RIO DE JANEIRO, Brazil (AP) — Two Latin American countries are investigating whether outbreaks of the mosquito-borne Zika virus are behind a rise in a rare and sometimes life-threatening nerve condition that can cause paralysis and leave victims on life-support.

The Zika virus has already been tentatively linked to a rash of microcephaly, a birth defect in which babies are born with unusually small heads. And while the mechanics of how the virus may affect infants remain murky, authorities in Brazil, Colombia and El Salvador are urging women to avoid the risk by postponing pregnancies.

The US Centers for Disease Control and Prevention advised pregnant women to reconsider travel to countries with Zika outbreaks, and on Friday it expanded the warning to 22 destinations, most in Latin America and the Caribbean.

The rise in cases of Guillain-Barre has also alarmed health officials region-wide. The nerve disorder causes muscle weakness that generally begins in the legs and spreads to the arms and face, and can cause numbness, trouble walking and even limb paralysis. While most people recover in weeks or months, in severe cases the muscles used for breathing weaken so much that patients require life-support.

Anyone of any age can get Guillain-Barre, although it is very rare. It is thought to be triggered by an infection — something as simple as food poisoning — and happens when the immune system attacks the body’s own nervous system.

Researchers have been wary of Zika since French Polynesia noted a jump Guillain-Barre and microcephaly cases in tandem with an outbreak of the dengue-like virus, though the populations were far smaller than in the recent outbreaks.

The World Health Organization said authorities in El Salvador reported 46 cases of Guillain-Barre in just five weeks, from December 1 to January 6. The full-year average for the country is 169 cases. Of 22 patients for whom there was information, at least 12 had experienced a rash-fever illness in the 15 days prior.

Brazilian officials are also probing a near-simultaneous rise in Guillain-Barre and Zika, which was first identified in the country last May. It is believed that Zika may have arrived through a tourist at the 2014 World Cup or an international canoeing event the same year.

Amid a Zika outbreak in the northeastern city of Salvador during last year’s rainy season, the Couto Maia Hospital saw an unprecedented rise in Guillain-Barre.

“Zika was really bad here from February to July and then all but disappeared in August. In May, June and July, we had 24 patients come in with Guillain-Barre, and none since August,” said Antonio Bandeira, an infectious disease specialist at the hospital. In a normal year, he sees just two or three such cases.

Most of the patients had also experienced Zika-like symptoms, which can include fever and red splotchy skin, Bandeira said.

Meanwhile the Hospital da Restauracao in Recife treated about six times the normal number of Guillain-Barre cases, neurologist Maria Lucia Ferreira said. Of the 94 patients treated there during the rainy season, 50 of them died.

However, the scope of the problem remains unclear, as Guillain-Barre has been so rare that Brazil’s Health Ministry does not track the exact number of cases.

Albert Ko, a professor of epidemiology at the Yale School of Public Health, said a link between Zika and Guillain-Barre is “plausible and highly likely.” But the difficulty of diagnosing Zika and the fact that Guillain-Barre can set in weeks later have made it tough to confirm the link.

“While many of us are convinced and believe it’s highly plausible that Zika virus caused this epidemic of Guillain-Barre, and can cause it anywhere the virus is being transmitted, we still lack really firm evidence to make that diagnosis,” said Ko, who has conducted research in north-eastern Brazil for two decades.

Zika originated in Africa and expanded to parts of Asia. When it was first detected in Brazil, health officials were not initially alarmed since the virus appeared to be like a less potent form of dengue. But then came the spike in microcephaly: Since October the country has recorded 3,893 suspected cases, compared with fewer than 150 for all of 2014.

Brazilian officials say they are convinced of a link. International health bodies say it is not yet scientifically established, but they are on alert. The CDC said Friday it issued its travel advisory “out of an abundance of caution.”

Earlier this week El Salvador recommended women avoid getting pregnant for the next two years, and some are taking that advice.

“We were very lucky. My son was born before this,” said Fatima Mejia, who took her 17-day-old infant to a clinic outside the Salvadoran capital for a check-up. “I’m not going to get pregnant until this passes. I’m not going to risk a child.”

In Colombia, Deputy Health Minister Fernando Ruiz said his country has recorded 13,531 suspected cases of Zika and that could hit a half-million this year. At least 560 involve pregnant women, though there have been no detected cases of microcephaly.

Ruiz said there have been 12 cases of people with Guillain-Barre who also experienced Zika-like symptoms.