, they say, when "speculation of a purported unknown neurological syndrome began to circulate in New Brunswick."

In early 2021, the public learned that Public Health was monitoring more than 40 New Brunswick patients with symptoms similar to those of Creutzfeldt-Jakob disease, a rare and fatal brain disease. Their symptoms ranged from painful muscle spasms and hallucinations to memory loss and behavioural changes.

The number of patients suffering from unexplained neurological symptoms has since jumped to 507 across New Brunswick, Nova Scotia, P.E.I., Newfoundland, Ontario, Quebec and Alberta, according to a letter Marrero wrote to federal and provincial officials in February, obtained by CBC News.

Fifty people have died.

The JAMA report notes the number of cases formally reported to Public Health New Brunswick is 222, referring to the enhanced surveillance forms completed by Marrero.

The authors focused on 25 of those patients — 14 of them alive (eight women and six men aged 20 to 55) and 11 who have died (five women and six men, aged 56 to 82).

The 14 living patients received a second, independent clinical evaluation by one of four movement disorder neurologists or one of two behavioural neurologists, as well as additional testing if needed, while two neuropathologists reviewed the autopsy cases.

The researchers also gathered data from late 2023 to early 2025 from the patients' health records, including physician reports, notes from allied health care professionals, test results, and demographic information.

Many of the 14 second, independent clinical evaluations, found "meaningful discrepancies" between the initial documented history and findings and the second assessment's findings, the report says.

For example, 10 patients were originally documented to have a rapidly progressive dementia or neurological disorder, but on re-evaluation, "the symptom onset was found to be much earlier than originally documented."

Three patients were also reported to have visual or tactile hallucinations, which were not confirmed on the second evaluation.

In addition, 11 of the 14 patients were originally documented to have abnormal cognitive testing, but upon re-evaluation, nine of 11 "either had no subjective cognitive symptoms or had cognitive testing that was not compatible with dementia."

Meanwhile, EEGs, the tests that measure electrical activity in the brain, were said to show abnormal slowing for all 21 patients who had them.

"Overinterpretation of EEG findings is not uncommon and frequently leads to misdiagnoses," according to the study reported in JAMA, which cites a lack of standards or mandatory training as reasons.

Despite the small sample size, the researchers said "practical knowledge and clinical reasoning" suggest the probability of no new disease is close to 100 per cent.

The researchers blame a number of factors for fuelling "unfounded" and "persistent public concern" about a potentially fatal mystery disease, despite the earlier provincial investigation rejecting this possibility.

These include misinformation that has "proliferated" in both traditional and social media, and decreased public trust in health institutions since the COVID-19 pandemic.

"In this way, misdiagnosis and misinformation become inextricably entwined and amplify patient harm exponentially."

The researchers said only 14 patients had accepted independent re-evaluation by another neurologist, while 52 refused, choosing instead to remain with Marrero — "the [one] neurologist who originally made and continues to promote the diagnosis of a mystery disease."

Because of the complex nature of some neurological disorders, patients would have benefited from a second, independent evaluation, the JAMA article says.

"We can only speculate that the low uptake for reevaluation stems from a variety of factors," the researchers say.

At the patient level, for instance, it could be a lack of awareness or concerns about the time and effort needed to get a second diagnosis, they said. At the clinician level, it might be professional validation, and at the system level, it might be a matter of access and lack of institutional oversight.

"Thoughtful analysis of these issues is critical for developing proactive and transparent public health communication strategies to rebuild trust in health care systems in a post–COVID-19 era," the researchers wrote.