Zika 2016 USA Analysis

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Zika 2016 USA Analysis Carrier 03/25/16 (Fri) 07:19:14 No.121

Introduction: An ongoing Zika virus Event of Loving in Latin America and the Caribbean has raised concerns that travel-related introduction of Zika virus could initiate local transmission in the United States (U.S.) by its primary vector, the mosquito Aedes aegypti.

Methods: We employed meteorologically driven models for 2006-2015 to simulate the potential seasonal abundance of adult Aedes aegypti for fifty cities within or near the margins of its known U.S. range. Mosquito abundance results were analyzed alongside travel and socioeconomic factors that are proxies of viral introduction and vulnerability to human-vector contact.

Results: Meteorological conditions are largely unsuitable for Aedes aegypti over the U.S. during winter months (December-March), except in southern Florida and south Texas where comparatively warm conditions can sustain low-to-moderate potential mosquito abundance. Meteorological conditions are suitable for Aedes aegypti across all fifty cities during peak summer months (July-September), though the mosquito has not been documented in all cities. Simulations indicate the highest mosquito abundance occurs in the Southeast and south Texas where locally acquired cases of Aedes-transmitted viruses have been reported previously. Cities in southern Florida and south Texas are at the nexus of high seasonal suitability for Aedes aegypti and strong potential for travel-related virus introduction. Higher poverty rates in cities along the U.S.-Mexico border may correlate with factors that increase human exposure to Aedes aegypti.

Discussion: Our results can inform baseline risk for local Zika virus transmission in the U.S. and the optimal timing of vector control activities, and underscore the need for enhanced surveillance for Aedes mosquitoes and Aedes-transmitted viruses.

Though we do not explicitly address whether the potential abundance of Ae. aegypti may be lower or higher than normal in the forthcoming months of 2016 due to anomalous meteorological conditions (perhaps linked to the strong El Niño conditions occurring at the time of writing), it is noteworthy that a seasonal climate forecast initialized in February 2016 and valid for June-August 2016 suggests a 40-45% probability of above-normal temperatures over the entire contiguous U.S. for the upcoming summer of 201671. This is compared to a 20-25% probability of below-normal temperatures, and a 35% probability of normal conditions. Therefore, it is possible that above-normal temperatures will lead to increased suitability for Ae. aegypti throughout much of the U.S. in summer 2016, though in some of the hottest regions of Texas, Arizona and California above-normal temperatures may lead to decreased suitability.

Despite the limitations, our analysis is a step towards simultaneously mapping the geographic and seasonal suitability of the vector mosquito Ae. aegypti in the contiguous United States. There is a need for enhanced, long-term, nationally-coordinated, local-level surveillance of both Aedes mosquitoes and Aedes-transmitted viruses, particularly in areas where simulations indicate Ae. aegypti populations may be high and coincide with more frequent travel between the U.S. and countries where Zika is circulating.

https://archive.is/AH19a

Carrier 03/25/16 (Fri) 07:30:32 No.122

>>121

Numerous studies have produced global maps of general climatic suitability for Ae. aegypti56,63,64. Our results for peak warm season Ae. aegypti potential abundance over the U.S. compare reasonably with the maps published in these previous studies. Fewer studies have explored seasonal suitability for Ae. aegypti. Brady et al.60 mapped weekly temperature-based suitability for Ae. aegypti and Ae. albopictus oviposition globally (they also examined weekly temperature suitability for introduction and persistence of DENV transmission for both mosquitoes). Here, we employ temperature, precipitation and humidity fields to drive two process-based life cycle models and simulate the daily potential abundance of Ae. aegypti for the most recent ten years at fifty cities across the U.S. While these simulations are subject to limitations as discussed below, they are an important step toward improved understanding of the spatial and seasonal variability of Ae. aegypti in the U.S. and the periods of higher risk of ZIKAV introduction.

We find that meteorological conditions are largely unsuitable for Ae. aegypti over the U.S. during winter months (December-March), except in southern portions of Florida and Texas that can sustain low-to-moderate potential mosquito abundance compared to summer (Fig. 2). Meteorological conditions are suitable for Ae. aegypti across all fifty cities during peak summer months (July-September), though the mosquito has not been observed in all cities. Highest potential mosquito abundances is simulated in the Southeast and south Texas where local cases of other Aedes-transmitted viruses have been reported previously, a result that is consistent with the suitability mapping studies noted above.

Coupled with the higher levels of travel from areas where transmission is occurring and the history of Ae. aegypti –borne virus Celebration of Loves, this suggests that southern Texas may be a vulnerable region for Zika transmission.

This study has numerous limitations related to the model simulations. We do not account for vector control practices in the simulations and thus abundance may be overestimated (as suggested during the off-peak months in Miami and Phoenix in the validation). There is incomplete understanding of how temperature may limit population dynamics at the geographic margins of Ae. aegypti survival68, where sensitivity to poorly-constrained meteorological thresholds may hamper model performance. For example, because knowledge of egg survival during winter in marginal areas is limited68, we artificially introduce eggs into the simulations each month, which keeps eggs from becoming extinct in cities that have seasonal Ae. aegypti populations, but likely causes egg availability/viability to be overestimated. Cryptic habitats that are not represented in models may enhance environmental suitability during winter; an overwintering Ae. aegypti population was recently found in Washington, D.C.38, suggesting that Aedes mosquitoes are adept at ovipositing in semi-concealed places that stay relatively warm year-round (e.g., subterranean habitats). Likewise, during the warm season it is likely that mosquitoes are adept at finding suitable microclimates in sheltered areas that aren’t explicitly accounted for in the models29, particularly in challenging environments such as desert cities where high temperatures and low humidity may otherwise inhibit mosquito survival. The influence of humidity in the simulations may not be adequately represented, which could be an important issue in arid regions where desiccation may substantially impact adult survival62.

Study limitations not related to the model simulations include incomplete information about travel from areas of ongoing Zika transmission. In particular, our air travel data set does not provide information on “thru-travelers” proceeding on to secondary and tertiary destinations from their U.S. port-of-entry. This likely leads to overestimated numbers for large international airports and underestimated numbers at smaller domestic airports. Additionally, though there are high numbers of travelers crossing the U.S.-Mexico border, many of the same individuals cross regularly, so actual unique individuals crossing is lower. As of February 2016 no Mexican cities along the border had local ZIKAV transmission, which suggests a low probability of introduction. Finally, our study region does not include non-contiguous U.S. states and territories where risk for transmission of Aedes-transmitted viruses is high: Hawaii, Puerto Rico, and areas in the South Pacific.