Updated 21/05/2021: 48 cases of the unknown neurological syndrome have now been reported, according to the New Brunswick Office of the Chief Medical Officer of Health. There have also been 6 deaths associated with the syndrome. 

Over 40 people have fallen sick with mysterious brain disease in the Canadian province of New Brunswick. Speaking at a press conference on March 18, public health officials in New Brunswick explained there’s a cluster of 43 cases — 35 confirmed cases and 8 suspect cases — of a progressive neurological condition of unknown origin. 

Dr Jennifer Russell, New Brunswick’s chief medical officer of health, notes that it is "most likely a new disease.”

"We have a lot of work ahead of us to determine the cause," Dr Russell added.

The disease is affecting a range of age groups, even affecting some younger people, and appears to be concentrated in the Acadian Peninsula in the northeastern corner of New Brunswick as well as the Moncton region in the southeast. No cases have been found elsewhere in Canada or beyond.

The first case emerged in 2015, but numbers appear to have been on the rise in recent years. As reported by CBC, Green Party Leader David Coon criticized the public health agency for their lack of transparency over the disease. He says that case numbers shot up in 2019 — when 11 cases were reported — and again in 2020 — when a further 24 cases were reported.

"So why are New Brunswickers only hearing about this cluster now?" he asked.

People afflicted with disease display a range of neurological symptoms, such as changes in behavior, sleep disturbances, unexplained pain, hallucinations, coordination problems, and severe muscle pains.

As Dr Russell noted, the symptoms bear some close similarities to Creutzfeldt-Jakob disease (CJD), a rare and fatal brain disease caused by prions, misfolded proteins that act as infectious agents. Some prion diseases can be picked up by consuming the brain tissue of an infected individual or animal. There are also a handful of unusual case reports of people contracting the illness after eating infected tissue, such as squirrel brains. 

However, CJD and all known prion diseases have been ruled out by early lab tests. This potentially means it's either a new variant of prion disease or perhaps a new disease altogether.

Environment toxins are also being investigated, according to CBC. One avenue for inquiry is B-methylamino-L-alanine (BMAA), a neurotoxin produced by bacteria linked to seafood. Another possibility is domoic acid, a neurotoxin produced by algae that accumulate in shellfish, sardines, and anchovies.

"We have not yet been able to come up with a causative agent, except that everything that we have analyzed so far suggests this is an environmental exposure of some kind that is acquired through food, water, air, professional activities, or leisure activities," Alier Marrero, a neurologist from the Dumont University Hospital Centre in Moncton whose investigating the case, told Medscape Medical News.

"There's also seems to be some geographic clustering, which suggests environmental exposure as well," Marrero added. "But since we don't have a picture of the whole province yet, it is slightly premature to talk about that."