Zika has become a disease to fear for pregnant women and women who may become pregnant—because babies who are infected with the Zika virus while in the womb can come into the world with devastating birth defects including the “tiny head” condition called microcephaly that can lead to severe developmental problems, seizures, blindness and more.

But there is new evidence that this mosquito-borne and sexually transmitted virus also damages adults—especially older adults. If so, it’s dangerous to all of us.

A PARALYZING DISEASE

Since Zika hit the headlines in the fall of 2015, there’s been a suspected association between the virus and the autoimmune disorder Guillain-Barré syndrome (GBS) in adults.

In GBS, the body attacks its own nerves, especially in the legs and arms. GBS can lead to muscle weakness, temporary paralysis and, if untreated, death. There’s no cure, although treatment can minimize complications. Recovery can take years. The elderly are more prone to GBS than younger people, and GBS symptoms last longer in older people as well.

Now there’s stronger evidence that Zika really can cause GBS. A recent study published in The New England Journal of Medicine evaluated 164,237 cases of Zika infection and 1,474 cases of GBS in seven South American and Central American countries in one year, from April 2015 to April 2016.

Findings: During Zika outbreaks, GBS cases increased anywhere from 100% to nearly 900%.

Perspective: On average, the risk of developing GBS is still very small even if you become infected with Zika, says Nikos Vasilakis, PhD, assistant professor in the department of pathology and member of the Center for Biodefense and Emerging Infectious Diseases at the University of Texas Medical Branch. He’s an expert on mosquito-borne diseases. In the study (Dr. Vasilakis was not associated with the study), about one in 300 people infected with Zika developed GBS. The real-world risk is likely much less, however, according to Dr. Vasilakis. Here’s why: The study covered people who had hospital-confirmed cases of Zika, but most people with Zika never even get symptoms. GBS, on the other hand, is almost always reported because the symptoms are so severe. In the US, there are only about 3,000 to 6,000 cases of GBS a year from all causes. Says Dr. Vasilakis, “Guillain-Barré is a very rare disease.”

Guillain-Barré isn’t the only neurological disease that Zika can cause in adults. It is associated with acute myelitis, an autoimmune condition. It may also lead to peripheral neuropathy—damage to the nerves in the arms and legs, leading to tingling or numbness in the hands and feet. The likely cause is inflammation as a result of the Zika viral infection. But this effect has been shown only in a case study, so it’s not known how common it is.

ZIKA AND ADULT BRAINS

The new GBS findings confirm a link that’s been suspected. New animal research also opens up the possibility that Zika may affect adults in ways we haven’t suspected before. It focused on adult brains.

In the fetus, Zika attacks neural progenitor cells—stem cells that form more specialized brain cells as a baby develops. Damage to these cells interferes with normal development, leading to birth defects.

In healthy adults, these stem cells have already become fully formed neurons. But our brains hang onto some of these stem cells, it turns out. They cluster in the hippocampus—the part of the brain involved in memory and learning—helping to develop new neurons as we learn new things. Having these stem cells as we age may help prevent dementia and depression.

Researchers at The Rockefeller University in New York and the La Jolla Institute for Allergy and Immunology in California wanted to see whether Zika also attacked these progenitor cells in adult brains. Using biomarkers in adult mice infected with the Zika virus, they tracked where the virus went in the brain. Result: The virus clustered around the progenitor cells, especially those in the hippocampus, attacking them—so that there were fewer new neurons. “It was very clear that the virus wasn’t affecting the whole brain evenly, like people are seeing in the fetus,” said Joseph Gleeson, MD, one of the study authors and head of the Laboratory of Pediatric Brain Disease at The Rockefeller University in New York City. “These cells are special and somehow very susceptible to the infection.” By attacking these adult stem cells, the authors suggest, Zika may affect the brain’s “neuroplasticity”—its ability to change. Over time, that could lead to cognitive decline.

Perspective: It’s a preliminary study in mice. Although human studies often show similar results to mouse studies, we can’t draw conclusions about Zika in people, who may be able to mount an effective immune response that prevents the virus from attacking the brain. However, a warning flag has now been raised—Zika could be much worse for adults than we thought.

HOW TO AVOID IT

Zika is an unusual virus. It’s spread not just by mosquito bites but also through sexual transmission (as well as blood transfusions). So far there is no evidence that it can be transmitted by deep kissing. It’s horrifically scary for women who are or want to become pregnant and for their partners. There is risk for adults getting GBS…and there may be risk to the adult brain as well.

One reassuring point: A Zika epidemic in the continental US is unlikely. Indeed, even in and around Miami, where there has been some local transmission—not just from travelers coming back from Zika-infected countries—it’s now coming under control. In the Wynwood section of Miami, which recently had a Zika outbreak, for example, transmission of the virus has been halted. Says Dr. Vasilakis, “We have air conditioning, window screens and mosquito-prevention strategies such as aerial spraying. They are very effective.”

If you’re traveling to another country that has Zika, though, check the Centers for Disease Control and Prevention (CDC) website for the latest guidance. Some tips…

  • If you’re pregnant, avoid any Zika area. These include much of Central and South America, parts of Southeast Asia and, in the US, the island of Puerto Rico and certain areas around Miami.
  • If you may become pregnant, or are a man whose partner may become pregnant, avoid nonessential travel to these areas. Because Zika is sexually transmitted—and the virus can be found in a man’s semen for up to six months after initial symptoms—it is recommended that if a man has symptoms of Zika infection, he should use condoms for six months after the start of his symptoms. If he has been in a Zika-infected area, even if he doesn’t have symptoms, he should use condoms for six months after his travel. (Symptoms may include fever, rash, joint pain, conjunctivitis [red eyes], muscle pain and headache. Flu, in contrast, doesn’t include red eyes or a rash.)
  • If you do travel to a Zika-infected area, try hard to avoid mosquito bites. The Aedes mosquito, which carries Zika, is active night and day, inside houses as well as outside. Wear insect-repellent clothing, and spray yourself with a repellent containing DEET or other effective mosquito repellents.

If you have a chronic condition that may affect your susceptibility to viral infections—and especially if you are immunocompromised—you may want to consider staying away from Zika areas entirely. This isn’t official CDC advice, but the same Aedes mosquito that carries Zika also carries other potentially debilitating diseases, including chikungunya and yellow fever. At the least, if you are considering such travel, talk with your own doctor before planning a trip.

Zika and the adult brain is an emerging story. As scientists learn more, we’ll keep you informed.

For more on Zika and travel, see Bottom Line’s article, “The Zika Virus: Should I Change My Travel Plans?”