Zika, sexual transmission, and birth defects evidence mounting

Fourteen suspected cases of sexually transmitted Zika virus – several of which involve pregnant women – are being investigated in the United States, the CDC said on Tuesday. The infection has been confirmed in two cases in which the only known risk factor was sexual intercourse with an infected male partner who had traveled recently to an area with active Zika virus transmission. No cases of women transmitting Zika to male sex partners have been reported. “We think mosquito-borne spread is the most common route of transmission, but we want to make people aware that sexual transmission is also a risk,” deputy incident manager for CDC's Zika response Jennifer McQuiston said.

The CDC is recommending that couples use a condom if the male partner has traveled to an area of active Zika transmission – especially if the female partner is pregnant – or to refrain from sex altogether. That poses a problem, because pregnancy alone should be enough of a deterrent for sex. Then again, the only reason to have sex with a pregnant woman is precisely the fact that you don’t have to wear a condom. “These recommendations might seem extreme to people, but the truth of the matter is we don't yet have good scientific data to say how long the virus may persist in semen,” McQuiston said. She added that a number of studies on this subject are in the planning stages, but in the meantime it’s better to err on the side of caution.

The risk of Zika sexual transmission entails the additional risk of giving birth to a child with microcephaly – a risk that has not been fully established either, nor is likely to be in the immediate future. For example, it took a good ten years before a link between rubella – a similar virus – and birth defects was proven beyond reasonable doubt. How long it takes to reach a conclusion “depends on how definitive you want to be,” director of the U.S. National Institute of Allergy and Infectious Disease Anthony Fauci said. “The absolute definitive proof will come from case-controlled studies, and those usually take months.” virus specialist at Australia's University of Queensland Ian MacKay agrees that confirming or denying a causal relationship is going to take “a long time.” “It's a huge amount of work, but a powerful study would be one lasting a year or more that enlists a cohort of mums-to-be months before becoming pregnant and intensively monitors them until the delivery of their baby,” he said.

In Brazil, where over 500 cases of microcephaly thought to be related to the Zika virus have been confirmed, at least three studies are being planned. In one of them, pregnant women who get the type of rash characteristic of Zika will be recruited and tested for the virus. “Then we follow them to establish what proportion will have a miscarriage or a stillbirth, and what proportion have a baby with microcephaly or other malformations,” professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine and lead researcher Laura Rodrigues said. “That will tell us how common microcephaly really is and how it is affected by the trimester in which the mother becomes infected.”   

In this case, as with the potential sexual transmission of Zika, it’s better to be safe than sorry. Sort of like when a head coach forbids his players to engage in sexual congress during a big tournament. Which is not how Brazil intends to protect athletes during the upcoming Olympic Games in Rio de Janeiro. “I want to reassure you that the government is working very closely with the international Olympic movement, with the local organizing committee, supported by the WHO, to make sure we have a very good work plan to target the mosquito, and to make sure that people who will come here either as visitors or athletes will get the maximum protection they need,” World Health Organization Director General Margaret Chan said on Tuesday. “The Zika virus is very tricky. We should expect to see more cases. We should expect this to be a long journey, but the government commitment led by President Rousseff is commendable. Based on what I have seen here, I can tell you: the mosquito is difficult, but it cannot beat Brazil.”

Related: Emergency vaccine for the Zika virus may be ready by year-end