SAN SALVADOR, El Salvador — Health officials say they’re trying to determine if an unusual jump in cases of a rare nerve condition sometimes severe enough to cause paralysis is related to the spread of the mosquito-borne Zika virus in at least two Latin American countries.

Fears the illness might be causing thousands of birth defects already has led authorities in Brazil, Colombia and El Salvador to take the drastic step of warning women against becoming pregnant. The U.S. Centers for Disease Control and Prevention on Friday expanded its warning for pregnant women thinking of visiting 22 destinations, most in Latin America and the Caribbean.

But concern also has been rising about a potential link to Guillain-Barre syndrome, a nerve disorder than can affect anyone. It causes muscle weakness, tingling in the arms and legs and sometimes temporary paralysis. Most people recover fully, but severe cases that affect muscles used to breathe can be life-threatening.

It’s thought to occur when someone’s immune system overreacts and attacks its own nervous system cells, often after various types of infections.

Researchers have been suspicious of the virus since French Polynesia noted a jump in cases of Guillain-Barre and microcephaly, in which a child is born with a small head, that accompanied a wave of Zika cases, though the populations were far smaller than in the recent outbreaks

The World Health Organization said Salvadoran authorities reported 46 cases of Guillain-Barre in just five weeks, from Dec. 1 to Jan. 6. The full-year average for the country is 169 cases.

It said that of 22 patients on which there was information, at least 12 patients had experienced a rash-fever illness in the 15 days before developing Guillain-Barre.

Brazilian officials too have said they’re investigating a link between Guillain-Barre and Zika.