This post has been updated.

U.S. and Brazilian health officials are investigating whether there may be an association between the mosquito-borne Zika virus that is spreading in the South American country and an increase in cases of a rare syndrome that causes paralysis.

The Centers for Disease Control and Prevention has sent a four-member team to Brazil to help the health ministry in its investigation of the growing number of cases of Guillain-Barré syndrome and a  potential connection to the Zika outbreak there.

Spokeswoman Christine Pearson said Thursday that the CDC team, dispatched at Brazil’s request, has been on the ground for less than a week. It includes a neuroepidemiologist and a medical epidemiologist.

Guillain-Barré syndrome occurs when individuals’ own immune system damages their nerve cells, causing muscle weakness and sometimes paralysis. Symptoms usually last for a few weeks. Most people recover fully from it, but some have long-term nerve damage. In rare cases, people have died, usually from difficulty breathing.

Brazilian officials have identified hundreds of cases of Guillain-Barré syndrome. A number of states have reported significant increases. Rio Grande do Norte state had reported 33 by August, compared to 23 in the whole of 2014, TV Globo said last month.

Brazil’s Ministry of Health said in December that it was investigating whether the syndrome could be caused by Zika. The ministry said the symptoms being seen start in the legs, before radiating through the trunk, arms and face. The condition is most dangerous when it involves respiratory muscles. “In this case, the syndrome can cause death if respiratory support measures are not adopted,” the ministry said.

Earlier this week, the Pan American Health Organization and World Health Organization announced an “increase of congenital anomalies, Guillain-Barré syndrome, and other neurological and autoimmune syndromes in areas where Zika virus is circulating.” PAHO warned member states to step up detection, prepare for a possible increase in neurological cases and reduce mosquito transmission.

Zika is carried by mosquitoes and typically causes only mild symptoms, but health officials globally have recently been alarmed because of a possible link between the virus and nearly 3,900 children born with suspected microcephaly in Brazil since October. The rare condition, marked by an abnormally small head, is associated with incomplete brain development.

Last week, CDC issued an advisory warning pregnant women against travel to Brazil or more than a dozen other countries and territories in South America, Central America and the Caribbean where local transmission of the virus has been confirmed.

The agency then followed up with guidelines for doctors who care for pregnant women, advising that they be tested for Zika infection if they show symptoms after visiting any of those areas. The guidelines note: “Guillain-Barré syndrome has been reported in patients following suspected Zika virus infection.”

In recent days, several countries have taken drastic measures to try to combat the Zika virus, with Brazil, Colombia, El Salvador and Jamaica all calling on women to consider delaying becoming pregnant.

In the United States, Florida, Illinois, New Jersey, Texas and Hawaii have confirmed Zika infections involving returning travelers who were likely bitten by mosquitoes while abroad. The CDC said there have been at least “a dozen or so” cases.

Ariana Eunjung Cha and Dom Phillips in Rio de Janeiro contributed to this report.

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