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Papal visits are a magnet for the Roman Catholic faithful. But public health officials are hoping that when Pope Francis visits Mexico later this week a specific segment of American Catholics who might normally flock south of the border will stay home: pregnant women.

Mexico is one of the many countries in Latin America and the Caribbean where the Zika virus is currently spreading. The virus is strongly suspected of being responsible for an increase in cases of microcephaly — an abnormally small head — among infants born to some women infected during pregnancy.


The messaging from the Centers for Disease Control and Prevention has been very clear: Pregnant women should do everything they can to avoid being infected with the virus. That means not traveling to affected countries if at all possible.

The CDC always issues advice to travelers in advance of mass gatherings like the World Cup, the Olympics, and even international Boy Scout jamborees. It just issued one for various Mardi Gras events, including Rio de Janeiro’s world-renowned carnival.

Mostly these alerts are focused on instructing people on how to be safe in crowds, reminding them their vaccinations should be up-to-date, and urging them to be careful about the food and water they consume. The alerts are known as Level 1 travel notices, which advise people to “practice usual precautions.”


But the CDC’s alert for Pope Francis’s visit to Mexico is different, acknowledged Dr. Martin Cetron, director of the division of global migration and quarantine. It’s a Level 2 alert, and it calls for “enhanced precautions.” In essence, while including the usual advice for most Americans, it is urging pregnant women to postpone travel to the country.

“I think the thing that makes it unique is that it’s occurring right at a time of intensified Zika transmission in the Americas,” Cetron told STAT.

“So in that setting we thought it would be really important to raise an additional level of awareness.”

Dario Lopez-Mills/AP
A woman sits on a sidewalk in front of a mural of Pope Francis during a drill for the route the pontiff will take in Mexico City. Dario Lopez-Mills/AP

The pope is scheduled to arrive in Mexico on Friday and depart next Wednesday. One of the stops on his visit will be Ciudad Juarez, just across the border from El Paso.

Cetron said that, given the imperfect state of Zika testing right now — some antibody tests confuse the Zika virus with dengue virus — it isn’t possible to know exactly where in Mexico the virus is spreading.

Areas along the Texas-Mexico border are known to have Aedes aegypti mosquitoes, the day-biters that spread Zika virus. These mosquitoes can also carry dengue virus, and there have been dengue outbreaks along the border in the past.

Where there has been dengue, there could be Zika, health officials warn.

Even if Zika hasn’t spread widely in northern Mexico, the pope’s visit could change that. People will travel from throughout the predominantly Catholic region — where the virus is spreading rapidly — to see the pope. If Ciudad Juarez doesn’t have Zika spread now, it very well may after Feb. 17, when the pope is scheduled to celebrate a Mass on at the city’s fairgrounds.

Are you at risk for contracting Zika virus? Your level of risk depends in part on your living conditions. Alex Hogan/STAT

“Any time you have a mass gathering and you have international people going to a place, there’s the risk of the interchange of infectious agents,” explained Dr. Aileen Marty, a mass gatherings medicine expert and a professor of infectious diseases at Florida International University in Miami.

In fact, many researchers believe that’s how the Zika virus arrived in Brazil in the first place — either in people attending the 2014 World Cup or a Va’a World Sprint Championship canoe race in August 2014.

Marty shares the CDC’s concern: “If you’re pregnant, please don’t go.”

Mass gatherings medicine is a specialty that looks at the health risks that can arise when throngs of people mass in one place at one time. It focuses both on the challenges of ensuring that people who attend an event — like the annual hajj pilgrimage to Mecca, Saudi Arabia, for instance — stay healthy. It also looks at the potential for these events to disseminate disease over great distances.

It happens. British Columbia experienced a measles outbreak in 2010 after Vancouver hosted the Winter Olympics. Genetic analysis of viruses from that outbreak suggested two people brought measles to Canada. One was likely from China; the other was infected with a strain that had previously been seen in Italy, India, and the United States.

The British Columbia measles outbreak only involved 82 cases; most people in Canada are protected against the measles virus. But the vast majority of people in the Americas have no immunity to the Zika virus.

The large crowds at stops on Pope Francis’s tour may well include people infected with the Zika virus.

“The US is most concerned with somebody bringing it home and it entering our mosquito population,” Marty said. “And they’re very concerned with pregnant women going there from the US and coming back and having babies with this problem.”

So far Pope Francis and the church he leads have been silent on the subject of the Zika virus and the calls from some countries for women to put off pregnancy for months and in some cases years.

Human rights advocates have questioned how workable that recommendation is in a region where the Catholic Church’s teachings on artificial contraception and abortion — the former is not allowed, the latter is a sin under all circumstances — are reflected in the laws of some nations.

Some religious scholars have acknowledged that the Zika outbreak will put pressure on the church to revisit its positions. Whether it does remains to be seen.