Zika from sex, the byway but not the highway

FlavivirusCan Zika virus be sexually transmitted? Perhaps in very rare cases, but the main mode of transmission is certainly via mosquitoes. That’s why I’ve shamelessly stolen a quote on this topic from Dr. William Schaffner of Vanderbilt University:

Mosquito transmission is the highway, whereas sexual transmission is the byway. Sexual transmission cannot account for this sudden and widespread transmission of this virus.

If you just read the news headlines, which many people do, you will think that Zika virus spreads like HIV. But it does not.

Let’s make a clear distinction between sexually transmitted viruses (like HIV – sex is the main mode of transmission, along with contaminated blood), versus sexually transmissible viruses. The latter includes viruses that now and then might be sexually transmitted under certain circumstances, but which normally are transmitted by another route. Zika virus is transmitted among humans by mosquitoes. If sexual transmission occurs, it is very, very rare, given the large number of Zika virus infections that have been documented.

Is Zika virus sexually transmissible?

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. Their data could not prove sexual transmission.

More recently infectious Zika virus was detected in semen of a French Polynesian male who had recovered from infection. The presence of virus in semen is compatible with sexual transmission, but the patient was not known to have transmitted infection to anyone.

The CDC has concluded that Zika virus was transmitted to an individual in Texas who had sex with a traveler returning from Venezuela. As of this writing I do not know exactly how the CDC came to this conclusion.

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.

9 thoughts on “Zika from sex, the byway but not the highway”

  1. I agree with Dr. Racaniello that CDC should not jump to the conclusion with limited evidence (that’s just unprofessional). People are tend to assume things once they saw some “relation” without substantial evidence. Like autism and vaccination.

  2. The worldwide spread of Zika from S Am. to Brazil and beyond involved air travel. Mosquitoes do not fly across oceans, and they certainly do not pinpoint islands in the middle of the Pacific. Any mosquito aboard an airplane full of travelers will almost certainly have had its bloodmeal by the end of a trip spanning oceans. It is highly likely therefore that sexual transmission has been the dominant mode of import into a country, after vector infection abroad. Infection rates in countries with Zika are very high, the disease is also mostly mild, and sexual transmission is a very likely occurrence given how widespread the virus is distributed throughout the body.

    See Foy et al., 2011; Musso et al., 2015.

    Foy, BD et al. 2011. Probable Non–Vector-borne Transmission of Zika Virus, Colorado, USA Emerg Infect Dis. 17(5): 880–882. doi: 10.3201/eid1705.101939

    Musso D et al., 2015. Potential sexual transmission of Zika virus. Emerg Infect Dis. 21(2):359-61. doi: 10.3201/eid2102.141363.

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  8. Hassan Uz Zaman Shamol

    Dear professor Racaniello, I’m a Microbiology Masters student from Bangladesh. Your blog and podcasts have been incredibly helpful to me throughout my undergrad years. Even more so this series on Zika viruses, seeing I have a seminar presentation on the topic in the coming week.

    I was wondering if you could explain this part in a bit more detail though?

    “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.”

    I’m having difficulty understanding how a positive PCR wouldn’t be decisive. Genome fragments might not be infectious, but how else would that have been found if not by infection (at least in the vast majority of cases)? Please comment.

    Thanks again,

  9. Biological Statistics

    It is a probabilistic statement, more like Bayesian. The statement is about having fragments of the virus in the test, but the fragments are non-infectious (non-lethal).

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