Diagnostic tests cannot easily discern Zika from related infections.
New Zika research released on Friday has found that the virus may spread sexually from a man to a woman even if the man had no symptoms of Zika infection.
The finding came from a report in Maryland where a man who was infected with Zika in the Dominican Republic but had no symptoms infected his female partner who had not traveled to a place where Zika is being transmitted.
The study, published in the U.S. Centers for Disease Control and Prevention's weekly report on death and disease, suggests that sexual transmission of Zika is no less likely in asymptomatic individuals than in others with symptoms.
Current recommendations for preventing sexual transmission of Zika in returning travelers now differ depending on whether the returning traveler is symptomatic and on whether the couple is planning to become pregnant, but that may need to be changed.
Separately, health officials in Puerto Rico have reported as many as 10 people who developed the paralyzing neurological disorder known as Guillain-Barre syndrome as a result of Zika infections.
The latest studies add to the evolving picture of the impacts of Zika, a virus previously considered to be mild but which has recently been shown to cause the serious birth defect known as microcephaly, as well as neurological illness in adults.
In Puerto Rico, where Zika arrived in December 2015, health officials have been systematically tracking cases of Guillain-Barre syndrome following reports in other countries showing an increase in cases related to Zika.
Guillain-Barre causes gradual weakness in the legs, arms and upper body, and in some cases, temporary paralysis.
Overall, the Guillain-Barre surveillance system identified 56 cases of the syndrome in people infected from Jan. 1 to July 31, 2016, officials from the Puerto Rican health department and the U.S. Centers for Disease Control and Prevention reported on Friday in the CDC's weekly report on death and disease.
Guillain-Barre is an autoimmune disorder in which the body attacks itself in the aftermath of an infection, typically occurring in the days following an illness.
Of the cases in Puerto Rico, 34 patients had evidence of a flavivirus infection, such as Zika, dengue or Chikungunya, and 10 had confirmed Zika virus infections. Diagnostic tests cannot easily discern Zika from related infections, but health officials suspect nearly all of the flavivirus infections seen were related to Zika because that is the predominant flavivirus currently circulating in Puerto Rico.
All 34 patients required intensive care, and 12 required a breathing tube and mechanical ventilation. One patient died of septic shock after treatment for Guillain-Barre.
In addition to the Guillain-Barre cases, there were seven patients with evidence of infection from Zika or a related virus that developed neurological disorders other than Guillain-Barre. The findings follow reports in other areas that Zika can directly infect adult nerve cells.
In the sexual transmission case, current guidelines for preventing sexual transmission of Zika suggest that couples in which one person returns from an area with active transmission but did not develop symptoms of Zika should wait eight weeks before attempting to conceive a child.
But men diagnosed with Zika should wait at least six months before attempting to have a child, and women with a Zika diagnosis should wait at least eight weeks.
Health officials said more study is needed to determine the risk of sexual transmission of Zika from asymptomatic individuals. As more is learned about how long Zika lasts in semen in infected men, -"recommendations to prevent sexual transmission of Zika virus will be updated if needed,-" officials wrote.
