A-borne disease spread by bites of infected species, Zika’s recent trajectory suggests it was only a matter of time before it got to the US.
This is because
- Its most prominent vector, , is widespread across the southern US ( ).
- Locally infected mosquitoes sustain local Zika infection (see below from ).
Little known for 60 years since its original isolation from a monkey in Zika forest in Uganda in 1947, < 20 human cases of Zika infection were confirmed over the next 60 years (). However, a quick succession of outbreaks across Polynesia, first in Yap island in 2007 then in French Polynesia in 2013, suggested Zika had somehow quietly spread beyond Africa sometime during those decades. 2015-2016 outbreaks in Brazil and Colombia signaled it had spread to the Americas (see below from and 4).
Keep in mind, Zika could keep coming to the US through other means as well. While it was primarily considered mosquito-borne, data from these successive large-scale outbreaks showed that it could also be transmitted sexually, vertically (from mother to fetus) and through blood (). This means that some degree of local infection could be sustained by returning infected travelers as well. After all, sexual transmission was suspected in what was perhaps the very first documented case of Zika infection in mainland US all the way back in 2008, when a US researcher, infected while working in the field in Senegal, apparently infected his wife but not his children after he returned to the US ( ).
2. Basu, Rupsa, and Ebenezer Tumban. “Zika Virus on a Spreading Spree: what we now know that was unknown in the 1950’s.” Virology journal 13.1 (2016): 165.
3. Musso, Didier, and Duane J. Gubler. “Zika virus.” Clinical microbiology reviews 29.3 (2016): 487-524.
4. Baud, David, et al. “An update on Zika virus infection.” The Lancet.
5. Foy, Brian D., et al. “Probable non–vector-borne transmission of Zika virus, Colorado, USA.” Emerging infectious diseases 17.5 (2011): 880.