During the past year, we have been learning a lot about the Zika virus.
Scientific and medical research has produced a constant stream of new information about what the virus looks like and which diseases it can cause in infants and adults. We have also learned more about Zika virus epidemiology — the various ways that the virus can spread. This figure is an illustrated summary of how this is happening in humans based on what we know so far.
Virologists refer to viruses like Zika as arboviruses — which is to say viruses that are transmitted by insects (arbo = arthropod-borne). Most arboviruses are carried by mosquitoes. This is the case with Zika, and it has been known for almost 50 years that the species Aedes aegypti transmits it to humans. The related Aedes albopictus species is similarly capable of passing on Zika. Other Aedes and Culex mosquito species are also known to carry Zika, but unlike their mosquito cousins, they appear to keep it to themselves.
Earlier this year, it was reported that the Zika virus can be active in the semen of infected men. Later, it became established that women can contract Zika not only from mosquitoes, but also from infected men. Zika is consequently now a member of the sexually transmitted virus club, along with other viruses like HIV, HPV and HSV. It is not known how long Zika may remain active in the semen, but the virus's genetic fingerprints have been detected there at 181 to 188 days after infection. To be cautious, the World Health Organization is currently recommending that anyone who has visited a place where Zika is being transmitted should abstain from unprotected sex during the following six months.
Men infected with Zika can also transmit it to other men. This was documented for the first time in January of this year, and follows the pattern of other sexually transmitted viruses that can be transmitted by men to either gender. The World Health Organization's 6-month recommendation applies equally in this scenario.
Zika also appears to be active in vaginal tract of infected women. In July of this year, the first clinical case was reported in which a woman passed Zika on to her male sexual partner. This may not be the most common path of infection, but it is possible nonetheless.
This mode of transmission is the most terrifying. It was initially suspected based on the uptick in cases of microcephaly that began occurring in Brazil in 2015. Early testing with infected women showed that the virus was present in spinal and brain tissue of the fetus as well as in the amniotic fluid. Evidence such as this persuaded the CDC to declare the cause (maternal Zika infection) and the effect (fetal microcephaly) to be linked, and this was supported by a recent case-controlled study in Brazil. Infection during any trimester of pregnancy is now known to be capable of causing developmental disorders.
The interesting thing about arboviruses is that they are capable of infecting and propagating themselves in two hosts that have very different physiologies. And so it is that Zika virus infection goes both ways between humans and mosquitoes. While there may not be much empathy for the mosquito in this scenario, it is still a cause of concern. This is because nothing migrates so prolifically as humans do; and infected humans can quickly and easily travel to new locations, where they can pass Zika on to local mosquito populations that didn't carry it before. This event spawns new cycles in new places, where the trajectory of transmission otherwise would have petered out. One such place where this scenario might be a factor is Miami Beach, Florida, where mosquitoes have now tested positive for the virus.
In August it was shown that like humans, infected mosquitoes can pass Zika on to their offspring. This was no great surprise, as this has been seen before with other mosquito-borne viruses. The study estimated that Aedes aegypti offspring have a 1 in 290 chance of getting Zika from their infected mosquito mothers, but this doesn't seem to happen at all in the Aedes albopictus mosquitoes. The implication of this is that Zika can persist at low levels in some mosquito populations in regions where humans or other primate hosts may be absent, or transient.
These are the general ways in which Zika is known to spread. And although only an estimated 19 percent of people who become infected with Zika may notice any symptoms, a person does not need to have those symptoms in order to transmit the virus to another person.
On very rare occasions, Zika may be transmitted person-to-person through other bodily fluids such as saliva, urine or tears. A new report from researchers at the University of Utah details how this might have been a factor in the curious case of an elderly man who appears to have acquired Zika while in Mexico, returned to his home in the Salt Lake Valley (where Zika mosquitoes are not present), and mysteriously transmitted it to a family member with whom he had been in contact. The elderly man had unusually high amounts of the virus in his blood, which may have made this mode of transmission possible, and unfortunately passed away.
This case illustrates that a lot of details are still unknown about the nature of Zika virus. But as more cases of Zika infection continue to emerge in the months to come, new information will certainly be gleaned about the virus and how it spreads.