— -- Researchers have for the first time determined that the Zika virus can be sexually-transmitted between men, according to officials from the U.S. Centers for Disease Control and Prevention (CDC).
CDC officials said this mode of transmission "might contribute to more illness than was anticipated when the outbreak was first recognized."
The CDC published its study in the Morbidity and Mortality Weekly report.
"Such cases highlight the need for clinicians to remain vigilant for and continue reporting any suspected cases of Zika virus infection to their state or local health departments," according to the report. "This includes suspected infections in symptomatic persons without travel history, but who report unprotected sexual contact with a person who has traveled to an area with active Zika virus transmission."
The transmission occurred when one man returned to Dallas, Texas, after a weeklong trip in Venezuela. Two days after arriving back in the U.S., the man developed classic symptoms of Zika virus including rash, fever and conjunctivitis. The man's partner of 10 years developed symptoms five days later, including fever, fatigue and headache.
Medical officials confirmed both men had the Zika virus through blood tests. They determined that sexual contact was the most likely cause for the second man's infection since he had not been in a country where the virus was being transmitted from mosquitoes to people.
Symptoms for both men cleared up in approximately a week, according to the report.
Earlier this week federal health officials declared there was enough evidence to conclude that Zika causes the birth defect microcephaly, which is characterized by an underdeveloped head and brain. Researchers and health officials said there is still much they do not know about the risks of the virus.