Can Zika virus be sexually transmitted?

Vincent Racaniell says in Virology blog "Perhaps in very rare cases, but the main mode of transmission is certainly via mosquitoes. If you just read the news headlines, which many people do, you will think that Zika virus spreads like HIV. But it does not."

The first hint of sexual transmission of Zika virus came from the story of two American scientists working in Senegal in 2008, where they were sampling mosquitoes. Between 6-9 days after returning to their homes in Colorado, they developed a variety of symptoms of infection including fatigue, headache, chills, arthralgia, and a maculopapular rash. The wife of one patient had not traveled to Africa, yet she developed similar symptoms three days after her husband. Analysis of paired acute and convalescent sera from all three patients revealed antibodies against Zika virus. The authors of the study do not conclude that transmission from husband to wife was via sexual activity – they suggest it as a possiblity.

Just as we are not sure that Zika virus causes microencephaly, we are not sure if it can be sexually transmitted.

What would be needed to prove that Zika virus is sexually transmissible?

Polymerase chain reaction (PCR) is used to diagnose many viral diseases. This assay detects small fragments of viral nucleic acid and can be very specific. However as we are trying to establish for the first time that Zika virus can be transmitted sexually, more than PCR must be done – infectious virus should be recovered from the donor and recipient. A positive PCR result does not mean that infectious virus is present in the sample, only fragments of the genome, which of course would not be infectious. It is important to correlate the presence of infectious virus with sexual transmission.

Not only should infectious virus be recovered from both donor and recipient, but the viral genome sequences should be nearly identical, providing strong evidence for sexual transmission. If the viral genome sequences were substantially different, this result could imply that the infection was acquired from someone else.

Looking for anti-viral antibodies in serum is a good way to confirm virus infection when virus is no longer present. However it is not as specific as PCR or virus isolation, and does not provide information about the genome of the donor and recipient virus.

Sexual transmission of Ebolavirus still remains speculative. There are several suspected cases, and many examples of PCR positive semen samples from men who have recovered from the disease. It’s not easy to prove that a virus can be transmitted sexually, especially when it is a rare event.

Just as we are not sure that Zika virus causes microencephaly, we are not sure if it can be sexually transmitted.