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Learning from the First Autochthonous Zika Outbreak

Amesh A. Adalja, MD, FACP, FACEP, FIDSA, August 5, 2016

The announcement that localized mosquito-borne transmission of Zika is occurring in southern Florida is an entirely expected, but nonetheless significant, event. As of this writing, 15 cases of local transmission have been identified. All but 1 case are associated with a specific neighborhood in the city of Miami called Wynwood. However, no mosquitoes in the US have tested positive for the virus to date. While US-based Zika outbreaks are likely to be delimited for many reasons, there are several important aspects of these cases that illustrate important points about Zika and similar diseases.

Hyper-locality

That this outbreak appears to be highly localized and almost entirely restricted to a specific neighborhood is in line with the behavior of the Aedes aegypti mosquito responsible for the Wynwood outbreak. Because this mosquito does not travel more than 150 meters from its birthplace, infected mosquitoes are highly concentrated in one area.

The hyper-locality of this outbreak does not preclude more cases in other parts of Florida (or other states), because the sheer number of infected travelers will provide other opportunities for distinct pools of mosquitoes to also become infected and spark local outbreaks. Indeed, in Florida’s prior dengue outbreaks, several counties, including Miami-Dade, reported cases.

Asymptomatic Cases

Of the 15 cases, 6 are described as being asymptomatic. Asymptomatic cases are traditionally thought to represent the bulk of Zika cases, with some debate existing over the actual rate with which symptoms occur. Asymptomatic cases, because of their cryptic nature, may play a special role in propagating the outbreak; people who are asymptomatic can escape diagnosis and thus are not being counseled about limiting exposure to mosquitoes so as not to facilitate more local mosquito infections. It will not be surprising if epidemiologic activities uncover even more asymptomatic individuals.

Travel Alerts

The Wynwood cases have also prompted an unprecedented domestic travel warning for pregnant women from the CDC, as well as the inclusion of the US on several countries’ Zika warning lists. The CDC warning is, to date, limited to a 1-square-mile area and does not apply to the rest of the city of Miami, the county of Miami-Dade, or the state of Florida. The granularity of the warning stands in contrast to those for other Zika-affected countries, which are mostly (if not exclusively) issued as blanket warnings. Presumably, exquisite detailing of the cases in Wynwood has facilitated a tightly delimited warning. Pregnant women who live in or frequent the affected area should be evaluated for Zika.

Experience Acquired

That Florida—a state that recently faced outbreaks of both dengue and chikungunya—is now facing another virus spread by Aedes mosquitoes is an event that has been anticipated since the virus appeared in this hemisphere. As the outbreak unfolds, crucial vector control and epidemiologic and health policy guidance steps will need to be taken to minimize the impact of the first autochthonous Zika outbreak in the 50 states. The experience acquired with managing the Wynwood outbreak should inform Zika management strategies in other US locales that may soon be in the same position.

 

Reference

Florida Department of Health. Department of Health daily Zika update. Updated August 2, 2016. http://www.floridahealth.gov/newsroom/2016/08/080216-zika-update.html. Accessed August 3, 2016.