When we were finalizing our book for publication, the West African Ebola epidemic was emerging (we hadn’t picked Ebola as one of our case studies), and our publishers asked if we could include some information about it in the book. Knowing that epidemic threats keep coming — some such as SARS apparently receding into the background, and others such as HIV staying with us as part of the infectious disease landscape — we wrote:
At the time of writing it is difficult to predict the trajectory of the Ebola epidemic. However, by the time this book is in the hands of a reader, it seems virtually inevitable that yet another infectious disease will be emerging.
Now, as we guessed, something else has come along — the Zika virus. While we are biologists with a general interest in infectious diseases, and not Zika specialists (i.e., please don’t treat this blog post as medical advice!), we have been following the news with our usual fascination, and of course concern for those affected. As frequently happens, the initial reports seem to have been slightly exaggerated, but there is no question that Zika is a big deal, and some researchers are predicting that it will establish and persist in the US mainland. The spread of Zika has prompted the World Health Organisation to declare a global emergency, and Brazil has warned pregnant women to stay away from this summer’s Olympic games. One of the biological details that interested us most about Zika is its ability to transmit sexually as well as through mosquito vectors. (This was one of the major factors in the recent decision to move major-league baseball games out of Puerto Rico, where Zika virus is spreading — baseball players, many of whom are heterosexual males with partners of child-bearing age, were concerned about catching Zika and spreading it to their partners.)
We know of plenty of parasites that spread by more than one mode of transmission — HIV, notoriously, has spread through needle-sharing, sexual contact, and transfusions. Even the next-most-recent epidemic threat, Ebola, is known to transmit sexually as well as by more casual contact with infected fluids. It isn’t surprising that a virus could spread sexually — next to direct injection into the bloodstream (whether naturally through mosquito mouthparts or artificially through needles), intercourse can be one of the most direct modes of contact between organisms, offering few physiological barriers to pathogen transmission. However, we don’t know of any other vector-borne diseases that can also spread through sexual contact: when a US virologist transmitted Zika to his wife after contracting it in Africa in 2008, he stated that at that time “human sexual transmission of an arbovirus [i.e., a virus spread by arthropods such as mosquitoes] has not been documented.”
On the other hand, our ignorance of arboviruses’ ability to transmit sexually could just be another aspect of our overwhelming ignorance of infectious diseases. When disease is endemic (persistent in a population) or rapidly spreading via mosquito bites, it’s hard to tell whether it also spreads sexually, because anyone who has had sex with an infected partner has probably also been bitten by lots of infected mosquitoes. It’s only (as in the case of the American scientist mentioned above, and in recent sexually transmitted Ebola and Zika cases) when people are infected in epidemic areas, travel home to non-epidemic areas, and then infect their partners that we can be fairly sure that sexual transmission has occurred. Even then, we have to rule out the possibility that after they returned home, travelers were bitten by mosquitoes that became infected and went on to infect their partners.
Sexual transmission is making headlines in Western media (we mostly follow North American media, but note that the first documented sexual transmission during the current epidemic was to a French woman whose partner was infected in Brazil) because it is the most likely way that people who don’t live in or travel to tropical areas could be exposed to Zika. It’s also possible, although in our opinion very unlikely, that sexual transmission coupled with virus evolution could eventually create a local transmission pathway that could keep Zika endemic in non-tropical areas. We really have no idea at this point how much sexual transmission could add to the R0 of Zika (which has been estimated very crudely as between 3 and 6 in Colombia, and is certainly much less in cooler regions and places with more air conditioning and insect screens). The contribution of transmission via sexual intercourse will probably be very small — an infected person would typically have at most one sexual partner during the week or so they are infectious, and the probability of transmission per sexual event is unknown, but probably low. In principle, though, adding sexual to vector-borne transmission could make Zika harder to contain, prolonging the sputtering outbreaks that occur in diseases where R0 is close to 1, large enough for dozens of cases but not for a full-fledged epidemic. We need to know more about actual rates of sexual transmission to predict the actual magnitude of the risk. More generally we wonder: is sexual transmission of arboviruses indeed rare, or simply undetected? If it is rare, then why is Zika sexually transmissible when its close relatives, Dengue and Yellow Fever, are (apparently) not?
Featured Image credit: Mygg / Mosquito by Erik F. Brandsborg. CC-BY-SA 2.0 via Flickr.