Zika Transmission in the United States: our Conventional Wisdom may Not Apply


The Zika-affected Area in Miami, Florida [IMAGE: CNN.com]

Friday morning the Florida Department of Health reported that four cases of Zika virus infection were in all likelihood acquired locally by mosquitoes. Yesterday morning Governor Rick Scott added another ten to that total, and the Centers for Disease Control and Prevention issued a travel warning for the affected area. This is a first for the continental United States; all of our previous cases have been in patients who travelled to a country with active transmission or by sexual contact with someone who had. We have known for some time that local transmission of Zika, which is associated with the devastating birth defect microcephaly among other presentations, was possible and even probable in the Southeastern U.S. because of the presence of the mosquito vector Aedes aegypti. Now that it has finally happened we are in a better position to discuss what the potential impact will be.


For months, public health authorities have been of the opinion that any mosquito transmission of Zika in the U.S. would be localized and fairly minimal. This opinion made a good deal of sense for a few reasons: 1.) Aedes mosquitoes don’t tend to fly very far; 2.) closely related viruses such as Dengue virus that are also spread by Aedes aegypti have historically been spread locally in the Southeast without widespread disease breaking out; and 3.) the U.S. tends to have more modifiers that discourage mosquito-human contact, such as window screens and air conditioning. Now that we have some evidence on Zika, however, I’m no longer sure that the conventional wisdom still does make sense.


While Governor Scott reports that no infected mosquitoes have been found, it is not clear to me whether this represents a true lack of infected mosquitoes, or incomplete testing in proximity to the infected patients. Specimens from neighborhood residents are now being collected and tested for evidence of Zika infection, and we will soon know how many others beyond the discussed fourteen patients have been exposed. What we do know is that all four patients are actively infected as of this writing, and that is what concerns me most. Fourteen infections occurring at the same time seems quite an elevated rate when compared to Dengue, which has not been mosquito-transmitted locally in Florida since 1934. Does this mean that another mosquito species (the more aggressive Aedes albopictus, most likely) is primarily spreading Zika? Perhaps. Does it mean that there is an animal reservoir of Zika, making it easier for it to hang around between human infections the way another of its cousins, West Nile virus, does? This is another possibility. Most importantly in my opinion, it means that the previous vision we had of very minor and limited Zika spread in the Southeastern U.S. may not be as applicable as we hoped. In the coming weeks, we will see very clearly how big the potential impact of Zika on the continental U.S. will be.