Through sexual contact

• Zika virus can be spread by a man to his sex partners.
• In known cases of sexual transmission, the men developed Zika virus symptoms. From these cases, we know the virus can be spread when the man has symptoms, before symptoms start and after symptoms resolve.
• In one case, the virus was spread a few days before symptoms developed.
• The virus is present in semen longer than in blood.

 

Now, an additional complication has arisen, where scientists have discovered a case where a woman was responsible for spreading the Zika virus to a man, in the United States. Many researchers now acknowledge that mosquito infection of the Zika virus, while the major vector for transmission, is not the only one. And it is concerning that sex, not generally acknowledged to spread Zika through intimate contact, is now recognized as yet another driver of the condition.

Women in Latin America, according to two reports, are likelier to be infected than are men, even though both genders are equally exposed to the presence of mosquitoes, and thus equally vulnerable to be infected with Zika by mosquito bites. The differences in the genders seem to appear and become evident at the age when sexual activity is initiated by natural processes, fading as women become older.

It is known that the Zika virus persists in semen for months which explains why it is that women attempting to conceive are given clear warning to avoid unprotected sex with men from areas where the virus is prevalent. Argentina, Canada, Chile, France, Germany, Italy, New Zealand, Peru, Portugal and the continental U.S. have all reported infections that were almost certainly passed through sex.

A study released in May by Brazilian and European biostatisticians found that in Rio de Janeiro with its 6.4 million population, "a massive increase of Zika in women compared to men existed". The study authors found that women generally visit doctors more frequently than do men, including pregnant women; to be tested for Zika. Even so, women remained 60 percent likelier than men to be infected.

And it was sexual transmission, according to Flavio C. Coelho, a Vargas Foundation biostatistician, and the lead author of the study, that "was the most probable cause". Donald A. Berry, a biostatistician at the University of Texas M.D. Anderson Cancer Center waved off the results of the study, with the admonition that confusion over dengue with similar symptoms is so common that these variables alone could have accounted for the infections differential between men and women.

Researchers at the Colombian health ministry and the U.S. Centers for Disease Control and Prevention studied birth defects linked to the virus, finding age and gender disparities. Young boys and girls in Colombia had been infected with the Zika virus at approximately similar rates. After age 15, once sexual activity began, female rates of infection rose steeply. Women in Colombia by age 25 to 29 proved three times likelier as opposed to men of the same age to be diagnosed with Zika.

But as the women became older the margin began to taper off, until after age 64 the infection rates stabilized, and were comparable in numbers. According to Margaret A. Honein of the C.D.C., women 45 to 64 years of age in Colombia were nearly twice as likely as men of that age group to be infected. And no evidence exists that sexual transmission was an issue in the first Zika outbreaks that took place on Yap Island in Micronesia and in French Polynesia in 2007 and 2013 respectively.