The Zika Virus has become a public health fear due to its believed effects on pregnant women and their unborn children.

However, it is now linked to two adult brain disorders, and researchers are struggling to understand exactly how it causes dangerous autoimmune responses in some patients.

Zika is associated with acute disseminated encephalomyelitis, or ADEM, according to a Brazilian team of doctors led by Maria Lucia Brito, a neurologist at Restoration Hospital in Recife.

The autoimmune response attacks the brain and spinal cord, according to a statement released by the American Academy of Neurology.

“Though our study is small, it may provide evidence that in this case the virus has different effects on the brain than those identified in current studies,” said study author Maria Lucia Brito Ferreira, MD, with Restoration Hospital in Recife, Brazil. “Much more research will need to be done to explore whether there is a causal link between Zika and these brain problems.”

Zika has also been linked to another brain-affecting autoimmune syndrome, Guillain-Barre.

The doctors observed 151 cases of suspected Zika between December 2014 and 2015 in Brazil. Six of the cases became neurologically symptomatic – two of which were ADEM, and four of which were Guillain-Barre.

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“At present, it does not seem that ADEM cases are occurring at a similarly high incidence as the GBS cases, but these findings from Brazil suggest that clinicians should be vigilant for the possible occurrence of ADEM and other immune-mediated illnesses of the central nervous system,” said James Sejvar, a CDC doctor and member of the American Academy of Neurology. “Of course, the remaining question is ‘why’ – why does Zika virus appear to have this strong association with GBS and potentially other immune/inflammatory diseases of the nervous system?”

The Zika Virus is spread by two species of mosquito, Aedes aegypti and Aedes albopictus. Both insects are native to Central and South America – and the territory of both extends into the southern half of the continental U.S. (the latter extends as far north as Minnesota and Maine).

Zika, originally discovery in 1947, doesn’t have severe health repercussions for the vast majority of those who are infected. Some 80 percent of persons infected are completely asymptomatic, and fatalities are rare. But the link between the virus and microcephaly cases has not been explored.

The virus is believed to cause a fetal deformation called microcephaly, when infants are born with smaller-than-usual brains.

Brazil health officials have reported nearly 4,000 cases of microcephaly recently – including as much as two percent of all newborns in its hardest-hit state, according to Reuters.

The CDC and other regional health officials are warning people who may potentially be susceptible to Zika – especially pregnant women – to take full precautions as the warmer weather of spring and summer approaches.

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