By Dennis Thompson HealthDay Reporter Latest Infectious Disease News

WEDNESDAY, April 8, 2015 (HealthDay News) -- An active ingredient in some psoriasis and multiple sclerosis medications has been linked to two cases of a rare and sometimes lethal brain infection.

The ingredient, dimethyl fumarate, appears to have contributed to the deaths of two European women. The women contracted progressive multifocal leukoencephalopathy, or PML, according to two letters published in the April 9 issue of the New England Journal of Medicine.

One case involved a 54-year-old woman with multiple sclerosis. She died in October 2014 from complications related to PML and pneumonia, following 4.5 years of treatment with a time-delayed form of dimethyl fumarate carrying the brand name Tecfidera, researchers reported.

The second case was a 64-year-old woman with psoriasis. She died in August 2014 from PML after being treated with a delayed-release dimethyl fumarate compound with the brand name Psorinovo for two years, according to the researchers.

These cases follow reports linking other drugs containing dimethyl fumarate with PML, including Tysabri and Fumaderm, the researchers said.

However, the latest case reports do not prove that dimethyl fumerate caused the PML infections.

And outside experts said cases of PML occur rarely enough that dimethyl fumarate should remain on the market, as an option for people with MS or psoriasis.

"It's something to be concerned about and something to pay attention to. But from what we know right now, the occurrence of PML appears to be pretty low for Tecfidera," said Bruce Bebo, executive vice president of research for the National Multiple Sclerosis Society.

PML is caused by the JC virus, which normally lies dormant in most people's bodies and causes no harm. But if a person's immune system becomes compromised, the JC virus can flare up and attack the white matter of the brain.

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Ingredient in MS, Psoriasis Drugs Linked to Two Deadly Brain Infections