Could a Large-Scale Zika Outbreak Occur in the United States?

water storage tanks

Socioeconomic conditions such as a lack of running water—and thus the need for standing water storage, where mosquito larvae thrive—are a key factor in the likelihood of large-scale outbreaks of Zika and other mosquito-borne diseases. (Photo credit: Max J. Moreno-Madriñán)

By Josh Lancette

While recent news cycles in the United States largely have been dominated by election coverage, the threat of Zika remains a concern both locally and globally, with a recent outbreak in Brownsville, Texas, a study finding more than half of Brazilian women are avoiding pregnancy because of Zika, and the CDC sending nearly $200 million in funding to states and territories in order to fight the disease. But is a large scale outbreak of the disease likely in the United States? An article published in the Journal of Medical Entomology suggests: probably not.

The authors of the paper, Max J. Moreno-Madriñán of the Indiana University–Purdue University Indianapolis and independent research entomologist Michael Turell, suggest that socioeconomic conditions are a leading factor in whether or not a large-scale outbreak could occur and that the current socioeconomic conditions in the United States make a large-scale outbreak unlikely. This argument stands in contrast to the idea that if the climate is right, an outbreak is likely.

Josh Lancette

Josh Lancette

“The commonly accepted association between tropical climates and the endemicity of [mosquito-borne] diseases has led to concerns of the possibility of their redistribution due to climate change and transmission arising from cases imported from endemic regions initiating outbreaks in the U.S.,” write the authors. “While such possibilities are indeed well founded, the analysis of historical records not just confirms the potential critical role of traveling and globalization but also reveals that the climate in the U.S. currently is suitable for local transmission of these viruses. It seems clear that the main factors keeping these diseases from occurring in the U.S. today are socioeconomic such as lifestyle, housing infrastructure, and good sanitation. While such conditions are maintained, it seems unlikely that local transmission will occur to any great degree, especially in the northern states.”

Aiding their argument, the authors point to the different socioeconomic conditions and practices between the United States and places where Zika outbreaks have occurred. For example, many houses where outbreaks are common don’t have air conditioners or screens on windows, allowing mosquitoes to travel in and out of homes and increasing possible exposure time. Furthermore, due to a lack of piped water systems, many households use containers to collect and store rainwater, which mosquitoes can then use as breeding sites if the containers are not protected properly. In the United States, however, having air conditioning, screens on windows, and reliable drinking water infrastructure are more common.

The authors also point to historical case studies of yellow fever, malaria, and dengue, which were prevalent in the United States in the 17th through 19th centuries.

“Because the socioeconomic conditions in this country are clearly different today, as compared with when yellow fever and malaria were endemic, we believe that the critical factors determining the absence of these diseases in the U.S. today are socioeconomic rather than climatic,” write the authors. “This perception has been supported by contemporary cross-sectional comparisons regarding dengue transmission in contiguous cities near the U.S.-Mexican border. In studies addressing the latter case, the lower dengue prevalence on the U.S. side was explained by the higher socioeconomic conditions in the U.S. side and not differences in climate.”

However, the authors are careful not to rule out climate as a primary factor in whether an outbreak is likely—they just rule it out as the only factor.

“Both climate and socioeconomic conditions are in one way or another related to the various factors influencing the transmission of these viruses and, as such, need to be considered when analyzing the potential threat of transmission in the U.S.,” they write.

Even though the United States might not be at immediate risk for a large outbreak, it doesn’t mean it won’t ever be at risk or that residents and agencies shouldn’t take the threat of Zika in the United States seriously. Small, localized outbreaks could continue to occur, especially in southern states such as Florida, Louisiana, and Texas, which have long warm seasons, areas with socioeconomic conditions that more closely resemble those in areas where Zika has had large outbreaks, and more-common travel connections to countries where Zika and other diseases are present.

“It is important to consider that if the isolation between humans and Aedes aegypti mosquitoes in the U.S. is primarily caused by lifestyle and living infrastructure associated to socioeconomic conditions, these could be threatened by massive natural disasters, or any other event that disrupts current infrastructure,” the authors write. “Consequently, it is important that appropriate disaster preparedness plans be in place to address this potential issue.”

While the paper may work to allay the fears of some in regards to a large Zika outbreak in the United States, the authors would prefer that the paper act as a reminder of the importance of improving global health to stop diseases both domestically and abroad, as socioeconomic factors can be influenced positively by human prevention, education, and investment efforts.

“It is important for developed countries to be concerned about the health and wellbeing of developing and underdeveloped countries,” write the authors. “The growing interconnection of our global society makes global public health-related issues, such as sanitation and the lack of a continuous supply of running water in developing countries, to be an important concern to developed countries as these developing countries may serve as a source of imported cases of disease.”

Read more: “Factors of Concern Regarding Zika and Other Aedes aegypti-Transmitted Viruses in the United StatesJournal of Medical Entomology

Josh Lancette is manager of publications at the Entomological Society of America.

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